Updated:   1 December 2017


In September 2016 a new section
"Lunar Feature of the Month" was added to "The Sky Tonight" webpage.

The features chosen are craters, mountain ranges, peaks, rilles or other objects, chosen at random, which have unique or spectacular attributes.

Each object is being illustrated with a recent photograph taken with our Alluna RC20 telescope. If a better picture is obtained at a later date, it will replace the original one.

As all large lunar features are named, the origin of the name will be given if it is important..

To help the observer to find these features, each set of ten is preceded by an image of the Full Moon on which the locations of the ten photographs immediately following are shown according to their numbers in this series. 


Key to features 1 to 10 below.


1:    September  2016

This month's feature is the huge crater Clavius.

Clavius was photographed from Starfield Observatory, Nambour on August 13, 2016.
South is to the top, east is to the left.

Clavius is one of the largest craters, with a diameter of 231 kilometres. It is completely circular, but due to its position at lunar latitude 43 degrees south, we see it considerably foreshortened. It is quite ancient, and since it was formed by the impact of a large meteor or small asteroid about 3.9 billion years ago, other more recent impacts have deformed parts of it. The walls are quite high and rugged, especially on the eastern side. Two large craters have deformed the southern and northern walls. Both are about 55 kilometres in diameter and are named after the American astronomer Lewis Rutherfurd (top) and the American optician Russell Porter. When Clavius was formed, lava welled up and flooded the bowl-shaped floor, hardening into a smooth, flat surface with some ripples.

The floor has now been impacted itself, and shows many craters of various sizes. Unusually, there is an elegant curved line of five craters on Clavius' floor of ever-diminishing size from east to west. Their sizes range from 28 kilometres to 6 kilometres. Many dozens of craterlets dot the floor, particularly on the south-western side. The eastern side has a smooth area between Rutherfurd and Porter, that is crossed by lines of damage caused by ejecta thrown north by the Rutherfurd impact. Numerous craterlets less than two kilometres in diameter can be seen in this area. Some mountains protrude above the lava floor of Clavius, especially near its centre. Their elevation is shown by the fact that, as the sunlight is coming from the east, their shadows are seen on their right-hand side, while craters and other depressions have their bright walls on the right.


This spectacular crater is named after Christoff Clau, known to science as Clavius (left), who was born in Germany in 1537 and died in Rome in 1612. At that time, Europe was using the Julian Calendar, devised by Sosigines in Alexandria in 46 BCE on the orders of Julius Caesar. That calendar worked on the length of a year as being exactly 365¼ days. Unfortunately, the actual year is about 11¼ minutes shorter than that, and this error means that the calendar creeps ahead of the seasons by one day in 128 years. As Easter is a movable feast, not fixed to the calendar but determined by the seasons (Easter Sunday is the first Sunday after the first Full Moon after the northern hemisphere's spring equinox - our autumn equinox), but Christmas is fixed to the calendar (December 25), this meant that Christmas was catching up to Easter and by 1582 it had caught up by nearly two weeks. If nothing were done, then eventually Christmas and Easter would occur simultaneously, which alarmed the Church at the time.

In 1582, Pope Gregory XIII appointed Clavius, a Jesuit priest, to bring the civil and seasonal calendars back into synchronisation by realigning the vernal equinox with a part of the civil calendar that had been traditionally associated with the beginning of spring, around March 20. 

Clavius was a highly esteemed mathematician and astronomer who had written a well-regarded text book, Commentary on ‘De Sphæra’ of Sacrobosco. Clavius, like some other 16th century astronomers, had devoted much effort to recovering the mathematical texts of Greek antiquity, and had translated Euclid’s Elements, which brought him fame as the ‘Euclid of his time’. He adhered steadfastly to the geocentric universe as described in Ptolemy's Almagest, teaching his students that the Earth was the centre of the universe and that the Sun, Moon, planets and stars revolved around the Earth once each day. He did this for forty years until the third edition of the Commentary on ‘De Sphæra’ in 1581, when he began to have some doubts, writing: “regarding Copernicus, it is not absolutely certain that the eccentrics and epicycles are arranged as Ptolemy thought.”

Clavius admired Copernicus as a mathematician, and used data from De Revolutionibus, Sacrobosco’s De Sphæra, and Reinhold’s Prutenic Tables as bases from which to work on the calendar. Although the total error as described above was 12.7 days, Clavius calculated that Sosigenes had ignored a pre-existing three-day error the other way, so by 1582 the error he had to deal with was only about ten days. The simplest solution was to simply drop those ten days in one hit, rather than by one day at a time. To prevent such errors accumulating again in the future, Clavius modified the rule for determining leap years. In the Julian calendar, every year that was divisible by four was a leap year. But that practice clearly inserted too many leap days. How many? Over 128 years, it added an extra day, so over a 400 year period, it would add three extra days. So every 400 years, three days needed to be removed to keep the civil calendar in alignment with the seasons. 

What three days could be given up? Clavius decided that even century years would no longer be designated as leap years, unless they were divisible by 400. This meant that the years 1600 and 2000 would still be leap years, but 1700, 1800 and 1900 would not. Pope Gregory thought that these suggestions by Clavius were splendid, and instructed all countries to adopt them. It was announced that the evening of October 4 would be followed by the morning of October 15.

The ten days from October 5 to 14, 1582 simply would not exist. All Catholic countries complied, and adopted this ‘Gregorian Calendar’ immediately, but European Protestant nations refused to change. They eventually complied in 1700. Britain changed in 1752 – by then the number of days to be dropped had risen to 11. The day after September 2 was September 14. The British people rioted, chanting “Give us back our eleven days!"  Sweden changed their calendar in 1753, Russia complied in 1918 and Turkey in 1928.

When Galileo published his first spyglass observations in Siderius Nuncius (The Starry Messenger) in 1610, in which he described the phases of Venus, the craters of the Moon, and the satellites of Jupiter, Clavius was by then the top astronomer at the Collegio Romano, the Pope's university in Rome. He read Galileo's book with great interest, and began to correspond with its author about spyglasses. He invited Galileo to come down from Florence to Rome, which Galileo did in March 1611 and was treated with great ceremony by the priests at the Collegio. Clavius talked with Galileo for many hours about the new discoveries. Clavius had recently completed his sixth updated version of De Sphæra, which was already at the publisher as the third volume of his five-volume collected works, the Opera Mathematica.

After Galileo’s visit, he managed to have a new section inserted into the De Sphæra volume, enumerating Galileo’s discoveries. He also included a statement that, while slightly ambiguous, indicates to us that Clavius may have been thinking that his life’s work teaching geocentric astronomy had been ‘barking up the wrong tree’. He wrote, “Some things are thus, astronomers ought to consider how the celestial orbs should be arranged in order to save these [new] phenomena.” Whether he was recommending switching to the little-known Tychonic system or the Copernican is unclear, as both systems could explain the phases of Venus and could accommodate the moons of Jupiter and odd appearance of Saturn. Although he had opposed both theories for religious reasons, most historians agree that he was having second thoughts and was admitting that the Ptolemæic system in the form he had taught for a lifetime was no longer tenable. Perhaps he was beginning to accept the Sun-centred system. We would have known for sure had not serious illness laid him low in mid-1611, leading to his death eight months later.

On April 14, 1611, a few days after Galileo had been fêted by the Collegio Romano, Rome’s Lincean Academy held a banquet in his honour in Rome. Galileo thrilled the five members and their many guests by letting them see sunspots and, after dinner, the satellites of Jupiter and the star clusters of the Milky Way through one of his spyglasses. They lingered long into the night, enjoying the novel views. To prove that what the spyglass revealed was genuine, he aimed his instrument at a church over a mile away. The guests were amazed to read clearly the inscription on the building’s façade. Galileo then presented his spyglass to the Academy. They entreated him to become the sixth member of their august group, which he did, regarding it as a great honour. Thereafter, he proudly signed his name, ‘Galileo Galilei, Linceo’.          

The host, Prince Federico Cesi, announced that the Academy would be honoured to be Galileo’s publisher from then on. He also said that one of the other men at the dinner, Giovanni Demisiani (a chemist, theologian and mathematician to Cardinal Ferdinand Gonzaga), had devised a better name for Galileo’s spyglass (which he called a ‘perspicillum’), one that more aptly conveyed the instrument’s amazing capabilities, and differentiated it from its crude, low-power cousins. The name, Cesi explained, was a melding of two words in Demisiani’s native Greek:  tele, meaning ‘far away’, and skopéin, meaning ‘to see’. Henceforth, Galileo’s instrument would be known as a telescope.  It would reveal astrology to be totally fraudulent, and would elevate astronomy to the purest science.

2:    October  2016

This month's feature is the large crater Gassendi.

Gassendi was photographed from Starfield Observatory, Nambour on August 14, 2016.
South is to the left, east is to the bottom.

Gassendi is a moderately large crater, with a diameter of 114 kilometres. It is completely circular, but due to its position towards the Moon's west-south-western limb, we see it considerably foreshortened. It is quite ancient, and since it was formed by the impact of a large meteor or small asteroid about 3.9 billion years ago, a large more recent impact has deformed its northern wall (on the right-hand side in the image above). This later crater is called Gassendi A, and is 33 kilometres across. Almost adjoining it on its north-western side is Gassendi B, which is 26 kilometres across. The floor of Gassendi is flat, with a group of mountains in the centre that average 1200 metres high. To the south is a large, flat lava plain called Mare Humorum (the Sea of Humours).

The Mare Humorum was caused by an asteroid striking the Moon in the epoch after Gassendi was formed. This huge impact blasted out a crater 391 kilometres across, fracturing the Moon's crust in the area. These fractures released pressure on the hot rocky layers below, which immediately liquified, allowing hot magma to come to the surface as lava, which filled up the crater that had been formed, resulting in the large, level lava plain that was discovered and named the "Sea" of Humours by Giovanni Riccioli in the mid-17th century.

As the lava spread out from the impact crater, much of it reached the southern wall of Gassendi, sweeping over it and bursting in to pool on the southern end of Gassendi's floor (to the left as seen in the image above). We can see a gap in Gassendi's southern ramparts where the wall has been completely demolished, and other parts of the southern wall have been smoothed over by the lava. As the lava cooled, ripples in it became solid, and can be seen close to the south and south-east walls of Gassendi. The eastern, northern and western walls, unaffected by the lava flow, are rugged.

The floor of Gassendi is crossed by numerous clefts and looks quite fractured. The smallest crater on the floor that can be detected in the image above, is between the smallest mountain in the central cluster and the western wall. Called Gassendi P, it is 2 kilometres in diameter.  A narrow valley or 'rille' starts near a cluster of hills near the lower right of the image, and winds its way towards the north (to the right-hand margin). Rilles are common on the Moon, and this one runs past the crater Herigonius (not in picture) and is called Rima Herigonius. It is 104 kilometres long. As it progresses it widens from about a kilometre to two kilometres across. Though it looks like a river valley, this cannot be so as there is no liquid water on the Moon. Rilles are probably volcanic in origin.


This spectacular crater is named after Pierre Gassendi (left), a French philosopher, priest, mathematician and astronomer who was born in Champtercier in 1592 and died in Paris in 1655. He corresponded with Kepler and Galileo, and wrote definitive biographies of Tycho Brahe, Nicolas Copernicus, Georg von Peurbach and Regiomontanus, which received much praise. Gassendi was an early user of the newly invented telescopes.

At the time, there were three views of the Universe for people to choose from. Firstly, there was the geocentric view that the Earth was the centre of the universe, and everything else, star and planets, Sun and Moon, revolved around the Earth each day. This was the view from the ancient Greeks that had been promoted by Ptolemy in his Almagest of 150 AD, and embraced by the Christian churches. Secondly, there was the Tychonic universe of Tycho Brahe, which had the Sun and Moon revolving around the fixed Earth at the centre of the universe, but the planets Mercury, Venus, Mars, Jupiter and Saturn revolved around the Sun, not the Earth. The sphere of fixed stars enclosed all. The third view, promoted by Nicolaus Copernicus, was heliocentric, in that the Sun was at the centre of the universe, with the Earth and planets in orbit around it. The Moon was in orbit around the Earth and the sphere of fixed stars enclosed all as with the geocentric view of Ptolemy.

