December 2023
Updated: 1 December 2023
Welcome to the night skies of Summer, featuring Capricornus, Aquarius, Cygnus, Pegasus, Andromeda, Cetus, Aries, Taurus, Orion, Canis Major, Jupiter and Saturn
Note: To read this webpage with mobile phones or tablets, please use them in landscape format, i.e. the long screen axis should be horizontal.
The Alluna RC-20 Ritchey Chrétien telescope was installed in March, 2016.
The 20-inch telescope is able to locate and track any sky object (including Earth satellites and the International Space Station) with software called TheSkyX Professional, into which is embedded a unique T-Point model created for our site with the telescope itself.
Explanatory Notes:
Rise and set times are given for the theoretical horizon, which is a flat horizon all the
way round the compass, with no mountains, hills, trees or buildings to obscure the view. Observers will have to make allowance for their own actual horizon.
Transient phenomena are provided for the current month and the next. Geocentric phenomena are calculated as if the Earth were fixed in space
as the ancient Greeks believed. This viewpoint is useful, as otherwise rising and setting times would be meaningless. In the list of geocentric events, the nearer object is
given first.
When a planet is referred to as ‘stationary’, it means that its movement across the stellar background appears to have ceased, not that
the planet itself has stopped. With inferior planets (those inside the Earth’s orbit, Mercury and Venus), this is caused by the planet heading either directly
towards or directly away from the Earth. With superior planets (Mars out to Pluto), this phenomenon is caused by the planet either beginning or ending its
retrograde loop due to the Earth’s overtaking it.
Apogee and perigee: Maximum and minimum distances of the Moon or artificial satellite from the Earth.
Aphelion and perihelion: Maximum and minimum distances of a planet, asteroid or comet from the Sun.
Eclipses always occur in pairs, a lunar and a solar but not necessarily in that
order, two weeks apart.
The
meridian is a semicircle starting from a point on the horizon that is exactly due north from the observer, and arching up into the sky to the zenith and
continuing down to a point on the horizon that is exactly due south. On the way down it passes through the South Celestial Pole which is 26.6 degrees above the
horizon at Nambour. The elevation of the South Celestial Pole is exactly the same as the observer's latitude, e.g. from Cairns it is 16.9 degrees above the
horizon, and from Melbourne it is 37.8 degrees. The Earth's axis points to this point in the sky in the southern hemisphere, and to an equivalent point in the
northern hemisphere, near the star Polaris, which from Australia is always below the northern horizon.
All astronomical objects rise until they reach the meridian, then they begin to set. The act of crossing or 'transitting' the meridian is called 'culmination'.
Objects closer to the South Celestial Pole than its altitude above the southern horizon do not rise or set, but are always above the horizon, constantly
circling once each sidereal day. They are called 'circumpolar'. A handspan at arm's length with fingers spread covers an angle of approximately 18 - 20 degrees.
Your closed fist at arm's length is 10 degrees across. The tip of your index finger at arm's length is 1 degree across. These figures are constant for most people, whatever
their age. The Southern Cross is 6 degrees high and 4 degrees wide, and Orion's Belt is 2.7 degrees long. The Sun and Moon average half-a-degree (30 arcminutes)
across.
mv = visual magnitude or brightness. Magnitude 1 stars are very bright, magnitude 2 less so, and
magnitude 6 stars are so faint that the unaided eye can only just detect them under good, dark conditions. Binoculars will allow us to see down to magnitude
8, and the Observatory telescope can reach visual magnitude 17 or 22 photographically. The world's biggest telescopes have detected stars and galaxies as faint as
magnitude 30. The sixteen very brightest stars are assigned magnitudes of 0 or even -1. The brightest star, Sirius, has a magnitude of -1.44. Jupiter can reach -2.4, and
Venus can be more than 6 times brighter at magnitude -4.7, bright enough to cast shadows. The Full Moon can reach magnitude -12 and the
overhead Sun is magnitude -26.5. Each
magnitude step is 2.51 times brighter or fainter than the next one, i.e. a magnitude 3.0 star is 2.51 times brighter than a magnitude 4.0. Magnitude 1.0
stars are 100 times brighter than magnitude 6.0 (5 steps each of 2.51 times, 2.51x2.51x2.51x2.51x2.51 = 2.515 =
The Four Minute Rule
How long does it take the Earth to complete one rotation? No, it's not 24 hours - that is the time taken for the Sun to cross
the meridian on successive days. This 24 hours is a little longer than one complete rotation, as the curve in the Earth's
orbit means that it needs to turn a fraction more (~1 degree of angle) in order for the Sun to cross the meridian again. It is called a 'solar day'. The stars,
clusters, nebulae and galaxies are so distant that most appear to have fixed positions in the night sky on a human time-scale, and for a star to return to
the same point in the sky relative to a fixed observer takes 23 hours 56 minutes 4.0916 seconds. This is the time taken for the Earth to complete exactly one
rotation, and is called a 'sidereal day'.
As our clocks and lives are organised to run on solar days of 24 hours, and the stars circulate in 23 hours 56 minutes approximately, there is a four minute
difference between the movement of the Sun and the movement of the stars. This causes the following phenomena:
1. The Sun slowly moves in the sky relative to the stars by four minutes of time or one degree of angle per day. Over the
course of a year it moves ~4 minutes X 365 days = 24 hours, and ~1 degree X 365 = 360 degrees or a complete circle. Together, both these facts mean that after the course of a
year the Sun returns to exactly the same position relative to the stars, ready for the whole process to begin again.
2.
For a given clock time, say 8:00 pm, the stars on consecutive evenings are ~4 minutes or ~1 degree further on than they were the previous night. This means
that the stars, as well as their nightly movement caused by the Earth's rotation, also drift further west for a given time as the weeks pass. The stars
of autumn, such as Orion, are lost below the western horizon by mid-June, and new constellations, such as Sagittarius, have appeared in the east. The stars
change with the seasons, and after a year, they are all back where they started, thanks to the Earth's having completed a revolution of the Sun and returned to
its theoretical starting point.
We can therefore say that the star patterns we see in the sky at 11:00 pm tonight will be identical to those
we see at 10:32 pm this day next week (4 minutes X 7 = 28 minutes earlier), and will be identical to those of 9:00 pm this date next month or 7:00 pm the month after. All
the above also includes the Moon and planets, but their movements are made more complicated, for as well as the Four Minute Drift with
the stars, they also drift at different rates against the starry background, the closest ones drifting the fastest (such as the Moon or Venus), and the most distant ones
(such as Saturn or Neptune) moving the slowest.
Observing astronomical objects depends on whether the sky is free of clouds. Not only that, but there are other factors such as wind,
presence of high-altitude jet streams, air temperature, humidity (affecting dew formation on equipment), transparency (clarity of the air), "seeing" (the amount of air
turbulence present), and air pressure. Even the finest optical telescope has its performance constrained by these factors. Fortunately, there is an Australian website that
predicts the presence and effects of these phenomena for a period up to five days ahead of the current date, which enables amateur and professional
astronomers to plan their observing sessions for the week ahead. It is called "SkippySky". The
writer has found its predictions to be quite reliable, and recommends the website as a practical resource. The website is at
http://skippysky.com.au and the detailed Australian data are at
http://skippysky.com.au/Australia/ .
Solar System
Sun:
The Sun begins the month in the non-zodiacal constellation of Ophiuchus, the Serpent-bearer. It transits Ophiuchus and crosses into Sagittarius, the Archer on December 19. Note: the Zodiacal constellations used in astrology have significant differences with the familiar astronomical constellations both in size and the timing of the passage through them of the Sun, Moon and planets.
The Moon is tidally locked to the Earth, i.e. it keeps its near hemisphere facing us at all times, while its far hemisphere is never seen from Earth. This tidal locking is caused by the Earth's gravity. The far side remained unknown until the Russian probe Luna 3 went around the Moon and photographed it on October 7, 1959. Now the whole Moon has been photographed in very fine detail by orbiting satellites. The Moon circles the Earth once in a month (originally 'moonth'), the exact period being 27 days 7 hours 43 minutes 11.5 seconds. Its speed is about 1 kilometre per second or 3679 kilometres per hour. The Moon's average distance from the Earth is 384 400 kilometres, but the orbit is not perfectly circular. It is slightly elliptical, with an eccentricity of 5.5%. This means that each month, the Moon's distance from Earth varies between an apogee (furthest distance) of 406 600 kilometres, and a perigee (closest distance) of 356 400 kilometres. These apogee and perigee distances vary slightly from month to month. In the early 17th century, the first lunar observers to use telescopes found that the Moon had a monthly side-to-side 'wobble', which enabled them to observe features which were brought into view by the wobble and then taken out of sight again. The wobble, called 'libration', amounted to 7º 54' in longitude and 6º 50' in latitude. The 'libration zone' on the Moon is the area around the edge of the Moon that comes into and out of view each month, due to libration. This effect means that, instead of only seeing 50% of the Moon from Earth, we can see up to 59%.