The Catholic Church had rejected the Copernican system and at the time was promoting the Tychonic system, for it seemed to explain the movements of the planets without requiring that the Earth move as well, for the Bible expressly said. "He set the earth on its foundations; it can never be moved." (Psalms 104:5). Although Gassendi followed his church's acceptance of the Tychonic world view, nevertheless he believed Kepler’s Copernican prediction that the planet Mercury would cross the face of the Sun on November 7, 1631. Setting up his telescope that morning, he made the first observation of a solar transit by a planet. He only saw the second half – the transit had started two hours before sunrise in Paris. Initially, when he saw a tiny black dot on the Sun's disc, Gassendi thought it was much too small to be a planet - he thought it was only a sunspot. Soon, however, its movement revealed it to be indeed Mercury.

Gassendi also wrote a dissertation on parhelia or sundogs, and was the first to propose a theory that light is composed of particles. This theory, which was published posthumously in the 1660s, was read by Newton in his student years, and was the basis for his later belief that light was composed of particles of matter, and not ‘waves’ passing through the æther as suggested by Huygens and Descartes.  

The argument over whether light was caused by particles or was a wave went on for over two centuries. We now know that light has both wave-like and particle-like properties, so everyone was partly right. For modern technical applications of any forms of electromagnetic radiation, both the wave and particle properties are indispensable. To make all of our telecommunications, radio, television, microwave ovens, mobile telephones, speed cameras and radar systems work, we need to invoke the radiation’s wave character. Digital cameras, gas discharge lamps (e.g. energy-saving light bulbs and street lighting), and (importantly) spectroscopy would not work at all were it not for the radiation’s particle nature. In March 2015,  scientists at the Swiss Federal Institute of Technology announced that they had photographed light acting as a wave and a particle simultaneously, a historic and significant breakthrough if confirmed. It is now thought that electrons also exhibit such duality.


3:    November  2016

This month's features are the two large adjoining craters in the image below. They are named Theophilus and Cyrillus.

Theophilus (top) and Cyrillus were photographed from Starfield Observatory, Nambour on October 7, 2016.
East (where the Sun is rising) is to the right, north is at the top.

These craters form a well-known threesome with Catharina (not shown), and Theophilus is obviously the newest, for it is more clearly defined and overlaps Cyrillus. All three craters were named by Giovanni Riccioli in the mid-17th century. He was a Jesuit priest who knew his history of astronomy and astronomers very well, and used this knowledge when applying names to the lunar features. The names were not chosen at random, and the three above were named after people connected with the lost Great Library of Alexandria which is described below.

Theophilus has a diameter of 104 kilometres and was formed between 1.1 and 3.2 billion years ago, much later than Cyrillus. It is bounded on the north by the Sinus Asperitatis (Bay of Asperity or harshness) and to the east by the Mare Nectaris (Sea of Nectar). Its walls rise about 1200 metres above those lava plains. The floor is flat, indicating that lava welled up from below shortly after the impact occurred, and pooled on the bowl-shaped floor, filling it to a moderatel extent. There is a cluster of four mountain peaks rising out of the flat floor to a height of about 1400 metres. On the north-west wall is a craterlet 8 kilometres across.

Cyrillus has a diameter of 100 kilometres, and its walls are highest in the south-east. It was formed between 3.85 and 3.92 billion years ago. The floor is rugged and fractured, with a large mountain and two smaller ones a little east of centre. A large meteor has struck the western interior slopes and created a 17 kilometre wide crater called Cyrillus A. The southern ramparts of Cyrillus are broken through by a wide valley which leads to Catharina about 80 kilometres to the south.


Background - the Great Library of Alexandria

The city of Alexandria was founded on the orders of Alexander the Great (356-323 BCE), a pupil of Aristotle, after his liberation of Egypt from the Persians in 332 BCE and the absorption of Egypt into the Greek Empire. The Egyptians welcomed him as a deliverer, and he was made Pharaoh. Standing on the shore of the Nile delta looking at the blue Mediterranean harbour under a brilliant Sun, Alexander saw at once that this could be the site of a great city. Named to honour him after his death aged only 32, it soon became the principal city of the world, a vast and sophisticated metropolis of marble, in comparison with which Rome and Athens were just provincial cities. Perhaps one fourth of Alexandria consisted of royal palaces and public buildings, but even private residences were made of stone. 

Though Alexandria became the literary, scientific, and commercial centre of the Greek world, what the ancient city is best remembered for today are its Temple to the Muses (Museum) and the Great Library of Alexandria, which were begun by Alexander’s General Ptolemy as a tribute to Alexander after he died. Ptolemy became Pharaoh Ptolemy I Soter (Saviour), the second Macedonian King of Egypt after Alexander, and is also famous for building the Pharos of Alexandria on a small island adjoining the city’s harbour. This 140 metre lighthouse was one of the tallest man-made structures on Earth for many centuries, and is regarded as one of the Seven Wonders of the Ancient World. Some remnants of it can still be seen.

Although the Museum was wonderful, it was the Great Library that became the jewel in the Greek crown, and a wonder of the ancient world. The gathering together of close to 1 million books and scrolls into a single collection in a world where books were rare and hand-copied was not an easy project, but it brought thousands of people into collaboration over many generations. Messengers were sent throughout the Mediterranean to collect everything of value. Writers, readers, translators, illustrators, artists, manuscript copiers, book traders, librarians, scientists, administrators and emperors all contributed to the greatest monument to human knowledge that ever existed until recently. In its halls, theatres and lecture rooms, it became possible for the first time for people to engage in systematic study.

Theophilus and Cyrillus

The earliest Christian thinkers had no problem at all in accepting the science of the ancient but pagan Greeks, Origen of Alexandria (AD 185-254) believing that the Bible was allegorical, and not in competition with natural laws; but by AD 300, writers such as Lactantius were ridiculing all pre-Christian knowledge, declaring that concepts such as the spherical shape of the Earth as taught by the Greeks were absurd and against the Holy Scriptures. This rejection and fear of science, along with most of the Greek heritage, became so pathological that in AD 391, Christian mobs acting for the Coptic Archbishop Theophilus wrecked the Great Library of Alexandria, burned as many books and scrolls as they could find, and killed the people working there.

The last scientist to remain after the destruction was a woman, Hypatia. She became head of what was left of the Great Library, and specialised in mathematics and astronomy. These particular areas of knowledge had come to be regarded by the Christian church as magic, heathen and therefore sinful. In great personal danger, Hypatia continued to teach and publish, and doggedly tried to pursue her studies. One day in the year AD 415, on her way home from work, she was set upon by followers of Cyrillus, the next Archbishop after Theophilus and later Coptic Pope of Alexandria. She was dragged from her chariot into a church, stripped, murdered and dismembered. The Christians flayed the flesh from her bones with abalone shells, oyster shells and shards of broken roof tiles. Her remains were burned and her works obliterated. A tall obelisk close to the site of her killing was later taken to London and named ‘Cleopatra’s Needle’ where it may still be found. Cyrillus (or Cyril), a nephew of Theophilus, was made an early Christian saint.  [ A 2009 movie, Agora, focuses on Hypatia’s final years, when she was studying heliocentrism and performing experiments of a kind that were later conducted by Galileo. A nearby crater to the three shown above is called Hypatia. The 117th Coptic Christian Pope still rules in Egypt today, Pope Tawadros (Theodoros) II of Alexandria. ]

We don't know what Hypatia looked like. Her image shown here is from a painting, The School of Athens, painted by Raphael between 1509 and 1511. This painting includes the images of 54 great thinkers, but for the representation of Hypatia, Raphael has used the face of Margherita, his mistress. The crater Catharina will be described next month.



4:    December  2016

Last month's features were the top two large adjoining craters in the image below,
Theophilus and Cyrillus.
This month's feature is the third, lower crater in this well-known threesome, Catharina.

Theophilus (top), Cyrillus and Catharina (bottom) were photographed from Starfield Observatory, Nambour on July 30, 2017.
East (where the Sun is rising) is to the right, north is at the top.

Of these three craters, Theophilus (top) is obviously the newest, for it is more clearly defined and overlaps Cyrillus. Catharina is the oldest of the three, appearing much more degraded and damaged by continual impacts by small meteorites over billions of years, All three craters were named by Giovanni Riccioli in the mid-17th century. He was a Jesuit priest who knew his history of astronomy and astronomers very well, and used this knowledge when applying names to the lunar features. The names were not chosen at random, and the three above were named after people connected with the lost Great Library of Alexandria which was described last month.

Catharina has a diameter of 100 kilometres and a depth of 3130 metres. It has been damaged by a later impact on its northern wall, which has produced a 46 kilometre wide crater, Catharina P. The walls of Catharina are quite steep in places, but the rugged floor is reasonably flat with no large, central mountains. The floor does contain some small hills and fissures, and is disrupted in the south by a 16 kilometre crater called Catharina S. Nearby, on the southern wall of Catharina, there is a small, bright, bowl-shaped crater 7 kilometres across, Catharina F.



The crater Catharina is named after Catherine of Alexandria, a Christian saint who was martyred in Egypt aged 18 in about AD 306, on the orders of the pagan Roman emperor Maxentius.

According to tradition, she was the daughter of Constus, the governor of Alexandrian Egypt which was then a province of the Roman Empire under the emperor Maximian. When Maximian retired in AD 305, his son Maxentius was passed over for succession as he had little military or administrative experience. By the following year, two joint emperors were in power, Constantine and Galerius. They allowed Maxentius to be regarded as emperor over central and southern Italy (excluding Rome), the islands of Sicily, Sardinia and Corsica, and the African provinces including Egypt.

Growing up in Alexandria, Catherine was a noted scholar, and after seeing a vision of the Madonna and Child, decided to follow Christianity and converted hundreds of people. When Maxentius began persecuting Christians, she went to him and rebuked him for his cruelty. He summoned fifty of his wisest philosophers to question her about her religious faith, but she described her beliefs in such eloquent terms that many of her inquisitors declared themselves Christians and were immediately put to death.

Catherine was then scourged and imprisoned, during which time over 200 people visited her, including Maxentius' wife Valeria. Catherine converted all of them to Christianity and they were subsequently martyred by Maxentius. The furious emperor condemned Catherine to death on the breaking wheel, a terrible form of torture. Here accounts differ slightly. One says that at her touch the wooden wheel shattered. Another says that as she was forced to the wheel, it suddenly began to spin rapidly, eventually flying to pieces. Maxentius then had her beheaded.

Her symbol in paintings is the spiked wheel. As well as having a crater on the Moon named after her, we remember her through the firework popularly known as the rapidly spinning "Catherine Wheel".



5:    January  2017

This month's feature is the crater Ptolemaeus.


Ptolemaeus was photographed from Starfield Observatory, Nambour on August 1, 2017.
East (where the Sun is rising) is to the right, north is at the top.

Ptolemaeus is a huge crater almost in the centre of the Moon's disc as we see it. Its walls reach up to 2400 metres high, yet its diameter is so large (averaging 155 kilometres) that a person standing in the centre of the crater would be unable to see even the highest peaks in the walls. Because the Moon is small compared with the Earth, the surface is more sharply curved, and the horizon is much closer than on Earth. With Ptolemaeus, all the walls would be below the horizon for a person standing at its centre.

The Jesuit priest who named most of the places on the Moon in the mid-17th century, Giovanni Riccioli, named this huge central crater after Ptolemaeus or Ptolemy, a Greek astronomer working at the Great Library of Alexandria in Egypt in the 2nd century AD, because Ptolemy played a central and important role in the history of astronomy. Because it is close to the centre of the Moon's disc, we see Ptolemaeus as if we were squarely above it, so there is no fore-shortening.

When Ptolemaeus was formed by a huge impact very early in the Moon's history (3.92 to 4.55 billion years ago), lava welled up from its centre and filled the floor, levelling it out. Since that time, only one large impact has marred the surface, although there are hundreds, if not thousands, of craterlets. The one noticeable crater inside Ptolemaeus is in its north-eastern quadrant, and is called Ammonius, after one of the last Greek philosophers, who lived in the 3rd century AD. It is 9 kilometres across and 1850 metres deep. North of Ammonius is a larger 'ghost crater' faintly seen above, which was completely swamped by the flow of molten lava.