The animation loop below shows the appearance of the Moon over one month. The changing phases are obvious, as is the changing size as the Moon comes closer to
Earth at perigee, and moves away from the Earth at apogee. The wobble due to libration is the other feature to note, making the Moon appear to sway from side
to side and nod up and down.
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New Moon:
December 13 09:32 hrs
diameter = 31.9'
Lunation #1249 begins
Last
Quarter:
January 4 13:31 hrs
diameter = 29.8'
New Moon:
January 11
21:58 hrs
diameter = 32.7'
Lunation #1250 begins
Lunar Orbital Elements:
January
January
Moon at 8 days after New, as on
December 21.
The photograph above shows the Moon when approximately eight days after New, just after First Quarter. A rotatable view of the Moon, with ability to zoom in close to the surface (including the far side), and giving detailed information on each feature, may be downloaded
here. A professional version of this freeware with excellent pictures from the Lunar Reconnaissance Orbiter and the Chang orbiter (giving a resolution of 50 metres on the Moon's surface) and many other useful features is available on a DVD from the same website for 20 Euros (about AU $ 33) plus postage.
Lunar Feature for this Month
Each month we describe a lunar crater, cluster of craters, valley, mountain range or other object, chosen at random, but one with interesting attributes. A recent photograph from our Alluna RC20 telescope will illustrate the object. As all large lunar objects are named, the origin of the name will be given if it is important. This month we will look at some unusual features near the impact crater Plinius.
Plinius is the largest crater in this image and has a diameter of 43 kilometres. It is located just right of centre, in the 'strait' where the Mare Serentitatis (Sea of Serenity, north-west of Plinius) joins the Mare Tranquillitatis (the Sea of Tranquility, south-east of Plinius. The 'strait' is between 190 and 200 kilometres wide. One oddity is that the two 'seas' of cooled basaltic lava are of slightly different composition and age. This has resulted in the Mare Tranquillitatis, being at least 70 million years older than Mare Serenitatis, is seen to be darker in colour. What is strange is that the boundary between the two maria is so clearly defined and nearly straight. The image was taken on 30 July 2017. North is to the top.
About 45 kilometres to the west (left) of Plinius and just above the centre of the image is a rocky finger of land pointing north-east across the 'strait'.
This promontory is called Promontorium Archerusia, and it points across to a mountain on the far side, called Mons Argaeus. Between that mountain
and the upper-right corner of the image is a cluster of mountains with valleys in between. The valley closest to the upper-right corner has a small, bright
mountain in it like a white dot. Just north of that 'dot' is the flat area where Apollo 17 landed in 1972,
marked by a yellow cross '
Between Plinius and the left-hand margin is a rugged curve of mountains which form the southern boundary of Mare Serenitatis. A 27 kilometre crater on
this curve of mountains is called Menelaus, the interior of which at the
time of imaging being largely filled with shadow. The uplands west of Menelaus adjoin the Montes Haemus (Haemus Mountains), which have previously been described in
Other impact craters worthy of mention include 17 kilometre Bessel (isolated at upper left in the vast plain of the Mare Serenitatis), 18 kilometre Dawes (midway between Plinius and Mons Argaeus), 27 kilometre Ross (100 kilometres south of Plinius), and 20 kilometre flat-floored Maclear (35 kilometres south-west of Ross and near the midpoint of the bottom margin). Just below the midpoint of the right-hand margin is another flat-floored crater, 23 kilometre Jansen, but its flat floor is punctured by a small craterlet only 3.6 kilometres across.
Plinius
Gaius Plinius Secundus (AD 23-79, generally called Pliny the Elder) was a Roman author, naturalist, natural philosopher (scientist), and naval and army commander of the early Roman empire. He lived in the first century after the birth of Jesus. He wrote an encyclopedia called Naturalis Historia (Natural History) which was published as 37 books organised in 10 volumes. In AD 79 he was killed in the town of Stabiae while attempting to rescue a friend and his family caught up in the eruption of nearby Mount Vesuvius, which wiped out the seaside town as well as Pompeii and Herculaneum. His encyclopedia was completed and published by his nephew Pliny the Younger.
Menelaus
In Homer's epic stories The Iliad and The Odyssey, Menelaus was a central figure in the Trojan War. As the King of Mycenaean Sparta, he led the Spartan contingent of the Greek army under his elder brother Agamemnon, King of Mycenae, when they went to war with Troy. This war occurred when Menelaus' grandfather died and Menelaus sailed to the funeral on the island of Crete. During his absence a Trojan prince named Paris either enticed Menelaus' wife Helen to run away with him or abducted her. When Menelaus returned and found her gone, he and Agamemnon declared war on Troy. They organised a huge army, put it on a fleet of 1000 ships, and sailed to Troy to bring her back. According to Homer, the war lasted ten years. It is a Greek myth, but could be based on some true events. We will never know for sure, but the archaeologist Heinrich Schliemann is believed by many to have located the ruins of Troy in 1873.
The area around the craters Plinius and Menelaus is located inside the rectangle.
Click here for the Lunar Features of the Month Archive
Geocentric Events:
2024
January 2: Venus 55 arcminutes north of the star Graffias (Beta2 Scorpii, mv= 2.59) at 04:36 hrs
(diameter = 14" )
January 2: Mercury at western stationary point at 13:03 hrs (diameter = 8.4" )
January 5: Moon 1.8º north of the star Spica (Alpha Virginis, mv=0.98) at 12:59 hrs
January 8: Limb of Moon 1.3º south of the star Dschubba (Delta Scorpii, mv= 1.86
) at 13:59 hrs
January 8: Limb of Moon 51 arcminutes north of the star Alniyat (Sigma Scorpii, m
v= 2.9) at 21.41 hrs
January 9: Moon 1.5º north of the star Antares (Alpha Scorpii, mv= 0.88) at
0:18 hrs
January 9: Moon 5º south of Venus at 3:10 hrs
January 10: Moon 6º south of Mercury at 3:18 hrs
January 10: Moon 3.4º south of Mars at 19:51 hrs
January 10: Moon 2.4º north of the star Kaus Media ((Delta Sagittarii, mv= 2.72) at 20:40 hrs
January 11: Moon 1.4º south of the star Nunki ((Sigma Sagittarii, mv= 2.02) at 7:37 hrs
January 12: Moon 2.1º south of Pluto at 12:36 hrs
January 13: Mercury at Greatest Elongation West (23º 27') at 5:08 hrs (diameter = 6.6" )
January 13: Mars 1.5º north of the star Kaus Borealis (Lambda Sagittarii, mv= 2.82) at 23:40 hrs
January 14: Limb of Moon 44 arcminutes south of the star Deneb Algedi (Delta Capricorni, mv= 2.85) at
2:57 hrs
January 14: Limb of Moon 52 arcminutes south of Saturn at 21:53 hrs
January 16: Limb of Moon 23 arcminutes south of Neptune at 6:04 hrs
January 19: Moon 2.8º north of Jupiter at 5:26 hrs
January 20: Moon 3.1º north of Uranus at 5:06 hrs
January 20: Mercury 2.6º north of the star Kaus Borealis (Lambda Sagittarii, mv= 2.82) at 5:50 hrs
(diameter = 6.0" )
January 20: Pluto in conjunction with the Sun at 23:44 hrs (diameter = 0.1" )
January 21: Moon occults the star Alcyone (Eta Tauri, mv= 2.85) between 1:34 hrs and
2:16 hrs
January 22: Mars 2.7º north of the star Nunki ((Sigma Sagittarii, mv= 2.02) at 3:50 hrs
January 22: Limb of Moon 9 arcminutes south of the star Elnath (Beta Tauri, mv= 1.65) at 17:17 hrs
January 25: Moon 1.6º south of the star Pollux (Beta Geminorum, mv= 1.15) at 6:35 hrs
January 25: Saturn 23 arcminutes south of the star Sigma Aquarius (mv= 4.8) at 11:36 hrs (diameter = 16" )
January 27: Mars 2.2º south of the star Pi Sagittarii (mv= 3.0) at 3:49 hrs
January 27: Uranus at eastern stationary point at 14:39 hrs (diameter = 3.6" )
January 27: Jupiter at eastern quadrature at 17:12 hrs (diameter = 40.2" )
January 28: Mercury 15 arcminutes north of Mars at 1:25 hrs
January 29: Venus 2.9º north of the star Kaus Borealis (Lambda Sagittarii, mv= 2.82) at 5:45 hrs
Mercury:
The innermost planet passed through superior conjunction (on the far side of the Sun) on October 20 and is now in the western twilight sky. It became visible close to the west-south-western horizon in mid-November, when it was passing by the red supergiant star Antares in Scorpius. Now in the constellation Sagittarius, Mercury is quite easy to find in the first three weeks of December. It will increase its angular distance from the Sun until it reaches Greatest Elongation East on the evening of December 4. The days centred on that date will also be when Mercury is at its brightest, shining at magnitude -0.47 and appearing through a telescope as a tiny half-Moon. On December 13, Mercury will be crossing the outlying stars in the bright globular cluster M22. The thin crescent Moon will be just above and to the left of Mercury between 7 pm and 7:30 pm on December 14. Mercury will reach inferior conjunction on December 23, after which it will become a morning object in the eastern pre-dawn sky.