The large crater to the north of Ptolemaeus is called Herschel, after the German-born Englishman William Herschel (1738-1832). It is a young crater 43 kilometres in diameter, with walls reaching heights of 3770 metres. There is a single mountain peak rising from the rugged floor, in an off-centre position.



Around the middle of the second century AD, Claudius Ptolemæus (Ptolemy, AD 90-168, no relation to the royal Ptolemies who were Macedonian-Greek Pharaohs of Egypt) was a Greek working at the Great Library. (The convention of Latinising one’s name, and then using either version, began with the Romans and continued until the end of the 17th century.) Ptolemy was the last of the great Alexandrian natural philosophers (scientists). He read the ancient Greek texts which were available, especially those of Hipparchus, and also the Babylonian records of observations that had been brought there in Alexander’s time. He realised that all the fragmented texts should be collated together, and preserved for posterity. Taking Euclid’s Elements as a model, in AD 150 he summarised the existing works of all the ancient Greek philosophers in mathematics, science and astronomy in a monumental 13-part book.

It was a textbook of astronomy as it was then known, not a revelation of discoveries that he himself had made. Although he claimed that some of what he wrote was based on his own personal observations, it now appears that some of these observations were from other unattributed sources, or possibly fabricated to fit his theories, a practice not unknown then or now. He called his book η Мαθηματικη Συνταξις or The Mathematike Syntaxis (The Mathematical Compilation). If Ptolemy had not produced this monumental work, most of our knowledge of the Greek philosophers would have been lost when the Great Library was destroyed.

Ptolemy amalgamated the ideas of the Greek thinkers into a complete cosmological model, based mainly on the teachings of Pythagoras, Plato, Aristotle, Apollonius, Aratus and Hipparchus. He reiterated that the Earth was an immovable globe at the centre of the universe, and that the Sun, Moon, planets, stars and the entire heavens revolved around it in circular orbits on eight transparent crystalline spheres, completing one revolution each day. This was the basis of his version of the geocentric view of the universe, which was adopted by astronomers without serious question for the next 1250 years, until the advent of Nicolaus Copernicus.


The Almagest

On December 22, AD 640, Muslim armies under Amrou Ibn el-Ass conquered Alexandria. They found the remains of the Library, and discovered thousands of ancient scrolls still hidden there. These were all burned in the local bath-houses to warm the water, except for the works of Aristotle. The destruction of the Great Library of Alexandria by both Christians and Muslims in the name of their religions was both thorough and tragic. Practically everything that had been painstakingly collected over centuries was gone forever. We are left with only fragments. All that remains of the building today is the cellar of the Serapæum, the library annexe, with a few mouldering shelves, but a new Library was built and opened in 2002.

Happily, some full sets of Ptolemy’s Mathematike Syntaxis were found by Arab soldiers and presented to Arabian astronomers as booty. These men immediately recognised them as great treasure, al-Jahiz (one of their most advanced thinkers), saying “The ancient writings are the key to wisdom.” They named Ptolemy’s work الكتاب المجسطي, (Al MegisteThe Great Book). It was translated from Greek into Arabic at Damascus and later at Baghdad's House of Wisdom, and was widely distributed throughout Arab lands. Eventually it found its way to Moorish Spain in the 12th century. There it was translated into Latin and disseminated into Europe, where it helped end the period of ignorance known as the Dark Ages. Known since those times as Ptolemy’s Almagest, it is the main source of all that we know and understand about the Greek astronomical heritage. After nearly 2000 years it is still being published, and an English translation running to nearly 700 pages can be purchased from Amazon.com for about $160.


William Herschel

William Herschel (1738-1832) started off as an amateur astronomer in Bath, England. After he discovered a new planet in 1781, he named it 'Georgium Sidus' (The Georgian Star) after King George III. The French named the new planet 'Herschel', but the German astronomer Johann Bode suggested that it be called 'Uranus', in keeping with the names of the other planets, which were named after Roman gods. It took the English seventy years to finally adopt the name 'Uranus'. King George made William the King's Astronomer, his only duties being to provide astronomical entertainment when required by the King's guests at Windsor Castle. William moved to Slough, within walking distance of Windsor, and devoted his life to building telescopes and using them effectively. His telescopes were simply the biggest and best in the world at the time, and were capable of looking deeper into space than any others. With these instruments, Herschel amassed a lifelong record of astronomical discovery that would have been worthy of an entire research institute, much less one man.

A partial list of William’s discoveries compiled by his son John in 1825 includes the discovery of Uranus and two of its satellites, Titania and Oberon; discovery of two satellites of Saturn, Enceladus and Mimas; the first measurement of the rotation period of Saturn’s rings; confirmation of the gaseous nature of the Sun’s surface; measurement of the heights of lunar mountains; the discovery of 848 binary stars and 2500 nebulae and star clusters; the identification of planetary nebulae, the resolution of the entire Milky Way into stars; the finding that stars differ widely in their intrinsic luminosities; and the determination that the Sun and its solar system are moving through space towards a point in the constellation Hercules, the ‘solar apex’. Interested in the link between light and heat, he used a prism to disperse a ray of sunlight into a spectrum, and then placed the bulbs of thermometers at each colour, finding that the red was the hottest. Struck by a flash of inspiration, he then moved the thermometer at the red end of the spectrum into the unlit area further from the violet end, and found the temperature there even hotter. He decided that this area must be lit by invisible light rays which produced heat, and called these rays ‘calorific rays’. Soon they became known as ‘infrared radiation’, one of William’s most important discoveries.

Whereas previous astronomers had been either painstaking observers like Tycho Brahe and John Flamsteed, excellent telescope makers like Galileo, or ground-breaking theorists like Kepler and Newton, William Herschel was unique in that he was all three combined. Sir Patrick Moore said that Herschel was probably the greatest observer who ever lived.

6:    February  2017

This month's feature is the crater Alphonsus.


Alphonsus was photographed from Starfield Observatory, Nambour on August 1, 2017.
East (where the Sun is rising) is to the right, north is at the top.

Ptolemaeus, described last month, is a huge crater almost in the centre of the Moon's disc as we see it. Adjoining it on its southern rim is another large crater, Alphonsus, which is 121 kilometres in diameter and has walls up to 2730 metres high. Some of the walls have terraces. The crater floor is fairly flat, but a central spine runs from north to south. Alongside this spine, a large mountain peak is a few kilometres north-east of the crater's centre. The mountain's base has been swamped by lava that has flooded the floor, so that only its peak pokes above the surface. The floor is broken by numerous faults and rilles, some of which are radial but some are roughly parallel to the walls.

In places on these faults and rilles, there are small craters with dark haloes around them. These are volcanic vents, obviously connected with the fault lines. Called ash volcanoes, they emit quantities of fine-grained dark-coloured ejecta which accumulates around the vent, sometimes filling in the closer parts of the fault trough. There are nine ash vents visible in the above image.

The medium-sized crater to the south west (bottom left) is Alpetragius, 41 kilometres across with a bowl-shaped floor. It is unusual in that it has a large dome-shaped mountain in its centre.

The Jesuit priest who named most of the places on the Moon in the mid-17th century, Giovanni Riccioli, named Alphonsus after King Alfonso X of Castille.



The rapid rise of Islam following Mohammed’s flight to Medina in AD 622 had a major impact on astronomy and the world. Muslim armies spread west from Mecca, subjugating Syria, Egypt, Libya, Roman Carthage, North Africa, Morocco, and east to Persia and the Indian border. When Muslim soldiers reached Egypt in AD 640, they destroyed what remained of the Great Library of Alexandria (it had been sacked by Christian mobs incited by the Coptic Archbishop Theophilus in AD 391). Yet many of the works of pagan Greek philosophers of the previous ten centuries were regarded as treasure, for the wisest Arab thinkers said, "The ancient writings are the key to wisdom." The scrolls containing the works of Euclid, Archimedes, Plato, Aristotle and Ptolemy were carried back to the Islamic capital of Damascus, and translated from Greek into Arabic. In the next three centuries the Muslim capital was moved to Baghdad and by AD 800 the Caliph al-Mansur had built a 'House of Wisdom' for the translation, copying and distribution of the ancient works. It was a research centre for the study of the humanities, mathematics and science, and was unrivalled in the medieval world. In this way the Arabic translations of the writings of the Greeks were disseminated across North Africa to Morocco, and when the Muslim armies crossed the Strait of Gibraltar and invaded the Iberian Peninsula, into Spain.

By AD 750 Muslim armies had occupied the southern and central parts of what we now call Spain, where they were known as 'Moors'. The northern provinces of León, Castile and Galicia remained Christian, with their own Kings. As time went on, the Moorish princes and caliphs became more interested in scholarship, art and architecture than warfare, and established libraries, academies and other seats of learning, particularly at Toledo, Seville, Córdoba and Granada. These communities were very progressive, Córdoba being one of the first medieval towns to have public street lighting. Muslim scientists were encouraged to base themselves in these towns, and they brought with them their Arabic translations of the ancient Greek texts such as Ptolemy's Almagest. The Moors built great citadels and mosques in Spain, the greatest one still existing being the Alhambra palace-fortress at Granada.

Alfonso VI (1040-1109), nicknamed ‘El Bravo (the Brave)’, was King of León from 1065, and King of Castile and Galicia from 1072. He wished to release Iberia from Muslim domination and enlisted one of his adversaries, Rodrigo Díaz de Vivar (1043-1099), known to history as ‘El Cid (The Lord and Master)’, to lead his army.  The Christian forces of the combined kingdom of León-Castile liberated Toledo in the Reconquista (re-conquest) of AD 1085. After that, Toledo continued to be a major cultural centre. Its vast Muslim libraries were made freely available to all. Alfonso established a translation centre in the city, in which Arabian books were translated from Arabic or Hebrew to Spanish by Arab and Jewish scholars, and from Spanish to Latin by Castilian scholars, thus letting long-lost Greek knowledge spread into Europe. Most of the southern half of the Iberian Peninsula including Andalusia remained in Moorish hands.  

Abu Ishaq Ibrahim al-Zarqali (Arzachel, AD 1028-1087) was a Muslim astronomer living in Toledo, and as Alfonso’s forces approached, he escaped to Córdoba where he continued his work. He laboured to adjust and correct Ptolemy’s geographical data, specifically the length of the Mediterranean Sea. He prepared accurate ‘zijes’ (a ‘zij’ was the Arabic name for a table of positions of stars and planets), using the more recent observations of Arab astronomers al-Khwarizmi and al-Battani to update Ptolemy’s work and correct for precession. It is quite possible that he implemented his own program of observation, or utilised even more sources, to modernise the older material. Such books of zijes or tables were known by their Spanish-Arabian name al-manakh (calendar), from whence was derived the word ‘almanac’. In 1087 he published the Almanac of Azarqueil.

Raymond of Toledo, Archbishop of Toledo from AD 1126 to 1151, continued the efforts of Alfonso VI by encouraging translation efforts at the library of the Cathedral of Toledo, where he led a team of translators that included Mozarabic Toledans, Jewish scholars, Madrasah teachers and monks from the Order of Cluny. They usually translated the ancient texts from Arabic into Castilian, and then from Castilian into Latin, although in some cases, where the expertise of the translator allowed it, they were translated directly from Arabic into Latin or Greek. The work of these scholars also made available any important texts from Arabic and Hebrew philosophers, if the Archbishop deemed them important enough for a complete understanding of the classical Greek authors, especially Aristotle. As a result of their activities, the cathedral became known as the Toledo School of Translators – a translation centre with an importance not matched in the history of western culture. The present cathedral was begun in 1226.

Gerard of Cremona, (AD 1114-1187), born in Cremona in northern Italy, came to the Toledo School of Translators in about AD 1144 in search of Ptolemy’s Almagest. Since he didn’t understand Arabic when he arrived, he had to rely on Jews and Mozarabs for both translation and teaching. He turned out to be the most productive of the Toledan translators at the time, with more than 87 books to his credit. He edited for Latin readers the Tables of Toledo, the most accurate compilation of astronomical and astrological data (ephemerides) ever seen in Europe up until then, These tables were completely up-to-date, and were partly based on al-Zarqali’s Almanac of Arzarqueil of AD 1087, and also on the works of Jabir ibn Aflah, the Banu Musa brothers, Abu Kamil, Abu al-Qasim, and Ibn al-Haytham (including his Book of Optics). The Tables of Toledomade a vital contribution to the rebirth of mathematical astronomy in Christian Europe.

Gerard was then given a huge task, to translate the Arabic version of the Almagest into Latin. Two Arabic translations of the Greek were then current, one by al-Hajjaj dated AD 827, and one by Ishaq ibn Hunayn dated AD 880. The second one had been revised by Thabit ibn Qurra in AD 901. Gerard had copies of both, and his translation into Latin combined the two. Where the two versions differed, Gerard wrote the alternative translation in a small hand, in the margin. The State Library of Victoria owns a priceless copy of Gerard's translation of the Almagest, hand-copied in Venice between 1200 and 1225.

In 1212 a coalition of Christian kings led by King Alfonso VIII of Castile drove the Muslims from the central Iberian Peninsula, forcing them into the small Moorish Kingdom of Granada in the south, which lasted for another 280 years. 

By 1252, King Alfonso
X (Latin form Alphonsus, AD 1221-1280, right), nicknamed ‘El Sabio (the Wise)’ of Castile, León and Galicia, ruled northern and central Spain. In Toledo he continued the tradition of translation enthusiastically, having many Arabic scientific and philosophical writings translated into Castilian Spanish by a team of 15 Christian and Jewish savants, including one Muslim convert to Christianity. He took a personal interest in their work, and even wrote introductions to some of the translated books.  

Knowing the value of the Tables of Toledo, he assembled a team of scholars and directed that they be translated into Castilian and updated, with all the errors and anomalies removed. He had new instruments made and personally supervised an extensive program of new observations that were needed. A famous but probably apocryphal story says that while he was doing this work, Alfonso commented that the Ptolemæic system was far too complicated mathematically, and "if the Lord had consulted me before embarking on creation thus, I should have recommended something simpler."

It was through the work of Arzachel, Gerard of Cremona and Alfonso that the astronomical ideas of the ancient Greeks (mostly sourced from Ptolemy's Almagest that had been filtered by translation into Arabic and combined with Arabian mathematical concepts such as the decimal system with its zero and place value, Hindu-Arabic numerals, algebra and trigonometry), were translated into Latin and spread throughout Europe, ending the Dark Ages and eventually leading to the Renaissance. This mixture of cultural ideas is why today we have the majority of constellations in the sky that are from the ancient Greeks, yet are populated with stars that have Arabic names, e.g. Aldebaran, Altair, Deneb, Rigel and Fomalhaut.



Nur ad-Din al-Bitruji ("Alpetragius", AD circa 1130-1204) was born in Moorish Andalusia in the south of the Iberian peninsula (Spain). He proposed a theory on planetary motion in which he wished to avoid both epicycles and eccentrics of Ptolemy, and to account for their varying paths and speeds by compounding the rotations of homocentric spheres. It appears that the system he proposed was an amalgamation of that of Eudoxus of Cnidus, combined with the motion of fixed stars developed byal-Zarqali (Arzachel). His knowledge of Eudoxus’ works would have come through the filter of Arabic translations of the Almagest.


7:    March  2017


This month's feature is the crater Petavius:

Petavius was photographed from Starfield Observatory, Nambour on July 9, 2016.
East (where the Sun is rising) is to the top, north is to the left.

Petavius is a large circular formation, but as it is near the Moon's limb (edge), it appears to us greatly foreshortened. It is one of the finest crater plains on the entire lunar surface and a grand object under low and medium illumination, but difficult to see approaching Full Moon. It is 182 kilometres in diameter measured from crest to crest, with massive, broad and very complex walls rising to peaks of 3300metres on the west where the wall is double, and 2100 metres on the east. There are traces of a once-complete double rampart, the inner one being lower and less regular than the outer and main wall. The interior is decidedly convex, the central portion being 240 metres higher than that adjoining the walls. In the centre of the floor is a grand, complex mountain group, the principal peak rising 1700 metres and casting a long shadow under a low sun. This, and the details of the group, are best seen when the Moon is two or three days after Full.

From this mountain group one of the finest clefts on the entire Moon runs towards the south-west wall, cutting through the inner wall into a valley between the walls. In places this great almost-straight cleft, which can be seen with a very small telescope, has raised banks, like a canal. The inner third of its length is about 4 kilometres wide, but then it narrows to about 2 kilometres and becomes deeper as it approaches the outer wall. There are a number of small dome-like hills on either side of this cleft.

The southern half of the floor of Petavius is quite smooth in comparison with the northern half, which is much more rugged and crossed by two winding clefts. One of these connects with the central end of the Great Cleft, and heads roughly north from a valley between the central peaks, before turning to the north-west and petering out. The other is finer, and crosses the crater floor midway between the peaks and the north-east rim. Whereas the Great Cleft and the north-heading one are radial to the crater's centre, this third, delicate cleft runs nearly parallel to the crater rim.

Three parallel clefts run from the central mountains in a south-east direction towards a small 5 kilometre crater, Petavius A, which is halfway from the centre to the main wall. These are quite delicate, as are the numerous craterlets which dot the floor of Petavius. South-west of Petavius A is a cluster of low domes, visible in the image above.

The surrounding region is very complex, consisting of ridges gradually radiating away from the rim of Petavius with shallow valleys between. The whole area around the crater has been swamped with superheated liquified rock, melted from the heat caused by the impact which created Petavius.

As Petavius is near the south-east limb of the Moon, we see the crater at an angle, which foreshortens its circular shape into an ellipse. On the southern wall of Petavius (on the right in the picture above, is an 11 kilometre wide crater, Petavius C. A peculiar double ridge 150 kilometres long passes through Petavius C and ends at the end of the Great Cleft. To the west of Petavius is the 60 kilometre wide crater Wrottesley which, like most large impact craters, also has a central mountain group. Petavius was created in the Lower Imbrian period (3.8 to 3.85 billion years ago), while the later crater Wrottesley, adjoining its western ramparts, dates from the Upper Imbrium period (3.2 to 3.8 billion years ago). 



The crater's name is the Latinised version of the name of a French chronologist, Denis Pétau (1583-1652). He was also a historian and theologian, but his connection with astronomy is quite obscure. The crater was named by Riccioli in 1651.



This crater is named after John Wrottesley (1798-1867), a 19th century English astronomer who did valuable work in cataloguing double stars and measuring star positions. The crater was named by Birt and Lee in 1865.


8:    April  2017


This month's features are the craters Eratosthenes, Copernicus and Stadius.

This area was photographed from Starfield Observatory, Nambour on October 10, 2016. East (where the Sun is rising) is to the right, north is to the top.  Eratosthenes is the crater at top right, Copernicus is at lower left. The debris field caused by rubble from the Copernicus impact is at centre, where the ghost crater Stadius can be faintly seen.


Copernicus one and two days later. As the Sun rises, the shadows diminish. A high peak on the eastern rim throws a conspicuous shadow. This pair of pictures demonstrates what astronomers call "poor seeing" and "excellent seeing", i.e. when the air is turbulent and when it is still. Even the best Earth-based optical telescope has its work restricted to some extent by the vagaries of atmospheric conditions, which is why there are quite a few orbiting observatories in space now.

Eratosthenes is the 60 kilometre wide crater at upper right, at the south-western end of a spectacular range of mountains called the Apennines. It is typical of a medium-sized crater, with a cluster of central peaks. It was formed between 1.1 and 3.2 billion years ago.

The crater Copernicus, with a diameter of 95 kilometres, is the largest crater in the image, at lower left. It is much more recent, only about 1 billion years old. The Sun was just on the point of rising over this crater when the main image was taken, and only the raised circular rim is illuminated by sunshine. The depressed interior is in deep shadow. Views of the crater one and two days later show how the appearance of all lunar features rapidly changes from night to night.

The oldest crater is 71 kilometre wide Stadius, found between the other two, and so ancient that numerous lava flows have filled it up and almost obliterated it, making what is called a 'ghost crater'.

These three impact craters are not far from the centre of the Moon, so they are seen almost directly from above, and are not much foreshortened. Although there are some volcanoes on the Moon with craters at their summits, by far the vast majority of deep, circular features with raised rims that cover much of the Moon's surface are caused by the impacts of flying pieces of rock onto the lunar surface. In 1791 Johann Schröter named these features 'craters', from the Latin word for 'cup', because of their profile. He also named certain valleys 'rilles', from the German word for 'grooves'.

In the Moon's distant past, when the Solar System was very young, there was a lot of material circling the Sun which came into collision with the newly-formed planets and their satellites or moons. This material was in the form of rocks ranging in size from sand grains and pebbles up to car-sized and even as large as Tasmania. Each planet's gravity swept most of them up, but there are still plenty of them still flying free in space. We call them SSSBs (Small Solar System Bodies). The small ones are commonly known as meteors when they strike our atmosphere and burn up by friction; larger ones may develop tails of dust and vented gas when approaching the Sun and are known as comets, and the biggest ones are sometimes called asteroids. The largest asteroid, Ceres, has a diameter about a quarter of that of our Moon, and is called a 'dwarf planet'.

If one of these objects flying through space hits a rocky planet with little of no atmosphere to slow it down, then the impact creates a huge crater. Mercury, Mars, the Moon and satellites of the outer planets have areas which are covered with overlapping craters. Even the Earth has had numerous strikes, and the evidence of many is still visible today, despite two or three billion years of weathering and erosion. Two large 'astroblemes' in Australia are Wolfe Creek crater in the Kimberley District and Gosse's Bluff in Central Australia, where a collection of smaller craters at Henbury can also be found.

On the Moon, a large impact crater like Copernicus is caused when a large rock flying through space - say about the size of Ayer's Rock or Nambour - hits the lunar surface when travelling at a typical 30 kilometres per second. This flying mountain has great mass and its speed gives it great kinetic energy. On impact with the Moon, the rock is stopped short in a tiny fraction of a second, and its kinetic energy is instantaneously converted into heat energy, which vaporises the rock in a stupendous blast that creates the crater. Great amounts of molten rock (called 'rock melt') are deposited all around the site of the impact, covering and blurring older features and clearly visible in the pictures. Rocks and boulders as large as city blocks are flung out in all directions for many hundreds (occasionally thousands) of kilometres. These rain down on the Moon's landscape, peppering the area with craterlets, many with shapes elongated radially from the impact. These types of craterlets are conspicuous in the central region of the picture above, some being in lines where a large block has bounced along the surface, leaving a craterlet each time it hit the ground.

The lunar surface was originally a light grey in colour, but as it ages, the continual rain of micrometeorites and particles from the Sun (mainly protons and ions) coats each soil particle with a very thin layer of metallic iron called nanophase iron (npFeo). This darkens each grain and hence the area of the surface in general, according to how long it has remained undisturbed. When there is a large impact nearby, material ejected from the new crater disturbs the surface in a pattern radial to the crater, bringing the light coloured subsoil to the surface and creating a pattern of light-coloured rays centred on the new impact crater. The craters Copernicus and Kepler have widespread ray systems. By far the most spectacular ray system on the near side of the Moon is centred on the southern crater Tycho, whose rays spread across the Moon for thousands of kilometres. They are very prominent around Full Moon. 

As well as the blast going outwards, it also sends a powerful pulse of energy downwards into the Moon. When this pulse hits the solid bedrock, it bounces back up to the surface, fracturing and lifting the floor of the crater just formed. This is the origin of the clusters of central peaks found in the middle of the floor of a majority of impact craters, such as Eratosthenes above. Copernicus also has a cluster of peaks, but they are only 1200 metres high. In the image above, their summits, being 2560 metres lower than the surrounding rim, are yet to be illuminated by the sun's rays.