Venus:
This, the brightest planet, was an 'evening star' for the first seven months of 2023 until it passed between the Earth and the Sun (inferior conjunction) on August 13 last. Then it returned to the pre-dawn eastern sky as a 'morning star' as we see it this month. On December 1, Venus will be a brilliant object high above the eastern horizon from 3 am on, in the constellation Virgo. At magnitude -4.2 it is much brighter than any other object in the night sky except for the Moon. Venus crossed into Virgo's head on November 3, and spent the rest of that month cruising along her body. It will reach her feet on December 11. At that point Venus will move into Libra, and then into Scorpius on January 2, 2024.
In December Venus is a beautiful sight in the morning sky, and observers with a small telescope will see it as a little gibbous Moon, becoming a little smaller and
a little 'fuller' each night as it leaves the Earth behind. The waning crescent Moon will be close by Venus on December 10 and January 9, 2024.
(The coloured fringes to the first, third and fourth images below are due to refractive effects in our own atmosphere, and are not intrinsic to Venus itself. The planet was
closer to the horizon when these images were taken than it was for the second photograph, which was taken when Venus was at its greatest elongation from the Sun.)
October 2023 May-July 2024 January 2025 March 2025 April 2025
C
Because Venus is visible as the 'Evening Star' and as the 'Morning Star', astronomers of ancient times believed that it was two different objects. They called it Hesperus when it appeared in the evening sky and Phosphorus when it was seen before dawn. They also realised that these objects moved with respect to the so-called 'fixed stars' and so were not really stars themselves, but planets (from the Greek word for 'wanderers'). When it was finally realised that the two objects were one and the same, the two names were dropped and the Greeks applied a new name Aphrodite (Goddess of Love) to the planet, to counter Ares (God of War). We use the Roman versions of these names, Venus and Mars, for these two planets.
Venus at 6.55 pm on September 7, 2018. The phase is 36 % and the angular diameter is 32 arcseconds.
Mars:
In this image, the south polar cap of Mars is easily seen. Above it is a dark triangular area known as Syrtis Major. Dark Sinus Sabaeus runs off to the left, just south of the equator. Between the south polar cap and the equator is a large desert called Hellas. The desert to upper left is known as Aeria, and that to the north-east of Syrtis Major is called Isidis Regio. Photograph taken in 1971.
Mars photographed from Starfield Observatory, Nambour on June 29 and July 9, 2016, showing two different sides of the planet. The north polar cap is prominent.
Brilliant Mars at left, shining at magnitude 0.9, passes in front of the dark molecular clouds in Sagittarius on October 15, 2014. At the top margin is the white fourth magnitude star 44 Ophiuchi. Its type is A3 IV:m. Below it and to the left is another star, less bright and orange in colour. This is the sixth magnitude star SAO 185374, and its type is K0 III. To the right (north) of this star is a dark molecular cloud named B74. A line of more dark clouds wends its way down through the image to a small, extremely dense cloud, B68, just right of centre at the bottom margin. In the lower right-hand corner is a long dark cloud shaped like a figure 5. This is the Snake Nebula, B72. Above the Snake is a larger cloud, B77. These dark clouds were discovered by Edward Emerson Barnard at Mount Wilson in 1905. He catalogued 370 of them, hence the initial 'B'. The bright centre of our Galaxy is behind these dark clouds, and is hidden from view. If the clouds were not there, the galactic centre would be so bright that it would turn night into day.
Mars near opposition, July 24, 2018
Mars, called the red planet but usually coloured orange, in mid-2018 took on a yellowish tint and brightened by 0.4 magnitude, making it twice as bright as previous predictions for the July 27 opposition. These phenomena were caused by a great dust storm which completely encircled the planet, obscuring the surface features so that they were only seen faintly through the thick curtain of dust. Although planetary photographers were mostly disappointed, many observers were interested to see that the yellow colour and increased brightness meant that a weather event on a distant planet could actually be detected with the unaided eye - a very unusual thing in itself.
The three pictures above were taken on the evening of July 24, at 9:05, 9:51 and 11:34 pm. Although the fine details that are usually seen on Mars
were hidden by the dust storm,
some of the larger features can be discerned, revealing how much Mars rotates in two and a half hours. Mars' sidereal rotation period (the time taken for one complete
rotation or 'Martian day') is 24 hours 37 minutes 22 seconds - a little longer than an Earth day. The dust storm began in the Hellas Desert on May 31, and after two months
it still enshrouded the planet. In September it began to clear, but by then the close approach had passed.
Central meridian: 295º.
The two pictures immediately above were taken on the evening of September 7, at 6:25 and 8:06 pm. The dust storm was finally abating, and some of the surface features were becoming visible once again. This pair of images also demonstrates the rotation of Mars in 1 hour 41 minutes (equal to 24.6 degrees of longitude), but this time the view is of the opposite side of the planet to the set of three above. As we were now leaving Mars behind, the images are appreciably smaller (the angular diameter of the red planet had fallen to 20 arcseconds). Well past opposition, Mars on September 7 exhibited a phase effect of 92.65 %.
Central meridian: 180º.
Jupiter: Jupiter is now found two handspans above the north-eastern horizon at 7 pm on December 1. It transits the meridian at 9:30 pm on that date, and at 7:25 pm on December 31. It is ideally placed for viewing before midnight. The waxing gibbous Moon will be close by Jupiter at 7:30 pm on December 22.
Jupiter as photographed from Nambour on the evening of April 25, 2017. The images were
taken, from left to right, at 9:10, 9:23, 9:49, 10:06 and 10:37 pm. The rapid rotation of this giant planet in a little under 10 hours is clearly seen. In the
southern hemisphere, the Great Red Spot (bigger than the Earth) is prominent, sitting within a 'bay' in the South Tropical Belt. South of it is one of the
numerous White Spots. All of these are features in the cloud tops of Jupiter's atmosphere.
Jupiter at opposition, May 9, 2018
Jupiter as it appeared at 7:29 pm on July 2, 2017. The Great Red Spot was in a similar position near Jupiter's eastern limb (edge) as in the fourth picture in the series above.
It will be seen that in the past two months the position of the Spot had drifted
when compared with the festoons in the Equatorial Belt, so must rotate around the planet at a slower rate. In fact, the Belt enclosing the Great Red Spot rotates around the
planet in 9 hours 55 minutes, and the Equatorial Belt takes five minutes less. This high rate of rotation has made the planet quite oblate. The prominent 'bay'
around the Red Spot in the five earlier images appeared to be disappearing, and a darker streak along the northern edge of the South Tropical Belt
was moving
south. In June this year the Spot began to shrink in size, losing about 20% of its
diameter. Two new white spots have developed in the South Temperate Belt, west of the Red Spot. The five upper images were taken near opposition, when the Sun was
directly behind the Earth and illuminating all of Jupiter's disc evenly. The July 2 image was taken just four days before Eastern Quadrature, when the angle
from the Sun to Jupiter and back to the Earth was at its maximum size. This angle means that we see a tiny amount of Jupiter's dark side, the shadow being
visible around the limb of the planet on the left-hand side, whereas the right-hand limb is clear and sharp. Three of Jupiter's Galilean satellites are visible, Ganymede to
the left and Europa to the right. The satellite Io can be detected in a transit of Jupiter, sitting in front of the North Tropical Belt, just to the left of its centre.