If the downward pulse of energy is powerful enough to fracture the bedrock, thereby releasing pressure on the hot rocks below, they immediately liquefy into molten magma which forces its way up to the crater floor, where it pools, sometimes overwhelming the cluster of new peaks and creating a flat floor to the crater. The large crater plain Ptolemaeus (described above and visible in the small Full Moon image above as a circular flat area almost in the Moon's exact centre) was created in this way. There have been no craters larger than a kilometre in diameter formed in the 408 years since Galileo first looked at the Moon through a telescope in 1609.



Since the time of Pythagoras in the sixth century before Christ, all educated Greeks and Egyptians knew that the world was shaped like a sphere. Its actual size was still open to conjecture. They believed that the Earth was fixed and unmoving at the centre of the universe (a 'geocentric' view), and that the Sun, Moon, planets and stars revolved around the Earth once per day, being carried on transparent crystalline spheres. A few thinkers in the third and second centuries BCE, such as Aristarchus and Seleucus, thought that the Sun was at the centre of the universe (a 'heliocentric' view), and that the Earth was just another planet revolving around it in a year and rotating on its axis once each day, but they were not taken seriously.

At the beginning of the sixteenth century AD, the Renaissance was under way. The printing press has been invented, Columbus had discovered the New World, Vasco da Gama had found a sea route to India, and people like Leonardo da Vinci and Michelangelo were changing the art world. Niklas Koppernigk (Latinised to Nicolaus Copernicus, 1473-1543) was by 1506 a well-educated Renaissance man. He was a physician, economist, diplomat, mathematician, classical scholar, military leader, governor and artist. He worked as a Canon at Frauenburg Cathedral in East Prussia (now Frombork in Poland), but did not take holy orders. His hobby was astronomy.

He had read the Epitome of the Almagest by Regiomontanus, which described the fixed Earth and transparent spheres surrounding it, and thought the whole system as described by Aristotle and Ptolemy was mathematical sleight-of-hand. Though it gave fairly good predictions of planetary movements for astrologers, the whole complicated system with its epicycles, deferents and an equant seemed to be based on a lie. The more he studied Ptolemy's original Almagest when it was finally published by printing press in 1515, the more he realised that all the complications would vanish if the Sun replaced the Earth at the centre of the planetary orbits, and each planet would then snap into place.

He decided to begin work on a book of his own, using the Almagest as a model. In his book, he would clearly and patiently piece his theory together, fully explaining it with mathematical proofs and rigorous logic. He did not know it, but his book would restore interest in the heliocentric system suggested by Aristarchus in the 3rd century BCE, and lead directly to new laws of physics.

The book took about twenty years to write, but Copernicus was afraid to publish it as he felt that the Church, which strongly supported the geocentric view, would see his idea of an Earth moving around the Sun as an attack on the holy scriptures.  After all, Psalm 105:5 said quite definitely, "He set the Earth on its foundations; it can never be moved." In his last years, he finally agreed to publish 500 copies of it, and a copy fresh off the press was placed in his hand as he lay comatose on his deathbed. The book was called De Revolutionibus (On the Revolutions). It was indeed 'revolutionary', and gave us the word to use for anything that challenges the existing order of things.

There was no theory of universal gravitation existing at the time to provide physical support for Copernicus’ system. Initially, the book made little impact. Reading it in Latin was heavy going, and not many people read it through completely; but before long most scholars were aware of the substance of the heliocentric theory, and a small but growing group of astronomers were convinced of its veracity and studied it in great detail. It was treasured by all who had a copy pass through their hands, and the wide margins of the copies still in existence are full of comments and notes. 275 of the 500 first edition copies are still with us, and priceless. In the end, De Revolutionibus was enough. It would be recognised as the greatest scientific book of the sixteenth century. It would change the world forever, but not until the invention of the telescope in 1608 proved that the Earth and other planets did orbit the Sun.



This crater is named after Eratosthenes, who was a custodian of the Great Library in Alexandria at the mouth of the Nile two centuries before the birth of Christ. Many of our Greek myths about the constellations come to us through his writings and those of Hyginus.

The story goes that Eratosthenes was told by a visitor to the Great Libray that at noon on the summer solstice (June 21), the Sun shone vertically down a well further up the Nile River at Syene, its reflection appearing in the water. Also, vertical sticks and tall columns were seen to ‘swallow their shadows’ around the time of the summer solstice. He checked this the following year and found that it was so, and that, at the same time on the same day, the Sun was 7.2º from the vertical at Alexandria.

Knowing the distance between the two observing sites to be 5000 stadia or about 800 kilometres, a stade or stadion being 157.5 metres (he hired professional ‘steppers’ to pace it out using a standard stride), he used this information to make a famous measurement of the diameter and circumference of the Earth, getting quite an accurate result. His method was simple: the 7.2º angle between the two towns is about one-fiftieth of a circle of 360º. This distance equals 800 kilometres.

Therefore the circumference of the whole circle (the Earth) equals  50 × 800 km  =   40 000 km. His calculation of the Earth’s circumference turned out to be very close to the true figure of 40  025 km (polar circumference), but it seems that he had more than his share of good fortune. The fact that Syene is slightly north of the Tropic of Cancer should have made the Sun shine down the well at a slight angle, not vertically. We are not told how deep the well was, and if it was exactly vertical. Would the use of their primitive measuring instruments at Alexandria have provided the quoted accuracy? How accurate were the measurements of the man or men who paced the distance? Did he use an 'Egyptian Stadion' or an 'Attic Stadion'? All of these factors would have introduced small errors into his observations and calculations, but luckily they appear to have cancelled each other out. In any case, he was the first man to measure the size of a planet, and all he used was sticks, eyes, feet and brains.


One of the first astronomical almanacs to be based solely on the heliocentric system of Copernicus was the Ephemerides novae at auctae of Jan Stade (Latinised to Johannes Stadius, 1527-1579), published at Cologne in 1554. Avidly read and enthusiastically used by Tycho Brahe and Michel de Nostradamus, this set of planetary tables fearlessly stated that its calculations were made using the heliocentric system.


9:    May  2017


This month's features include the craters Aristarchus and Herodotus, and the unusual Schröter's Valley.

Aristarchus is the bright 41 km crater at right, Herodotus is the flooded 36 km crater to its left.
Vallis Schöteri
(Schröter's Valley) is the remarkable feature to their north, and is 165 km long. The valley
begins at a small crater in the highlands between Aristarchus and Herodotus, widens out and then narrows
again heading north, before zig-zagging to the north-west and then turning to the south-west, where it narrows
further and peters out.  It has been  described as like a snake, in particular as a cobra, and its widest area
near its starting point has become  known as the 'Cobra Head'. A delicate rille 200 metres wide like a dry
water course meanders along the margins of the valley, visible above for most of its length. There is a complex of
narrow, winding rilles in the north-eastern corner of  this image.

This area is in the north-western quadrant of the Moon, and is considerably fore-shortened. The brightest crater is Aristarchus, 41 kilometres in diameter and being less than 500 million years old is fairly recent. It is one of the brightest lunar craters, and can be seen in the dark part of the waxing crescent Moon, being faintly illuminated by Earthshine (sunlight reflected off the Earth, which would be nearly Full if the Moon is New). It stands on a rocky plateau, and its walls rise about 600 metres above the surrounding lava plain of the Oceanus Procellarum (Ocean of Storms). The interior walls have rugged terraces, and there is a small central mountain which is only about 350 metres high. The western interior slopes have a number of dark, radial bands running up them, two of which can be seen in small telescopes. The interior of Aristarchus is bowl-shaped, except for a small flat section at its centre, where lava has welled up and pooled.

Herodotus is a much older crater, being over three billion years old. It is 36 kilometres in diameter and its dark interior is in striking contract to that of Aristarchus. The narrow walls enclose a flat floor, which was once a lake of molten lava. The northern part of the crater wall has been struck by another meteor, leaving a 4 kilometre crater called Herodotus N. Between Aristarchus and Herodotus is an upland region, probably volcanic in origin.

Schröter's Valley begins in this upland area, in a deep grater  8 kilometres across (the Cobra Head), which adjoins a larger crater about 10 kilometres across (sometimes called the Cobra's Hood). There is a large landslip on the western side of this larger crater. The Valley heads north, then north-west, then south-west, for a length of about 165 kilometres. Its width varies between 6 and 10 kilometres, tapering down to about 500 metres at its western end. The fine rille which runs the length of the floor is 200 metres wide. Schröter's Valley is without doubt one of the strangest and interesting formations on the Moon.



Aristarchus of Samos (ca. 310-230 BCE), born ten years after the death of Aristotle, spent time conducting research at the new Great Library of Alexandria. He was one of the first to apply mathematical principles to astronomy, and taught that all the seven planets were spherical as was the Earth; what is more, the Earth rotated on its axis daily. Interested in improving mapping, he worked towards the concepts of latitude and longitude, which were developed further by Eratosthenses.

Aristarchus obtained reasonable estimates of the Moon’s size and distance by logical reasoning and sound application of Euclid’s geometry, after observing a lunar eclipse, thus:  A full shadow of the Earth on the Moon has an apparent radius of curvature equal to the difference between the apparent radii of the Earth and the Sun as seen from the Moon. This radius can be seen to equal 0.75 degree, from which (with the solar apparent radius of 0.25 degree) we get an apparent radius of the Earth of 1 degree. This gives an Earth-Moon distance of 60 Earth radii or 384 000 km. This result can be compared with the Earth-Moon distances this month under Lunar Orbital Elements above.

Aristarchus expressed the belief that the Sun was twenty times further away than was the Moon. He also deduced that the Sun was far bigger than the Earth, and this may have led him to the conclusion that the Earth should revolve around the Sun, rather than the reverse. Developing this idea further, he proposed a true heliocentric system, the first person to do so. He taught that the Earth rotated on its axis each day, and travelled around the Sun once in a year. In this he anticipated Copernicus by nearly two millennia.

Aristarchus pictured a universe many times larger than any that had previously been imagined. The geocentric theories of his predecessors cosseted the Earth snugly inside the planets’ crystalline shells like a cosmic egg, but Aristarchus placed the Sun at the centre of a huge void, with the Moon close to Earth, the planets much farther away, and the stars at such great distances as to be almost beyond comprehension. We do not know for sure if he realised that the stars were at different distances, or if he agreed with the accepted wisdom of the time (harking back three centuries to Anaximander) that they were all located on a remote black ‘sphere of fixed stars’.

The idea of a spinning Earth was ridiculous to the geocentrists, and indeed to practically everyone. Stand outside at night, they said, and watch the stars slowly wheeling overhead. The Earth feels solid and motionless under your feet. If it were spinning, you would feel it. Also, they knew that the Earth was large, and if it rotated once in a day, then mountains, rivers and cities would be careering around at many hundreds of kilometres per hour. Gale-force winds would rake the landscapes. Houses, animals and people would be flung off and whirled away up into the sky and out of sight. Surely, common sense dictated that the Earth doesn’t move, but the objects in the sky do. The Sun rises and sets, does it not? Geocentrists also claimed that, if the Earth were truly moving, a ball thrown vertically upward would not fall back into the hand of the thrower, but would be left behind as the Earth carried the thrower away. The concept of inertia was not known at the time.

The heliocentric theory of Aristarchus was rejected by all major thinkers, with one exception, the mathematician Seleucus of Seliucia (190-150 BCE). Living near Babylon, Seleucus only heard about it 100 years after Aristarchus had proposed it. According to Plutarch, Seleucus believed the Sun-centred, revolving Earth world view was a fact, and was the first to prove it so through reasoning. We do not know what arguments he used. The world was not yet ready for a heliocentric solar system, and the Earth-centred universe would be the accepted model for another 1800 years. 


Herodotus (484-420 BDE) was a Greek historian in the fifth century before Christ. He was a contemporary of Philolaus and Socrates, in the generation before Plato. Cicero has called him "the Father of History", as he was the first writer to record historical events critically as a factual narration, rather than following the tradition of Homer in romanticising and embellishing the stories, as in the legend of the Odyssey. His work is the earliest Greek prose that has survived intact.


Johann Hieronymus Schröter (1745-1816)was a wealthy German lawyer in Hanover who became interested in astronomy. In 1779 he acquired a three feet long (almost one metre) achromatic refractor with a 2¼ inch (57 mm) lens by Dollond to observe the Sun, Moon and Venus. Herschel’s discovery of Uranus in 1781 inspired Schröter to pursue amateur astronomy more seriously. He resigned his post and moved out of Hanover to the darker and clearer skies of the countryside, becoming chief magistrate and district governor of Lilienthal.

In 1786 he paid 600 Reichstaler (equivalent to six months earnings) for a 2.14 metre focal length, 16.5 cm aperture reflector by Schrader, with eyepieces allowing up to 1200 magnification. He paid another 26 Thaler for a screw-micrometer.

Soon he had become an accomplished selenographer, and in 1791 he published an important early study on the topography of the Moon entitled Selenotopographische Fragmente zur genauern Kenntniss der Mondfläche, (Fragments of the Moon’s Topography for a more accurate Knowledge of the Lunar Surface). By this time, Latin had been phased out as the language of science, and such books were being written in the common language of their authors. This book introduced the Latin word ‘crater’ (cup), and the German word ‘Rille’ (groove) to the lunar terminology.

Schröter knew his history of astronomy and named 76 new features, including the craters Alhazen, Bernoulli, Bradley, Cassini, Euler, LaCaille, Tobias Mayer, Mercator, Picard and Rømer. He named two craters Hooke and Newton, but, knowing of the bitter enmity between these two men in their lifetimes, he placed them as far apart on the Moon as he could, Hooke away up in the northern hemisphere  and Newton near the South Pole.

Also named were Mount Huygens, Mount Hadley, Mount Pico, Mont Blanc and the Leibniz Mountains. Like most astronomers, he preferred Riccioli’s names over those of Hevelius and Langrenus, and this fact influenced the official acceptance in the 20th century of most of Riccioli’s names by the International Astronomical Union. He discovered Schröter's Valley in 1787, which was named after him by later astronomers.


10:    June  2017


This month's feature is the Apollo 11 landing site, Tranquility Base.

This area was photographed from Starfield Observatory, Nambour on July 30, 2017. The landing site is shown by an ' x '.
Three craterlets have been officially named after the astronauts. Armstrong is 4.6 km in diameter,
Collins is 2.4 km, and Aldrin is 3.4 km.
 The lunar module's landing approach was from the east (right).


Tranquility Base is the place where human beings first stepped onto another world. Three astronauts from the USA lifted off from the Kennedy Space Center in Florida on July 16, 1969 in a space vehicle comprising a three-stage Saturn V rocket on which was placed the three-part spacecraft that would travel to the Moon. The three parts were the Service Module (SM, unmanned) to which was attached the Command Module (CM, named "Columbia") in which the astronauts travelled, and the Lunar Module (LM, at the time called the 'Lunar Excursion Module' or LEM, named "Eagle") which was designed to descend to the Moon's surface with two astronauts on board. The LM was housed at lift-off in a 'garage' or adapter on top of the third Saturn V stage and behind the SM. During trans-lunar orbit the SM + CM had to separate from the third rocket stage, use its thrusters to turn through 180 degrees, dock with the LM, and then withdraw it from its 'garage'. The SM + CM +LM then headed for the Moon as a unit, the LM leading. The third Saturn V stage went into orbit around the Sun.

After travelling for four days, the Commander Neil Armstrong and Lunar Module Pilot Edwin 'Buzz' Aldrin climbed through from the CM into the LM, and the two vehicles separated. The Command Module Pilot Michael Collins remained in the CM in lunar orbit and did not take part in the landing. The landing site is on a great lava-plain called Mare Tranquillitatis or the Sea of Tranquility. It is within a degree of the Moon's equator.

On a few orbits over the area before the descent, Armstrong and Aldrin were able to identify the landmarks they needed to pinpoint the chosen level landing area. Some of these landmarks were the twin craters of Ritter (32 kilometres diameter) and Sabine (31 kilometres), both seen near the top of the above image about a third of the way in from the left margin. Another was a long rille, double in places with a flat floor, running for nearly 200 kilometres from just south of Sabine in an east-south-easterly direction. This is called the Rimae Hypatia or Hypatia Rille - it was unofficiallty dubbed by NASA "US Highway 1". Just north of this "highway" near its eastern end was a small but bright crater, Moltke (7 kilometres across). It can be seen above, just below the actual landing site, which is shown with a yellow asterisk  *.

Three craterlets in a line north of the landing area were unofficially named Armstrong, Collins and Aldrin, in order from right to left as that was the direction of the LM's flight path. These craters appear in the image above, just below the printed names. The first was 5 kilometres across, the other two were 3 kilometres. These names are now official, the International Astronomical Union (IAU) having made an exception to the rule that craters can only be named after a person posthumously.

The LM, using the engine in its lower half as a brake, slowed down and landed on the Moon at 6:17:40 am on July 21 (Australian Eastern Standard Time). The touch-down was at the far western end of the planned landing area, about 15 kilometres south of the craterlet Collins.  Six hours and 38 minutes later, Armstrong placed his left foot on the Moon's powdery surface at 12:56:15 pm AEST. Queensland school children watched his "one small step for man" on television during their lunchtime and most of the afternoon's lessons were set aside so that they could participate in the historic event. Armstrong and Aldrin spent 21½ hours on the lunar surface, taking pictures, deploying experiments and collecting 21.55 kilograms of rocks and soil. Seven of those hours were spent in resting inside the LM after their strenuous activities.

At 3:54 am AEST on July 22 the ascent stage of the Lunar Module lifted off the Moon and docked with the orbiting Command Module, after which the LM was left in orbit around the Moon and subsequently crashed. The LM's descent stage remains on the Moon where it has been photographed occasionally by mapping satellites, such as the Lunar Reconnaissance Orbiter on March 7, 2012. The three astronauts, now together again in the CM, fired the SM's rocket motor to return them to the Earth. Prior to re-entry into the Earth's atmosphere, the SM was jettisoned and the CM turned so that its heat shield was facing forward. The CM splashed down in the mid-Pacific Ocean 24 kilometres from the recovery ship, USS Hornet on July 25 at 2:51 am AEST, after a total trip lasting 8 days 3 hours and 19 minutes. Of the 3000 tonne Saturn V + Apollo space vehicle, only the 5 tonne Command Module returned safely to the Earth. Currently, Michael Collins is 86 and Buzz Aldrin is 87. Neil Armstrong departed this life in 2012 aged 82.


Key to features 11 to 15 below.

11:    July 2017


This month's feature is the large walled-plain named Plato.

This area was photographed from Starfield Observatory, Nambour on August 2, 2017.
East (where the Sun is rising) is to the right, north is to the top.
The area is dominated by the large oval formation at centre, which was named by Riccioli and Grimaldi
in 1651 after the ancient Greek philosopher Plato.

Plato is a huge walled plain with a diameter of 104 kilometres. As it lies at lunar latitude 51 degrees North, we see Plato greatly foreshortened, but if we could see it from directly above, it would be almost perfectly circular. It has struck the Moon in a rugged, mountainous area called The Alps, lying between the Mare Frigoris (Sea of Cold) and Mare Imbrium (Sea of Rains). The floor of Plato is remarkably flat, and is evidently the solidified surface of a great lake of molten lava, which welled up and half-filled the crater after the initial impact fractured the underlying bedrock. In the western half there is a very low lava dome or shield volcano that is 30 kilometres wide and 40 kilometres long. The dark floor is dotted with dozens of small craterlets. The five largest of them can be detected above – they average two kilometres in diameter and have interior shadows.

The bright walls average 2000 metres in height, with some peaks being much higher. Under a low Sun, the peaks throw long shadows across the floor. The western wall exhibits a huge landslip, where a large mass has slumped down and now intrudes upon the floor. Two smaller landslides can be seen in the north-western wall.  A narrow but prominent valley leads from the interior floor through the south-eastern wall, then immediately turns south-west and runs parallel to the wall, eventually reaching the plain of Mare Imbrium.

To the south-west of Plato, in the lower left corner of the image, is a bright mountain range protruding from the cooled lavas of the Mare Imbrium. It is called the Montes Teneriffe or Teneriffe Mountains (after the volcanoes that rise out of the Atlantic Ocean to form the Canary Islands). Two of the mountains in the northern spur exceed 2000 metres in height, and the furthest one to the east reaches 2500 metres.


Naming the Moon

As far as we know, the first person to name lunar features was William Gilbert (1544-1603), an Englishman, who made a sketch in 1600 of the Moon, showing dark and light areas. He assumed that the dark areas were land, and the light ones were seas, an assumption that he shared with Leonardo da Vinci and the young Kepler, but which was the opposite to most other observers, who thought that the 'seas' were the dark areas. Gilbert invented thirteen names for features in his drawing, which actually are familiar places on the Moon today. One dark oval area he named  "Brittannia" (sic), thinking it was an island. It was later named "Mare Crisium", as later observers considered it to be a 'sea'. Gilbert died of bubonic plague aged 59 in 1603, five years before the first telescopes made their appearance. His naked-eye drawing was not published until 1651, by which time it was merely a curiosity. However, Gilbert's real claim to fame is for his work on magnetism, and as one of the originators of the word 'electricity'. 

The first realistic map of the Moon, made with a good quality telescope, was produced by Michael van Langren (Langrenus, 1598-1675) in 1645. He used the names of famous and historic people to identify lunar features, and some large dark areas were named after places on Earth, two being the Belgian Sea and the Caspian Sea. Only three of his names are still in use, for the craters Endymion, Pythagoras and Langrenus (which he named after himself).

Only two years later, 1647, Johannes Hevelius (1611-1687) in East Prussia produced a book, Selenographia, which contained maps of the Moon made with a 3.6 metres long telescope which could magnify 50 times. He also showed maps of the Moon at different phases. The maps were hand-tinted, and the colours chosen seem to indicate that his telescope was unable to convince him that the ‘seas’ on the Moon were not really water, for he has had them coloured blue. This project took four years, and entitles him to be called a founder of lunar cartography. He began this work before van Langren's map was published, and may not have been aware of its existence or of van Langren's names for lunar features.

Hevelius decided to name all of the lunar features after places on the Earth, but in their classical Latin and Greek forms. Why did he choose the names he did? People had argued since antiquity that the Moon is a mirror of the Earth. They had believed the patterns of dark and light on the Moon’s surface to be seas and continents that paralleled those on the Earth. Just as humans, animals and plants inhabit the Earth, so ‘selenites’ and other life-forms were presumed to inhabit the Moon. This parallel between the Earth and the Moon was important to Hevelius, and he compiled his lunar maps from multiple, detailed telescopic observations, adopting some of the symbolic conventions of geographical mapping. He did not seem to fully understand the interplay of light and shadow on the Moon’s surface, and mapped the shadow-casting ridges and crater rims as if they were chains of terrestrial mountains. For some reason, he called most hollowed-out depressions (craters) ‘Mons’, meaning ‘mountains’, e.g. the crater Tycho was named Sinai Mons (Mount Sinai), the crater Copernicus was named Mons Ætna (Mount Etna), and the crater Ptolemæus was named Mons Sipylus. The three terrestrial mountains just named are all volcanic in origin, but only Mount Etna has an actual crater, which may be why he chose it for the great formation of Copernicus. He called the dark crater-plain Plato illustrated above ‘Lacus Niger Major’ (Large Black Lake), which is an apt description given his 50x telescope. 

On his map of the Moon's western side he shows ‘M. Ætna’ (Mt Etna) in the island of ‘Sicilia’ (Sicily), sitting in the middle of the ‘Mare Mediterraneum’ (Mediterranean Sea). The islands of Corsica and Sardinia are nearby, with Majorca and Minorca not far away. At the southern end of the Mare Mediterraneum, we see ‘Mare Ægyptiacum’ (Egyptian Sea), with ‘Fl. Nilus’ (‘Fluvius Nilus’, the Nile River), flowing into it as three streams. The islands of Crete and Cyprus are also shown. By applying familiar names to the lunar features, perhaps Hevelius aimed to make it a more understandable and recognisable place, and to reduce any strangeness the observer might experience.

Like van Langren before him, Hevelius exhorted other astronomers to adopt his system of naming lunar landmarks, and threatened litigation if anyone else should devise different systems of names in opposition to his. However, this was an empty threat, as there was no legal requirement for any competitor to comply with such requests.