Jupiter reached opposition on May 9, 2018 at 10:21 hrs, and the above photographs were taken that evening, some ten to twelve hours later. The first image above was taken at 9:03 pm, when the Great Red Spot was approaching Jupiter's central meridian and the satellite Europa was preparing to transit Jupiter's disc. Europa's transit began at 9:22 pm, one minute after its shadow had touched Jupiter's cloud tops. The second photograph was taken three minutes later at 9:25 pm, with the Great Red Spot very close to Jupiter's central meridian.
The third photograph was taken at 10:20 pm, when Europa was approaching Jupiter's central meridian. Its dark shadow is behind it, slightly below, on the clouds of the North Temperate Belt. The shadow is partially eclipsed by Europa itself. The fourth photograph at 10:34 pm shows Europa and its shadow well past the central meridian. Europa is the smallest of the Galilean satellites, and has a diameter of 3120 kilometres. It is ice-covered, which accounts for its brightness and whitish colour. Jupiter's elevation above the horizon for the four photographs in order was 50º, 55º, 66º and 71º. As the evening progressed, the air temperature dropped a little and the planet gained altitude. The 'seeing' improved slightly, from Antoniadi IV to Antoniadi III. At the time of the photographs, Europa's angular diameter was 1.57 arcseconds. Part of the final photograph is enlarged below.
Jupiter at 11:34 pm on May 18, nine days later. Changes in the rotating cloud patterns are apparent, as some cloud bands rotate faster than others and
interact. Compare with the first photograph in the line of four taken on May 9. The Great Red Spot is ploughing a furrow through the clouds of the South
Tropical Belt, and is pushing up a turbulent bow wave.
Jupiter at opposition, June 11, 2019
Jupiter reached opposition on June 11, 2019 at 01:20 hrs, and the above photographs were taken that evening, some
twenty to twenty-two hours later. The first image above was
taken at 10:01 pm, when the Great Red Spot was leaving Jupiter's central meridian and the satellite Europa was preparing to transit Jupiter's disc.
The third photograph was taken at 10:41 pm, when Europa was about a third of its way across Jupiter. Its dark shadow is trailing it, slightly below, on the clouds of the North Temperate Belt. The shadow is partially eclipsed by Europa itself. The fourth photograph at 10:54 pm shows Europa and its shadow about a quarter of the way across. This image is enlarged below. The fifth photograph shows Europa on Jupiter's central meridian at 11:24 pm, with the Great Red Spot on Jupiter's limb. The sixth photograph taken at 11:45 pm shows Europa about two-thirds of the way through its transit, and the Great Red Spot almost out of sight. In this image, the satellite Callisto may be seen to the lower right of its parent planet. Jupiter's elevation above the horizon for the six photographs in order was 66º, 70º, 75º, 78º, 84º and 86º. As the evening progressed, the 'seeing' proved quite variable.
There have been numerous alterations to Jupiter's belts and spots over the thirteen months since the 2018 opposition. In particular, there have been major disturbances affecting the Great Red Spot, which appears to be slowly changing in size or "unravelling". It was very fortuitous that, during the evenings of the days when the 2018 and 2019 oppositions occurred, there was a transit of one of the satellites as well as the appearance of the Great Red Spot. It was also interesting in that the same satellite, Europa, was involved both times.
Jupiter's moon Europa has an icy crust with very high reflectivity, which accounts for its brightness in the images above. On the other hand, the largest
moon Ganymede (seen below) has a surface which is composed of two types of terrain: very old, highly cratered dark regions, and somewhat younger (but still
ancient) lighter regions marked with an extensive array of grooves and ridges. Although there is much ice covering the surface, the dark areas contain clays
and organic materials and cover about one third of the moon. Beneath the surface of Ganymede is believed to be a saltwater ocean with two separate layers.
Jupiter is seen here on 17 November 2022 at 8:39 pm. To its far right is its largest satellite, Ganymede. This "moon"
is smaller than the Earth but is bigger than Earth's Moon. Its diameter is 5268 kilometres, but at Jupiter's distance its angular diameter is only 1.67 arcseconds.
Despite its small size, Ganymede is the biggest moon in the Solar System. Jupiter is approaching eastern quadrature, which means that Ganymede's shadow is not behind it as in the shadows of Europa in the two
sequences taken at opposition. In the instance above as seen from Earth (which is presently at a large angle from a line joining the Sun to Ganymede), the
circular shadow of Ganymede is striking the southern hemisphere cloud tops of Jupiter itself. The shadow is slightly distorted as it strikes the spherical
globe of Jupiter. If there were any inhabitants of Jupiter flying across the cloud bands above, and passing through the black shadow, they would experience
an eclipse of the distant Sun by the moon Ganymede.
Above is a 7X enlargement of Ganymede, showing markings on its rugged, icy surface. The dark area in its northern hemisphere is called Galileo Regio.
Saturn:
The ringed planet is located in the constellation of Aquarius, and will remain there until it crosses into Pisces on April 19, 2025. Saturn reached eastern quadrature (high overhead at sunset) on November 23.
Left: Saturn showing the Rings when edge-on. Right: Over-exposed Saturn surrounded by its satellites Rhea, Enceladus, Dione, Tethys and Titan - February 23/24, 2009.
Neptune:
The change in aspect of Saturn's rings is caused by the plane of the ring system being aligned with Saturn's equator, which is itself tilted at an angle of 26.7 degrees to Saturn's orbit.
As the Earth's orbit around the Sun is in much the same plane as Saturn's, and
the rings are always tilted in the same direction in space, as we both orbit the Sun, observers on Earth see the configuration of the rings change from wide open (top large picture) to half-open
(bottom large picture) and finally to edge on (small picture above). This cycle is due to Saturn taking 29.457 years to complete an orbit of the Sun, so the
complete cycle from
"edge-on (2009) →
view of Northern hemisphere, rings half-open (2013) → wide-open (2017) → half-open (2022) →
edge-on (2025) → view of Southern
hemisphere, rings half-open (2029) → wide-open (2032) → half-open
(2036) → edge-on (2039)"
Uranus:
Neptune, photographed from Nambour on October 31, 2008
Pluto: The erstwhile ninth and most distant planet
will reach conjunction with the Sun on January 20, so it is now too close to the
Sun to be safely observed. Pluto's diameter is 0.13 arcseconds, less than one
twentieth that of Neptune. Located in Sagittarius, it is close to the border
with Capricornus, and will cross into the latter constellation on January 4,
2024. It is a 14.1 magnitude object, very small and faint. A telescope with an
aperture of 25 cm is capable of locating Pluto when the seeing conditions are right.
The waxing crescent Moon will be close to Pluto on December 17.
The movement of the dwarf planet Pluto in two days, between 13 and 15 September, 2008. Pluto is the one object that has moved.
Width of field: 200 arcseconds
This is a stack of four images, showing the movement of Pluto over the period October 22 to 25, 2014. Pluto's image for each date appears as a star-like point at the upper right corner of the numerals. The four are equidistant points on an almost-straight line. Four eleventh magnitude field stars are identified. A is GSC 6292:20, mv = 11.6. B is GSC 6288:1587, mv = 11.9. C is GSC 6292:171, mv = 11.2.
D is GSC 6292:36, mv = 11.5. (GSC = Guide Star Catalogue). The position of Pluto on October 24 (centre of image) was at Right Ascension = 18 hours 48 minutes 13 seconds, Declination = -20º 39' 11". The planet moved 2' 51" with respect to the stellar background during the three days between the first and last images, or 57 arcseconds per day, or 1 arcsecond every 25¼ minutes.
Meteor Showers:
Phoenicids
December 6 and 7
Waning crescent Moon, 34% sunlit ZHR =
6
Radiant: Near the star Achernar
Geminids
December 13 and 14
New Moon, 4% sunlit ZHR =
90
Radiant: Near the star Castor. Associated with Asteroid 3200 Phaethon.
This will be quite a rich meteor shower.
Ursids
Us
Although most meteors are found in swarms associated with debris from comets, there are numerous 'loners', meteors travelling on solitary
paths through space. When these enter our atmosphere, unannounced and at any time, they are known as 'sporadics'. On an average clear and dark evening, an
observer can expect to see about ten meteors per hour. They burn up to ash in their passage through our atmosphere. The ash slowly settles to the ground as
meteoric dust. The Earth gains about 80 tonnes of such dust every day, so a percentage of the soil we walk on is actually interplanetary in origin. If a
meteor survives its passage through the air and reaches the ground, it is called a 'meteorite'. In the past, large meteorites (possibly comet nuclei or small asteroids) collided with the Earth and produced huge craters which still exist
today. These craters are called 'astroblemes'. Two famous ones in Australia are Wolfe Creek Crater and Gosse's Bluff. The Moon and Mercury are covered with such
astroblemes, and craters are also found on Venus, Mars, planetary satellites, minor planets, asteroids and even comets.
Comets:
ZHR = zenithal hourly rate (number of meteors expected to be observed at the zenith in one hour). The
maximum phase of meteor showers usually occurs between 3 am and sunrise. The reason most meteors are observed in the pre-dawn hours is because at that time
we are on the front of the Earth as it rushes through space at 107 000 km per hour (30 km per second). We are meeting the meteors head-on, and the speed at
which they enter our atmosphere is the sum of their own speed plus ours. In the evenings, we are on the rear side of the Earth, and many meteors we see at that
time are actually having to catch us up. This means that the speed at which they enter our atmosphere is less than in the morning hours, and they burn up less
brilliantly.
This comet was discovered on 2 March 2022 at the Zwicky Transient Facility (ZTF) at the Hale Observatory on Mount Palomar. It was found on CCD images taken by the
famous 48-inch Schmidt Telescope. It
Both of these comets appeared recently in orbits that caused them to dive towards the Sun's
surface before swinging around the Sun and heading back towards the far reaches of the Solar System. Such comets are called 'Sun grazers', and their close approach to
the Sun takes them through its immensely powerful gravitational field and the hot outer atmosphere called the 'corona'. They brighten considerably during their
approach, but most do not survive and disintegrate as the ice which holds them together melts. While expectations were high that these two would emerge from
their encounter and put on a display as bright comets with long tails when they left the Sun, as they came close to the Sun they both broke up into small
fragments of rock and ice and ceased to exist.
Comet 46P/Wirtanen In December 2018, Comet 46P/Wirtanen swept past Earth, making one of the ten closest approaches of a comet to our planet since 1960. It
was faintly visible to the naked eye for two weeks. Although Wirtanen's nucleus is only 1.2 kilometres across, its green atmosphere
became larger than the Full Moon, and was an easy target for binoculars and small telescopes. It reached its closest to the Sun (perihelion) on December 12, and then
headed in our direction. It passed the Earth at a distance of 11.5 million kilometres (30 times as far away as the Moon) on December 16,
2018.
Comet Lulin
This comet, (C/2007 N3), discovered
Comet Lulin at 11:25 pm on February 28, 2009, in Leo. The brightest star is Nu Leonis, magnitude 5.26.
The
LINEARNearly all of these programs are based in the northern hemisphere, leaving gaps in the coverage of the southern sky. These gaps are the areas of sky where amateur astronomers look for comets from their backyard observatories.
To find out more about current comets, including finder charts showing exact positions and magnitudes, click
here. To see pictures of these comets, click here.
The 3.9 metre Anglo-Australian Telescope (AAT) at the Australian Astronomical Observatory near Coonabarabran, NSW.
Deep Space
Sky Charts and Maps available on-line:
There are some useful representations of the sky available here. The sky charts linked
below show the sky as it appears to the unaided eye. Stars rise four minutes earlier each night, so at the end of a week the stars have gained about half an
hour. After a month they have gained two hours. In other words, the stars that were positioned in the sky at 8 pm at the beginning of a month will have the
same positions at 6 pm by the end of that month. After 12 months the stars have gained 12 x 2 hours = 24 hours = 1 day, so after a year the stars have returned
to their original positions for the chosen time. This accounts for the slow changing of the starry sky as the seasons progress.
The following interactive sky charts are courtesy of Sky and Telescope magazine. They can simulate a view of the sky from any location on Earth at any time of day or night between the years 1600 and 2400. You can also print an all-sky map. A Java-enabled web browser is required. You will need to specify the location, date and time before the charts are generated. The accuracy of the charts will depend on your computer’s clock being set to the correct time and date.
To produce a real-time sky chart (i.e. a chart showing the sky at the instant the chart is generated), enter the name of your nearest city and the country. You will also need to enter the approximate latitude and longitude of your observing site. For the Sunshine Coast, these are:
latitude: 26.6o South longitude: 153o East
Then enter your time, by scrolling down through the list of cities to "Brisbane: UT + 10 hours". Enter this one if you are located near this city, as Nambour is. The code means that Brisbane is ten hours ahead of Universal Time (UT), which is related to Greenwich Mean Time (GMT), the time observed at longitude 0o, which passes through London, England. Click here to generate these charts.
_____________________________________
Similar real-time charts can also be generated from another source, by following this second link:
The first, circular chart will show the full hemisphere of sky overhead. The zenith is at the centre of the circle, and the cardinal points are shown around the circumference, which marks the horizon. The chart also shows the positions of the Moon and planets at that time. As the chart is rather cluttered, click on a part of it to show that section of the sky in greater detail. Also, click on Update to make the screen concurrent with the ever-moving sky.
The stars and constellations around the horizon to an elevation of about 40o can be examined by clicking on
The view can be panned around the horizon, 45 degrees at a time. Scrolling down the screen will reveal tables showing setup and customising options, and an Ephemeris showing the positions of the Sun, Moon and planets, and whether they are visible at the time or not. These charts and data are from YourSky, produced by John Walker.
The charts above and the descriptions below assume that the observer has a good observing site with a low, flat horizon that is not too much obscured by buildings or trees. Detection of fainter sky objects is greatly assisted if the observer can avoid bright lights, or, ideally, travel to a dark sky site. On the Sunshine Coast, one merely has to travel a few kilometres west of the coastal strip to enjoy magnificent sky views. On the Blackall Range, simply avoid streetlights. Allow your eyes about 15 minutes to become dark-adapted, a little longer if you have been watching television. Small binoculars can provide some amazing views, and with a small telescope, the sky’s the limit.
This month,
These descriptions of the night sky are for 10 pm on December 1 and 8 pm on December 31. Broadly speaking, the following
description starts in the west and heads east, then south, then high in the south.
The Great Spiral M33 in Triangulum. Above Triangulum is the zodiacal constellation of Aries, now past the meridian in the north-north- west. There are three main stars in Aries, the brightest being a second magnitude orange star called Hamal. The other two are named Sheratan and Mesarthim
. M33 lies two-fifths of the way between Mirach and Hamal. The faintest of these three
stars is Mesarthim, magnitude 4.5. 8.8 degrees south-east
of this star is the brilliant planet Jupiter, much brighter than anything else in this
half of the sky except for the Moon. 16 degrees (almost a handspan) to the south-east (above and to the right) of Hamal is the planet Uranus,
but it is not visible without at least a pair of good binoculars or a small
telescope. At 9:23 pm at mid-month, Uranus will be crossing the meridian (the north to south line passing through the
zenith). It will be two handspans north of the zenith when this occurs (called 'culmination'). Cetus (the Whale) lies a little north-west of the zenith. Though this part of the sky has no really bright stars, a little more
than a handspan west of the zenith is a mv 2.2 star. This is Beta Ceti, the brightest star in the constellation. Its common name is Diphda,
and it has a yellowish-orange colour. By rights, we would expect the star Menkar or Alpha Ceti to be brighter, but Menkar is actually more than half a
magnitude fainter than Diphda. Menkar may be seen high in the north-east, halfway between Diphda and Aldebaran. Cetus is a large constellation, and to the unaided eye it appears unremarkable. But it does contain a most interesting star, which
even medieval people noticed. Hevelius named it Mira, the Wonderful (see below)
because in the space of almost a year its brightness varies from magnitude 3 to
magnitude 10 and back to magnitude 3. This year
Mira reached its peak magnitude on June 13, and is near its minimum brightness this month. Between Cetus and Pegasus is the zodiacal constellation of Pisces
(the Fishes). The spectacular constellations are in the eastern half of the sky tonight. Taurus, with its two star clusters the
Pleiades (also known as the Seven Sisters, or Subaru, or Santa's Sleigh) and the Hyades, is high in the north-north-east (see below). The brightest
star in Taurus is an orange star dominating the Hyades cluster, but not a member of it. This is Aldebaran, a K5 star with a visual magnitude of 0.87.
Below the Pleiades we can see part of the far-northern constellation of Perseus. The two brightest stars are Mirphak (Alpha
Persei) and Algol (Beta Persei). Algol is the higher above the northern horizon of the two. Since ancient times, Algol shows regular variations in brightness.