Giovanni Battista Riccioli (1598-1671) discovered the first binary star (Mizar in Ursa Major) in 1650. He worked with his former pupil Francesco Grimaldi (1618-1663) at Bologna. Both were Jesuit clerics in the Catholic Church. They shared an interest in astronomy, and decided to work together in producing an accurate map of the Moon, among other things. This map, or selenograph was the result of Grimaldi’s telescopic study of the Moon and was drawn by himself. It is fairly accurate, although for some reason the spectacular crater Cassini is missing.

Riccioli, with some help from Grimaldi, contributed a new series of names for lunar features, in which mountains and valleys were named as such, and the bowl-shaped depressions (not yet called ‘craters’) were named after famous astronomers and philosophers, some still alive. The 'seas' ('maria') were named after various meteorological ideas of the time, for it was thought that the Moon influenced the weather. We still use Riccioli’s names today, and those of Langrenus and Hevelius are mostly dispensed with, but there are some exceptions.

Riccioli published the map in his two-volume work on astronomy entitled Almagestum novum, astronomiam veterem novamque complectens or New Almagest, in 1651. By necessity, as an official of the Catholic Church, when writing about astronomical matters he had to comply with Church doctrine. He therefore opposed the Sun-centred world view of Copernicus, Kepler and Galileo, though he ventured to praise the heliocentric theory as a useful hypothesis for calculating planetary positions. Riccioli and Grimaldi experimented with falling bodies in an attempt to refute Galileo’s findings. Instead, they found that Galileo was correct, which impressed them greatly.

It appears that, when they began assigning names to the lunar craters, Riccioli and Grimaldi had a hidden agenda. Despite their professed opposition to Copernicus’ theory, they named the Moon’s most spectacular crater after him, and a nearby prominent crater after Kepler. Another relatively close crater was named after Aristarchus, the originator of the heliocentric theory. Not far away, they named a crater after Galileo. Twelve craters were named after other Jesuit astronomers, such as Bettinus, Clavius, Curtius, Cysatus, Furnerius, Gruemberger, Kircher, Moretus, Scheiner, Schomberger, Simpelius, and Zucchius, but they are in a far-off part of the Moon, near its south pole and the crater Tycho, as the Society of Jesus supported the Tychonic world view. When questioned, they said that they had “flung the heliocentrists away into the Ocean of Storms”, far from the Jesuits.

But Riccioli and Grimaldi named two large adjoining craters near the Moon’s limb after themselves. Where are those craters? Not in the far south of the Moon with other Jesuits, but near its Equator, bordering the Oceanus Procellarum (Ocean of Storms), close to the crater Galilæus and in the same quadrant of the Moon as Copernicus, Kepler and Aristarchus. This is considered to be covert support for the Copernican theory, which as Jesuits they knew that they could not publicly express.

As telescopes improved, smaller and smaller features became detectable, and so later astronomers such as Schröter, Mädler, Birt, Lee, Neison, Schmidt, Franz, Krieger, König, Fauth, Lamèch and Wilkins applied new names to them. With the mapping of the far side by satellites, official bodies such as NASA were allowed to name features, as long as they abided by rules laid down by the International Astronomical Union or IAU.


Plato (ca. 429-348 BCE, a student and friend of Socrates, described the spherical nature of the Earth in one of his Dialogues, Timaeus. He also taught that the world (universe) was composed of tiny atoms comprising four fundamental elements – earth, water, air and fire, with the least heavenly element, earth, at the bottom. He said that water flowed over the earth, air floated above both, and fire rose through the air to the sphere of the Moon, above which the planets and stars moved within a fifth element, the quintessence or æther.;    

The word ‘æther’ in Homeric Greek means ‘ever-moving, pure, fresh air’ or ‘clear sky’, and was believed in Greek mythology to be the pure essence where the gods lived and which they breathed, different from the impure air breathed by mortals which only reached as high as the Moon. On rare occasions the æther was seen as a glow around the Sun’s disc during solar eclipses. Its name comes down to us today in the words ‘ethereal’ (‘heavenly’) and ‘ethernet’. Æther had no qualities at all (was neither hot, cold, wet nor dry), was inviolate, everlasting and incapable of change (with the exception of change of place), and by its nature moved in perfect circles. It was also personified as a god itself, Æther, the son of Erebus and Nyx.

Plato taught that the seven planets (which included the Sun and Moon) and the fixed stars revolved around the Earth on eight crystalline spheres, as first postulated by Pythagoras. The spheres were made of a solid, transparent form of quintessence or æther, and the planets and stars were denser, luminous nodules on the spheres. Gaseous quintessence enclosed all the spheres and was perfect and immutable. The celestial bodies moved forever in perfect circles at constant speed.

Like others of his time, Plato said that seeing was not necessarily believing: the eye could be deceived – optical illusions proved that. Only the human mind could reveal how things really were, by the application of logic and reasoning to create mathematical models of reality. This was the basis for his acceptance of a system of concentric, transparent spheres for carrying the planets.

Plutarch, a student at Plato’s Academy some 500 years later, tells us that, in his old age, Plato changed his geocentric beliefs and entertained some of the Pythagoreans’ latest ideas. He came to accept that the diurnal (daily) movement of the heavenly bodies was due to the axial rotation of the Earth and was interested in Philolaus’ concept that the Earth orbited a central fire. Maybe he even thought that Heracleides’ idea that the Earth was one of a family of planets orbiting a central Sun could be possible. All we really know for sure is that Plutarch quotes Plato as saying that he “repented of giving to the Earth the central place in the universe, which did not belong to it.”


12:    August  2017


This month we look at some of the interesting features in the vicinity of Plato.

This area was photographed from Starfield Observatory, Nambour on October 10, 2017.
East (where the Sun is rising) is to the right, north is to the top.
The area is dominated by the large walled plain Plato at upper left, and the impact crater Cassini at lower right.
 Between the two is a rugged mountainous area called the Alps, to the west of which is a large basin
 filled with solidified lava, called Mare Imbrium (the Sea of Rains).
Some of the more notable features are named.

Plato in close-up. Five craterlets with diameters of 2 to 3 kilometres are visible inside it, as well as several smaller ones.

The Vallis Alpes (Alpine Valley) is 134 km long. Along its length runs a delicate rille which varies between
500 and 700 metres across, and averages 78 metres deep. Although it looks like a water course, there is
 no running water on the Moon. It is probably volcanic in origin. Both images above were photographed at
Starfield Observatory through the Alluna 20 inch Ritchey Chrétien telescope on August 2, 2017.


This area includes parts of two lava plains, which as they are basically cold basalt appear darker in colour than some other areas. In ancient times the Moon was thought to be a perfect, silver mirror, reflecting the seas and continents of the Earth. This idea persisted until the 16th century. By 1600, some people including Leonardo da Vinci in Italy and William Gilbert in England believed that the darker areas were reflections of our continents and the lighter areas were reflection of the Earthly seas and oceans, but most thinkers thought that the opposite was true, that the darker areas were seas ("mare" singular and "maria" plural) and the sole larger one an ocean ("oceanus"). The lighter areas were lands ("terrae"). This second view was the one held by the first makers of Moon maps, Langrenus, Hevelius, Riccioli and Grimaldi.

As Riccioli and Grimaldi have given us most of the names of places on the Moon that they could see with their primitive telescope, the darker areas in the pictures above are the Mare Imbrium (Sea of Rains) and the Mare Frigoris (Sea of Cold). The Mare Imbrium is a great circular plain on the Moon, caused by a massive asteroid impact - see "The Imbrium Event" below. All of the seas and the solitary ocean were formed by such impacts and are also dry, cold plains of solidified lava. The terrae around them are much more ancient and are totally covered with fairly large craters, most overlapping older craters, but the Seas are more recent, and show fewer large craters, although they also are peppered with thousands of craterlets.

The impact of asteroids, meteoroids, rocks, and micrometeoroids with the Moon’s surface over the eons has left it covered with a thick layer of dust, soil and shattered rocks. This layer averages 4 to 5 metres deep over the maria, but can be up to 30 metres deep in the valleys of the ancient highlands or terrae. This layer is called the regolith. The dusty upper surface is fine and powdery, like talcum powder, and holds astronaut’s bootprints very well.  The Apollo 16 astronauts landed near the crater Descartes, at a small recent crater with rays, called North Ray Crater. The regolith near this crater was found to be only a few centimetres thick. Many samples of the regolith have been brought to Earth for analysis. Those from sites on maria are generally rock fragments o f basalt, with pyroxene and plagioclase. Those from highland sites are mainly broken plagioclase rocks and crystals.

Beneath the regolith of the maria there is a layer of basalt, averaging 20 kilometres deep, formed when the initial asteroid fractured the surface down to a depth of 60 kilometres, to the Moon's mantle. These fractures released pressure on the superheated rocks in the top layers of the mantle. They immediately liquefied into molten basaltic magma which forced its way to the surface and spread out over the landscape as lava, swamping or partially swamping existing craters and mountains, and spreading out over thousands of square kilometres. The lava cooled and hardened to form the "seas" that are such a notable feature of our side of the Moon.

In the pictures above, the lava has covered the bases of the Teneriffe Mountains, Mount P ico and Mount Piton to a great depth, although their lofty peaks protrude above the plain of Mare Imbrium. The crater Plato was caused not long after the Imbrium Event, but just south of Plato can be seen the faint outline of a similar but slightly larger crater, that was completely submerged under the lava except for some of the ramparts of its wall which were high enough not to be swamped - these ramparts are now called the Teneriffe Mountains (1.4 kilometres high) and Mount Pico (2.4 kilometres high).

Three impact craters on Mare Imbrium of more recent origin are Piazzi Smyth (22 km), Kirch (12 km) and spectacular Cassini (60 km).

Between Mare Imbrium and Mare Frigoris (Sea of Cold) is a rough chain of lofty mountains, trending from north-west to south-east. Johannes Hevelius called this mountainous area the "Montes Alpes", and Riccioli named it the "Terra Grandinis (High Land"). This was one case where the name given by Hevelius was preferred over Riccioli's alternative, and the range is usually known simply as the Alps. Its highest mountain is Mont Blanc (White Mountain) and is 3.6 kilometres high.

The Alps are divided in two by a straight valley called the Vallis Alpes (Alpine Valley). It is 134 kilometres long and is a linear fault or graben that was filled with lava. Along its length runs a delicate rille that is shown in the picture above and on our  Gallery  webpage. 


The Imbrium Event

The Moon and planets coalesced out of debris in the solar proto-nebula about 4.6 billion years ago. Most of the craters on the Moon were formed early in its history, during the first 500 million years. At that time, there was a lot of solid material in orbit around the Sun, and the planets attracted much of it to themselves by their own gravity. There were major collisions of asteroid-sized rocks with all the rocky planets and moons, and they still show the scars. Earth, despite the effects of weather and erosion, still has some large impact craters or astroblemes, two in Australia being Wolfe Creek Crater and Gosse’s Bluff.

After the main bombardment had ended, most of the Moon was covered in craters overlapping each other. The south polar region of the Moon and the far side still have this appearance. Some large asteroids later struck our side of the Moon, the huge impact craters being filled with lava which produced the large, dark-coloured level area which give the Moon its patchy appearance to the naked eye. Early astronomers thought that the dark areas were water, and in the 17th century the Jesuit astronomers Grimaldi and Riccioli, working with primitive telescopes, named them, giving us 3 marshes, 17 lakes, 11 bays, 20 seas and one ocean. Of course, there is not one drop of liquid water on the Moon - all of the dark areas are plains of cooled lava.

The final large asteroid collided with the Moon about 3.85 billion years ago. We don’t know its size, but it was probably half as big as Tasmania. It struck the northern hemisphere, blasting out a crater over 800 kilometres across. Magma immediately rose from the Moon’s mantle through fractures in the crust and flooded out across the surface, swamping the destroyed area with great lava flows which filled the floor of the great new crater. This cataclysm is known as the 'Imbrium Event'.