It usually shines at magnitude 2.1, but every 2 days 20 hours 49 minutes it dims to magnitude 3.4 for 10 hours before recovering its original brightness. Because of
this clock-like change, early astronomers called it the 'Demon Star'. It marks the head of Medusa, the Gorgon, which is carried in the hand of Perseus. The Dutch-born Englishman John Goodricke (1764-1786) was a young amateur astronomer. He was profoundly deaf and also
restricted in speech
after suffering from scarlet fever as a young boy. Goodricke is best known for his explanation of the variations in the star Algol (Beta Persei) in 1782.
Although several stars were already known to vary in apparent magnitude, Goodricke was the first to propose a mechanism to account for this. He suggested
that Algol is actually a pair of stars circling each other, and the variation in brightness is caused when one passes behind the other. Such a star system is now
known as an eclipsing binary. He presented his findings to the Royal Society in May 1783, and for this work, the Society awarded him the Copley Medal for that year.
Goodricke is also credited with discovering the periodic variation of the namesake, Delta Cephei, of the type of variable
stars called Cepheids, in 1784. It was the second Cepheid found, the first, Eta Aquilae, being found by Goodricke’s friend Edward Pigott earlier the same year.
Goodricke proposed that Cepheids were unstable stars that regularly swell up and then fall back - 'pulsating variables' (see
Mira, the Wonderful
below).
The Stars and Constellations for this
Soon after darkness falls, setting on the west-south-western horizon is the zodiacal constellation Capricornus,
the Sea Goat. Above it is Aquarius,
the Water Bearer, which is due west.
There are no stars brighter than third magnitude in this constellation, but it does contain many interesting objects, including a group
of four stars known as the 'Water Jar'. Also, this year the bright ringed
planet Saturn is located in the centre of Aquarius, but will have set by
11 pm at mid-month. Right on the boundary of
Aquarius with the next zodiacal constellation of Pisces, the Fishes,
observers with telescopes can find the faint planet Neptune.
This planet is currently 1.6 degrees south-west of the magnitude 6 star 20
Piscium. Pisces crosses the meridian at 7 pm in mid-December.
The Great Square of Pegasus is tilted over on its lower left corner, and is setting in the north-west, being followed by
Andromeda, the Chained Maiden. The constellation of Triangulum,
the Triangle, is above the north-north-western horizon. These constellations contain the well-known spiral galaxies M31 (in Andromeda,
illustrated at the bottom of this webpage), and M33 (in Triangulum, illustrated below).
The three main stars in Andromeda are, from left to right, Alpheratz, Mirach, and Almach, and above them is the faint constellation of
Triangulum, a narrow triangle of stars. M31 lies about eight degrees (one third of a handspan) below and slightly to the left of Mirach tonight. These large spirals
are members of the Local Group of galaxies (our Milky Way is a third member), and can be easily seen with binoculars. They
are the nearest galaxies that can be observed from the large observatories in the Northern Hemisphere. This month they are
best seen as soon as darkness falls, for then they will be closer to due north and therefore at their highest elevation.
He was elected a Fellow of the Royal Society on April 16, 1786, but never learned of this honour, as he was very ill and died four days later, probably from pneumonia, aged only twenty-one years and seven months. Yet he is still famous in the present day.
Between the Hyades and the north-eastern horizon, the large pentagon of Auriga, the Charioteer has risen. The brightest star in this constellation is Capella, which is directly above the north-north-eastern horizon. Just above Capella is a little triangle of stars, known as 'The Kids'. The top star of Auriga's pentagon, Elnath, points towards Orion. Actually, Elnath is the second-brightest star in Taurus, and forms the tip of one of the Bull's horns.
At 10 pm at the beginning of the month, the two bright stars of Gemini, Castor and Pollux, are just about to rise above the theoretical north-eastern horizon. These two first magnitude stars are known as 'The Twins'. If your horizon in this direction is very low, you may be able to glimpse them, both stars rising together about five degrees apart. Pollux is the more easterly of the two. If your horizon is not low, you will have to wait a little to see these stars appear. These two stars are the twins' heads, and their feet point towards Orion. Two-thirds of the way from their heads to their feet, each twin is marked by a star at his hip, third magnitude Mebsuta for Castor, and fourth magnitude Mekbuda for Pollux.
Orion (see below) is about halfway up the sky in the east, and a bright star in Canis Minor (the Lesser Dog), Procyon, is now about half-a-handspan above the eastern horizon. Around midnight at mid-month, Orion will cross the meridian (the north-south line across the sky, passing through the zenith), at which time the constellation is said to ‘culminate’. The three stars marking Orion's Belt will be about twenty-five degrees (a little more than a handspan) north of the zenith. Sirius, the brightest star in the night sky, is south-east of Orion, in the constellation Canis Major (the Greater Dog).
Very low in the south, Alpha Crucis (or Acrux), the brightest and most southerly star of Crux (Southern Cross) is beginning to
rise. Between Crux and Sirius is a large area of the Milky Way filled with interesting objects. This was once the constellation Argo Navis, named for Jason’s famous ship
used by the Argonauts in their search for the Golden Fleece. The constellation Argo was found to be too large, so modern star atlases divide it into three sections -
Carina (the Keel), Vela (the Sails) and Puppis (the Stern). Above the horizon, just to the left of due south and above Acrux, is the small constellation Musca, the Fly. Musca is
a circumpolar constellation, i.e. it is always in our sky, being too close to the South Celestial Pole to set. Out of the 88 constellations, it is the only insect. High in the south-east and two handspans from Sirius is the second brightest star in the night sky, Canopus (Alpha Carinae). On the border of
Carina and Vela is the False Cross, larger and more lopsided than the Southern Cross. Both of these Crosses are actually more like kites in shape for, unlike
Cygnus (the Northern Cross) they have no star at the intersection of the two cross arms. At the present time, both Crosses are upside-down. At about thirty-five degrees above the south-western horizon, the flattened triangle of Grus, the Crane, is swinging down. To its right is the
bright star Fomalhaut, a bright, white first magnitude star in the faint constellation Piscis Austrinus, the Southern Fish. Fomalhaut means 'the fish's
mouth'. A planet revolving around Fomalhaut was recently photographed by the Hubble Space Telescope. Midway between -0.71 magnitude Canopus and
1.16 magnitude Fomalhaut is the 0.46 magnitude star Achernar, in the constellation Eridanus, the River. Eridanus winds its faint way across the sky from Achernar to Cursa, these two stars marking the ends of the river. Cursa
(Beta Eridani) is a 2.79 magnitude star about four degrees to the north-west of Rigel (see below). To the right of Achernar, the faint constellation of Phoenix may be seen. Its brightest star is
Ankaa, a mv 2.39 star which is about one third of the way from Achernar to Diphda. About a handspan south of Achernar is a small, faint glowing patch. This is the
Small Magellanic Cloud
(SMC). A handspan to its left is a larger glow, the Large Magellanic Cloud (LMC). Both of these glowing clouds are
small galaxies in their own right, separate from our Milky Way galaxy. They are described in greater detail in
Two close galaxies
below. The LMC bridges two faint constellations, Dorado and Mensa. The SMC is in the constellation Tucana, the Toucan.
The zodiacal constellations visible tonight, starting from the south-western horizon and heading across to the north-east horizon, are Capricornus,
Aquarius, Pisces, Aries, Taurus and Gemini.
If you would like to become familiar with the constellations, we suggest that
you access one of the world's best collections of constellation pictures by clicking
The Winged Horse and the Chained Maiden:
About half-way up the sky from the north-north-western horizon (at the end of twilight on December 1) is a large star group called the Great Square of Pegasus. The Square (actually slightly rectangular) is unusual for the sparcity of bright stars within it. Pegasus was the winged horse of Greek legend, and is used as a symbol by the Mobil Oil Company and TriStar Pictures. By 8.00 pm the Square will be tilted over to the west and it will begin to set in the north-west by 11.00 pm.
The stars making the four corners of the Square, starting from the top left and moving clockwise, are Markab, Algenib, Alpheratz and Scheat. Three of these are in the constellation of Pegasus, but Alpheratz is actually in the constellation Andromeda.
Between Alpheratz and the northern horizon, a very faint glowing oval may be seen if the sky is dark enough. You would need to be away from light pollution caused by bright lights. Binoculars will help you to find it, but it is visible faintly without optical aid. This oval is the Great Nebula in Andromeda (M31), a large spiral galaxy of over 100 million stars. The Andromeda spiral is almost a twin of our own galaxy, the Milky Way. Lying at a distance of 2.6 million light years, it is the furthest object that can be seen with the unaided eye (see The Andromeda Galaxy and the President of the United States
at the bottom of this page).The Hunter and his Dogs:
Two of the most spectacular constellations in the sky may be seen rising above the eastern horizon soon after sunset in December. These are Orion the Hunter, and his
greater dog, Canis Major. Orion straddles the celestial equator, midway between the south celestial pole and its northern equivalent. This means that the centre of
the constellation, the three stars known as Orion's Belt, rise due east and set due west. A smaller constellation, the lesser dog Canis Minor, accompanies them.