The circular area resulting from this gigantic blast is known to us today as the Mare Imbrium, the Sea of Rains. The area is circled by great mountain ranges, the Alps, the Caucasus, the Apennines and the Carpathian Mountains, which are in fact the original ramparts of the impact crater. Because of its l ocation halfway between the Moon's equator and its north pole, it appears foreshortened to us, looking oval in shape. Protruding from the lava floor of Mare Imbrium are isolated mountain peaks, such as Pico and Piton. Their bases are kilometres below the Mare surface. As different flows of lava occurred, ripples in the surface were caused, which are visible today as wrinkle ridges.

Some craters were formed after the initial lava flows had solidified, and then partially or completely flooded by later flows. These are called ghost craters. When the initial Imbrium Event occurred, large blocks of rock the size of mountains were sent crashing for hundreds of kilometres across the lunar surface, particularly heading south-south-east. These caused lines of smaller crate5s and great longitudinal scars to be formed as the blocks bounced along, all of this damage radiating from the centre of the Imbrium Event impact site. Most of these scars are seen in the area north, north-west and north-east of the central crater Ptolemæus.

Since those times, new craters have been formed on what was once the level, smooth Mare Imbrium. We can see some in the pictures above. One large later collision occurred at the northern edge of the Mare, creating the crater Plato. This was severe enough to fracture the crust and allow lava to well up and almost fill the crater, so that instead of being bowl-shaped it has a flat floor at about the same level as the nearby Mare. Being made of the same basaltic lava, the floor is the same dark grey colour as the Mare, while the surrounding mountains are much lighter in tone.



13:    September  2017

This month's feature is an unusual feature in the Moon's southern hemisphere, the Rupes Recta or "Straight Wall".

The "Straight Wall" and its surroundings on the Moon, photographed from Nambour on August 2, 2017.

Rupes Recta, or the Straight Wall, is a long escarpment in the Moon's Mare Nubium (Sea of Clouds). 100 km long and 1 to 1.5 kilometres wide, it is a fault in the lunar crust. Despite its appearance under a low Sun as above, it is neither particularly high nor steep. The eastern or right-hand side is about 250 metres higher than the  western, near the wall's mid-section. The slope is actually quite gentle, some sources quoting it as being as little as seven degrees. It can be detected with quite a small telescope. At the time near First Quarter as above, the wall is in shadow and appears as a dark line. Two weeks later, around Last Quarter when the Sun is coming from the west, it appears brighter than its surroundings - a bright line.

Also in the image are some craters. To the west (left) of the Wall, is the 17 km crater Birt, the eastern rim of which is deformed by the later impact that created 6.8 km Birt A. Starting in a craterlet just to the west of Birt is a curved valley or rille called Rima Birt, which is 51 km long and turns to the north-west before fading out. It has a gap in its middle and a slight S-bend.

The large crater to the right of the image is 60 km Thebit. It has a rille on its floor which is unusual in that it is composed of four straight sections joined at abrupt angles, almost like half of a hexagon. Most rilles are sinuous. Thebit A (20 km) has deformed the western rim of the larger crater, and is itself deformed by Thebit L (12 km). In Thebit A quantities of soil and rocks have slumped down the walls and covered much of the floor.



William Radcliffe Birt (1804-1881) was an English amateur astronomer who worked extensively with Sir John Herschel in the middle of the 19th century. He was connected with the Kew Observatory in London. Mainly interested in meteorology and atmospheric phenomena, he was also very interested in mapping the Moon. The crater Birt was named after him by Schmidt in 1878.


The crater Thebit is named after Thabit ibn Qurra (AD 826 to AD 901). Thabit was born in Assyria (Turkey) but moved to Baghdad which was at the time the greatest seat of Arab learning. It was there that a House of Wisdom had been established in AD 800, for the translation into Arabic of the works of the Greek and Alexandrian philosophers. Thabit was a physician, mathematician, astronomer and translator, and made important discoveries in algebra, geometry and astronomy. He translated Ptolemy's Almagest from its original Greek into Arabic, and made corrections to Ptolemy's work where he thought they were needed. For example, Ptolemy had described Hipparchus' discovery of Precession of the Equinoxes, but Thabit and others saw that there was an irregularity in the precession, a slow wobble which varied back and forth over an average, causing planetary tables to need constant correction. Thabit described this wobble in his De motu octavae sphaerae (On the motion of the eighth sphere), the eighth sphere being the sphere of 'fixed' stars.

To account for this irregularity, which was later called Trepidation(unease), later astronomers sometimes added an extra crystalline sphere between the sphere of precession and the Primum Mobile. They named this new sphere ‘Trepidationis’. Not until 1728 was it found that this irregularity is caused by a small 18.6-year swaying of the Earth’s axis called nutation, which is superimposed upon the large 26 000-year swing of 46.87º diameter that causes precession of the equinoxes. In comparison with precession, the wobble in nutation is very tiny, with maximum variations of ±17 arcseconds in longitude and ± 9 arcseconds in obliquity. Although many astronomers did not think that this minuscule movement warranted the inclusion of a Trepidationis Sphere, the map-maker Andreas Cellarius believed that it did in his star atlas Harmonia Macrocosmica of 1660.

Copies of Thabit's version of the Almagest were carried to Spain by the Moors, and one was used at the Toledo School of Translators by Gerard of Cremona in the 12th century when he was given the task of translating the Arabic Almagest into Latin for dissemination into Europe. The translation of such ancient Greek texts (at first by means of the Arabic versions) provided the stimulus for the ending of the Dark Ages in Europe. By the 14th century, when the original Greek versions of the ancient texts were rediscovered at Constantinople, their translation into Latin helped bring about the Renaissance.

14:    October  2017


This month's features are a pair of craters in the Moon's southern hemisphere, Pitatus and Hesiodus.

The largest crater in this image is Pitatus which is 100 km in diameter. To its west (left) is its 44 km diameter neighbour Hesiodus.

Both of these craters have flat floors with multiple rilles around their circumferences. Pitatus has an off-centre mountain massif on its floor, while that of Hesiodus has two newer impact craters, both bowl-shaped. There is an unusual valley joining the floors of Pitatus and Hesiodus. A 12 km 'ghost crater', partially submerged by the lava flows from the north, is about 30 km north of Pitatus, with another to the west. From the western ramparts of Hesiodus, the 309 km long rille called  Rima Hesiodus runs to the south-west and off the image. There are three craterlets visible on this narrow and shallow valley. They are so well-aligned on the rille that they are probably volcanic vents associated with the creation of the rille. It is unlikely that random impacts from space could have landed together exactly on the rille.


This image shows the crater Hesiodus (above centre) when the Sun was higher, reducing the shadows
 and flattening the contrast. Adjoining Hesiodus to the south-south-west is the 15 km crater Hesiodus A.
This crater is remarkable in that it contains two concentric rings like a bulls-eye. The rings only become
 visible when the Sun is high enough to shine over the steep walls and illuminate them. The picture was taken on
September 2, 2017. The rings are only partially visible in the larger picture above, which was taken on August 2, 2017.


Pitatus is named after Pietro Pitati, a 16th century Italian mathematician and astronomer. As the Julian calendar of his time had become out of phase with the seasons and was then running 10 days late, some form of calendar reform was required. Pitati suggested that three out of every four even-century years be made non-leap-years to keep the calendar true. The man who corrected the calendar in one bold step four decades later in 1582, Christoph Clavius, accepted this suggestion, declaring that the even century years 1600, 2000, 2400 etc should remain as leap-years, but the intervening even centuries (1700, 1800, 1900, 2100 etc) should not. This avoided any more accumulating errors. To remedy the existing 10-day lag, Clavius dropped 10 days from the Julian Calendar. Pope Gregory XIII ratified this, and the day after Thursday, October 4, 1582 was decreed to be Friday, October 15, 1582. This was the beginning of the Gregorian Calendar which we use today. Catholic countries complied immediately, but others were slower to act. Great Britain and her colonies did not adopt it until 1752, and Turkey was the last in 1926.


Hesiodus is named after Hesiod, an ancient Greek poet who was active in the 8th century BCE and was a contemporary of Homer. Hesiod is a major source for our knowledge of Greek mythology, religious customs, agriculture, early astronomy and time-keeping. Some call him the world's first economist.




15:    November  2017 


This month features one of the most spectacular ranges of mountains on the Moon, the Montes Apenninus, and the crater Eratosthenes which is at the range's western end.


Sunrise on the Apennines: the crater Eratosthenes and the Montes Apenninus on the Moon, photographed from Nambour on August 1, 2017.

The Montes Apenninus or Apennine Mountains is a range of peaks in the Moon's northern hemisphere at about latitude 20º North. It forms a gentle curve on the south-east margin of Mare Imbrium (Sea of Rains), and is just a few kilometres short of being 1000 km long. Some of the peaks reach up 5.4 kilometres above their bases on the lava plain to the north. The Jesuit priest who named most of the features on the Moon, Riccioli, named the range "Terra Nivium" (Land of Snows), and the picture above shows that at lunar sunrise the peaks can have the appearance of being snow-capped.

A contemporaneous observer, Johannes Hevelius, had already named this line of mountains after the Apennine Range in Italy, and an adjoining range to the north after the Alps. Although the Alps are not shown in the image above, they can be seen in  the photograph below. Hevelius in fact named all the most prominent lunar features after well-known places on the Earth. For example, on his maps of the Moon you will find the Mediterranean Sea with the islands of Sicily, Sardinia, Corsica and Majorca, and a large crater (Copernicus) named Mt Etna. People thought that this was a confusing idea and his system of names failed to win popularity. When Riccioli produced his new nomenclature a few years later, in which craters were named after famous and historic people, the names given by Hevelius were superseded by Riccioli's. But these two ranges were exceptions - later observers preferred to keep the Moon's Alps and Apennines.

Both of those ranges form the eastern rim of Mare Imbrium, a great circular 'sea' on the Moon - although it looks oval from Earth - that was caused by the tremendous impact of a large asteroid (see below).



Eratosthenes (ca. 276-194 BCE, left), was a custodian of the Great Library in Alexandria, at the mouth of the Nile. Many of our Greek myths about the constellations come to us through his writings and those of Hyginus. The story goes that Eratosthenes was told by a visitor that at noon on the summer solstice (June 21), the Sun shone vertically down a well further up the Nile River at Syene, its reflection appearing in the water. Also, vertical sticks and tall columns were seen to ‘swallow their shadows’ around the time of the summer solstice. He checked this the following year and found that it was so, and that, at the same time on the same day, the Sun was 7.2º from the vertical at Alexandria.

Knowing the distance between the two observing sites to be 5000 stadia, about 800 kilometres, a stade or stadion being 157.5 metres (he hired professional ‘steppers’ to pace it out using a standard stride), he used this information to make a famous measurement of the diameter and circumference of the Earth, getting quite an accurate result. His method was simple: the 7.2º angle between the two towns is about 1/50 of a circle of 360º. This distance equals 800 kilometres.

It is also possible that, rather than having been told about a well by a visitor, Eratosthenes had read the old Egyptian data (then more than 2000 years old) that the Sun did not cast any shadows at noon on the summer solstice at obelisks on Elephantine, an island in the Nile adjacent to Syene. His calculation of the Earth’s circumference turned out to be very close to the true figure of 40 025 km (polar circumference), but it seems that he had more than his share of good fortune. The fact that Syene is slightly north of the Tropic of Cancer should have made the Sun shine down the well at a slight angle, not vertically. We are not told how deep the well was, and if it was exactly vertical. Would the use of their primitive measuring instruments at Alexandria have provided the quoted accuracy? How accurate were the measurements of the man or men who paced the distance? All of these factors would have introduced small errors into his observations and calculations, but luckily they appear to have cancelled each other out. In any case, he was the first man to measure the size of a planet, and all he used was sticks, eyes, feet and brains.

Eratosthenes is credited with the invention of the armillary sphere. Although not the first Greek to do so, he measured the obliquity of the ecliptic (its inclination to the equator) and realised that this figure indicated the amount of tilt of the Earth’s axis. His result of 23½º was only 7 arcminutes from the correct figure. He drew a map of the Hellenistic world for use by navigators. It was later used as a basis for a map by Claudius Ptolemy in his Almagest , and was not superseded until the great voyages of discovery at the end of the fifteenth century.




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