Orion:
This is one of the most easily recognised constellations, as it really does give a very good impression of a human figure. From the northern hemisphere he appears to stand upright when he is high in the sky, but from our location ‘down under’ he appears lying down when rising and setting, and upside down when high in the sky. You can, though, make him appear upright when high in the sky (near the meridian), by observing him from a reclining chair, with your feet pointing to the south and your head tilted back.
Orion rising as darkness falls in December
Orion rises south-east of Taurus. If you have a fairly low eastern horizon, you will
see most of the constellation rising above the eastern horizon as soon as it gets dark at the beginning of December. The bright
star Rigel is
highest in the sky. The red supergiant Betelgeuse is the last of Orion's bright stars to rise.
A little north of a line joining these stars is a tiny triangle of stars marking Orion’s head. In the centre of the constellation are three stars in a line forming Orion's Belt. These are, from west to east, Mintaka, Alnilam and Alnitak. These three stars are related, and all lie at a distance of 1300 light years. They are members of a group of hot blue-white stars called the Orion Association.
To the south of the Belt, at a distance of about one Belt-length, we see another faint group of stars in a line, fainter and closer together than those in the Belt. This is Orion’s Sword. Orion’s legs are marked by brilliant Rigel and fainter Saiph. Both of these stars are also members of the Orion Association.
The Saucepan, with Belt at left, M42 at lower right.
Orion is quite a symmetrical constellation, with the Belt at its centre and the two shoulder stars off to the north and the two leg stars to the south. It is quite a large star group, the Hunter being over twenty degrees (about a handspan) tall.
The stars forming the Belt and Sword are popularly known in Australia as ‘The Saucepan’, with the Sword forming the Saucepan’s handle. This asterism appears upside-down tonight, as in the photographs above. The faint, fuzzy star in the centre of the Sword, or the Saucepan’s handle, is a great gas cloud or nebula where stars are being created. It is called the ‘Great Nebula in Orion’ or M42*. A photograph of it appears below:
The Sword of Orion, with the Great Nebula, M42, at centre
By 8.00 pm in early December, a brilliant white star will be seen rising about one handspan to the south and lower than Orion.
This is Sirius, or Alpha Canis Majoris, and it is the brightest star in the night sky with a visual magnitude of -1.43.
It marks the heart of the hunter's dog, and has been known for centuries as the Dog Star. As he rises, the dog is on his
back with his front foot in the air. The star at the end of this foot is called
Mirzam. It is also known as Beta Canis
Majoris, which tells us that it is the second-brightest star in the constellation. Mirzam is about one-third of a handspan above
Sirius. The hindquarters of the Dog are indicated by a large right-angled triangle of
stars located to the right of Sirius and tilted. The end of his tail is the lower-right corner of the triangle, about one
handspan south (to the right) of Sirius. The star at the root of the Dog's tail is called
Wezen, and the one at the tip of his
tail is called Aludra. The star Adhara marks his back foot. The Dog's head is shown by a faint outline of fourth and fifth magnitude stars. Both Sirius and Rigel are bright white stars and each has a tiny, faint white
dwarf companion. Whereas a small telescope can reveal the companion to Rigel quite easily, the companion to Sirius the Dog Star,
(called ‘the Pup’), can only be observed by using a powerful telescope with excellent optics on a night of good 'seeing', as
it is very close to brilliant Sirius and is usually lost in the glare.
Canis
Major as it appears in the east soon after sunset in December
This small constellation contains only two main stars, the brighter of which is Procyon (Alpha Canis Minoris). This yellow-white star of mv= 0.5 forms one corner of a large equilateral triangle, the other two corners being the red Betelgeuse and white Sirius. Beta Canis Minoris is also known as Gomeisa, a blue-white star of mv= 3.1. Procyon rises in the east soon after 9.00 pm at the beginning of December.
Taurus, the Bull:
What is Orion hunting? About a handspan to the left of Orion's Belt is a bright orange star, Aldebaran. It is the brightest star in the constellation Taurus, the Bull. This orange star appears at the foot of a cluster of stars with the appearance of a capital A or inverted V. This cluster is called the Hyades, and seemed to the ancients like the face of a bull, with Aldebaran as his angry orange eye. Being in the southern hemisphere, we see it upside down. Actually, Aldebaran is not physically connected with the Hyades cluster, being much closer, but in the same line of sight.
Half a handspan to the left of the Hyades cluster, and a little higher above the horizon, is another cluster, smaller and fainter. This is the Pleiades, a small group like a question mark. It is often called the Seven Sisters, although excellent eyes are needed to detect the seventh star without binoculars or telescope. The group is also known as ‘Santa’s Sleigh’, as it appears around Christmas time. All the stars in this cluster are hot and blue. They are also the same age, as they formed as a group out of a single gas cloud or nebula. There are actually more than 250 stars in the Pleiades, and this cluster marks the Bull's shoulder.
T
he Pleiades is the small cluster at centre left, while the Hyades is the much larger grouping at centre right.
Wisps of the nebula which surrounds the Pleiades can be seen around the brighter stars in the cluster.
Mira, the Wonderful:
The amazing thing about the star Mira or Omicron Ceti is that it varies dramatically in brightness, rising to magnitude 2 (brighter than any other star in Cetus), and then dropping to magnitude 10 (requiring a telescope to detect it), over a period of 332 days.
This drop of eight magnitudes means that its brightness diminishes over a period of five and a half months to one six-hundredth of what it had been, and then over the next five and a half months it regains its original brightness. To the ancients, they saw the familiar star fade away during the year until it disappeared, and then it slowly reappeared again. Its not surprising that Hevelius named it 'Mira', meaning 'The Wonderful' or 'The Miraculous One'.
We now know that many stars vary in brightness, even our Sun doing so to a small degree, with a period of 11 years. One type of star varies, not because it is actually becoming less bright in itself, but because another, fainter star moves around it in an orbit roughly in line with the Earth, and obscures part of it on each pass. This type of star is called an eclipsing variable and they are very common.
The star Mira though, varies its light output because of processes in its interior. It is what is known as a pulsating variable. Stars of the Mira type are giant pulsating red stars that vary between 2.5 and 11 magnitudes in brightness. They have long, regular periods of pulsation which lie in the range from 80 to 1000 days.
In 2022, Mira reached a maximum brightness of magnitude 3.4 on July 16 and then faded slowly, dropping well below naked-eye visibility (magnitude 9) by February this year. It then brightened rapidly, and reached this year's maximum on June 13. Now it has faded again, and will reach its minimum brightness early in 2024. A small telescope will be needed in order to find it. By next Easter it will be brightening again and will reach maximum brightness on May 10, 2024. Unfortunately it will then be a morning sky object, rising on that day at 4:48 am.
This month, keen observers with telescopes will be able to detect Mira 66º (3.5 handspans) above the northern horizon at 8:30 pm at mid-month, but it is quite faint at tenth magnitude.
Mira near minimum, 26 September 2008 Mira near maximum, 22 December 2008
Astronomers using a NASA space telescope have spotted an amazingly long comet-like tail behind Mira as the star streaks through space. The Galaxy Evolution Explorer - "GALEX" for short - scanned the well-known star during its ongoing survey of the entire sky in ultraviolet light. Astronomers then noticed what looked like a comet with a gargantuan tail. In fact, material blowing off Mira is forming a wake 13 light-years long, or about 20,000 times the average distance of Pluto from the sun. Nothing like this has ever been seen before around a star. More, including pictures
Double stars:
Estimates vary that between 15% and 50% of stars are single bodies like our Sun, although the latest view is that less than 25% of stars are solitary. At least 30% of
stars and possibly as much as 60% of stars are in double systems, where the two stars are gravitationally linked and orbit their mutual centre of gravity. Such double
stars are called binaries. The remaining 20%+ of stars are in multiple systems of three stars or more. Binaries and multiple stars are formed when
a condensing Bok globule or protostar splits into two or more parts. Binary stars may have similar components (Alpha Centauri A and B are both stars like our Sun), or they may be completely dissimilar, as with
Albireo (Beta Cygni, where a bright golden giant star is paired with a smaller bluish main sequence star).
The binary stars
Rigil Kentaurus (Foot
of the Centaur, or Alpha Centauri) at left, and Beta Cygni (Albireo), at right.
Rigel (Beta Orionis, left) is a binary star which is the seventh brightest star in the night sky.
Rigel A is a large
white supergiant which is 500 times brighter than its small companion, Rigel B, yet Rigel B is itself composed or a very close pair of Sun-type stars that orbit
each other in less than 10 days. Each of the two stars comprising Rigel B is brighter in absolute terms than Sirius (see above). The Rigel B pair orbit Rigel
A at the immense distance of 2200 Astronomical Units, equal to 12 light-days. (An Astronomical Unit or AU is the distance from the Earth to the Sun.) In the centre of
the Great Nebula in Orion (M42) is a multiple star known as the Trapezium (right). This star system has four bright white stars, two of which are binary stars with fainter
red companions, giving a total of six. The hazy background is caused by the cloud of fluorescing hydrogen comprising the nebula.
Acrux, the brightest star in the Southern Cross, is also known as Alpha Crucis. It is a close binary, circled by a third dwarf companion.
Alpha Centauri
(also known as
Rigil Kentaurus, Rigil Kent or Toliman) is a binary easily seen with the
smallest telescope. The components are both solar-type main sequence stars, one
of type G and the other, slightly cooler and fainter, of type K. Through a small
telescope this star system looks like a pair of distant but bright car
headlights. Alpha Centauri A and B take 80 years to complete an orbit, but a tiny third
component, the 11th magnitude red dwarf Proxima
takes about 1 million years to orbit the other two. It is about one tenth of a
light year from the bright pair and a little closer to us, hence its name. This
makes it our nearest interstellar neighbour, with a distance of 4.3 light
years. Red dwarfs are by far the most common type of star, but, being so small
and faint, none is visible to the unaided eye. Because they use up so little of
their energy, they are also the longest-lived of stars. The bigger a star is,
the faster it burns its nuclear fuel and the shorter its life.
Close-up of the star field around Proxima Centauri
Knowing the orbital period of the two brightest stars A and B, we can apply Kepler’s Third Law to find the distance they are apart. This tells us that Alpha Centauri A and B are about 2700 million kilometres apart or about 2.5 light hours. This makes them a little less than the distance apart of the Sun and Uranus (the orbital period of Uranus is 84 years, that of Alpha Centauri A and B is 80 years.)
Albireo (Beta Cygni) is sometimes described poetically as a large topaz with a small blue sapphire. It is one of the sky’s most beautiful objects. The stars are of classes G and B, making a wonderful colour contrast. It lies at a distance of 410 light years, 95 times further away than Alpha Centauri.
Binary stars may be widely spaced, as the two examples just mentioned, or so close that a telescope is struggling to separated them (Acrux, Castor, Antares, Sirius). Even closer double stars cannot be split by the telescope, but the spectroscope can disclose their true nature by revealing clues in the absorption lines in their spectra. These examples are called spectroscopic binaries. In a binary system, closer stars will have shorter periods for the stars to complete an orbit. Eta Cassiopeiae takes 480 years for the stars to circle each other. The binary with the shortest period is AM Canum Venaticorum, which takes only 17½ minutes.
Sometimes one star in a binary system will pass in front of the other one, partially blocking off its light. The total light output of the pair will be seen to vary, as regular as clockwork. These are called eclipsing binaries, and are a type of variable star, although the stars themselves usually do not vary.
Why are some constellations bright, while others are faint ?
If we look at ninety degrees to the plane, either straight up and out of the galaxy or straight down, we are looking through comparatively few stars and gas clouds and so can see out into deep space. These are the directions of the north and south galactic poles, and because we have a clear view in these directions to distant galaxies, these parts of the sky are called the intergalactic windows. The southern window is in the constellation Sculptor, not far from the star Fomalhaut. This window is well-placed for viewing this month, and many distant galaxies can be observed in this area of the sky. The northern window is between the constellations Virgo and Coma Berenices, roughly between the stars Denebola and Arcturus. It is below the horizon in the evenings this month.
Some of the fainter and apparently insignificant constellations are found around these windows, and their lack of bright stars, clusters and gas clouds presents us with the opportunity to look across the millions of light years of space to thousands of distant galaxies.
Some fainter constellations:
Between the two Dogs is the constellation Monoceros the Unicorn, undistinguished except for the presence of the remarkable Rosette Nebula. South of Orion is a small constellation, Lepus the Hare. Between Lepus and the star Canopus is the star group Columba the Dove. Between the zenith and the south-western horizon are a number of small, faint constellations, Phoenix, Hydrus, Reticulum, Indus and Pavo. Clustered around the South Celestial Pole are Dorado, Octans, Apus, Chamaeleon, Mensa and Volans.
Finding the South Celestial Pole:
The South Celestial Pole is that point in the southern sky around which the stars rotate in a clockwise direction. The Earth's axis is aimed exactly at this point. For an equatorially-mounted telescope, the polar axis of the mounting also needs to be aligned exactly to this point in the sky for accurate tracking to take place.
To find this point, first locate the Southern Cross. Project a line from the orange star at the top of the Cross (Gacrux) to the star at its base (Acrux) and continue straight on towards the south (to the left) for another four Cross lengths. This will locate the approximate spot. There is no bright star to mark the Pole, whereas in the northern hemisphere they have Polaris (the Pole Star) to mark fairly closely the North Celestial Pole.
Another way to locate the South Celestial Pole is to draw an imaginary straight line joining Beta Centauri in the south-west to Achernar in the south-east. Bisect this imaginary line to locate the pole.
Neither of these methods will work in Queensland during December evenings as both the Southern Cross and Beta Centauri are below the horizon in the evening. However, they will have risen by 1 am.
Interesting photographs of this area can be taken by using a camera on time exposure. Set the camera on a tripod pointing due south, and open the shutter for thirty minutes or more. The stars will seem to move during the exposure, being recorded on the film as short arcs of a circle. The arcs will be different colours, as the stars are. All the arcs will have a common centre of curvature, which is the south celestial pole.
A wide-angle view of trails around the South Celestial Pole, with Scorpius and Sagittarius at left, Crux and Centaurus at top, and Carina and False Cross at right.
Star trails between the South Celestial Pole and the southern horizon. All stars that do not pass below the horizon are circumpolar.
Star Clusters:
The two clusters in Taurus, the Pleiades and the Hyades, are known as Open Clusters or Galactic Clusters. The name 'open cluster' refers to the fact that the stars in the cluster are grouped together, but not as tightly as in globular clusters (see below). The stars appear to be loosely arranged, and this is partly due to the fact that the cluster is relatively close to us, i.e. within our galaxy, hence the alternate name, 'galactic cluster'. These clusters are generally formed from the condensation of gas in a nebula into stars, and some are relatively young.
The photograph below shows a typical open cluster, M7
*. It lies in the constellation Scorpius, just above the scorpion's sting. It lies in the direction of our galaxy's centre. The cluster itself is the group of white stars in the centre of the field. Its distance is about 380 parsecs or 1240 light years. M7 is not visible tonight.
Galactic Cluster M7 in Scorpius
Outside the plane of our galaxy, there is a halo of Globular Clusters. These are very old, dense clusters, containing perhaps several hundred thousand stars. These stars are closer to each other than is usual, and because of its great distance from us, a globular cluster gives the impression of a solid mass of faint stars. Many other galaxies also have a halo of globular clusters circling around them.
The largest and brightest globular cluster in the sky is
Although Omega Centauri is below the horizon in the evenings this month, there is another remarkable globular, second only to Omega, which is in a good position. Close to the SMC (see Two close galaxies below), binoculars can detect a fuzzy star. A telescope will reveal this faint glow as a magnificent globular cluster, lying at a distance of 5.8 kiloparsecs. Its light has taken almost 19 000 years to reach us. This is NGC 104, commonly known as 47 Tucanae. Some regard this cluster as being more spectacular than Omega Centauri, as it is more compact, and the faint stars twinkling in its core are very beautiful. This month, 47 Tucanae is best placed for viewing at 8 pm.
The globular cluster 47 Tucanae
Observers aiming their telescopes towards the SMC generally also look at the nearby 47 Tucanae, but there is another globular cluster nearby which is also worth a visit. This is NGC 362, which is less than half as bright as the other globular, but this is because it is more than twice as far away. Its distance is 12.6 kiloparsecs or 41 000 light years, so it is about one-fifth of the way from our galaxy to the SMC. Both NGC 104 and NGC 362 are always above the horizon for all parts of Australia south of the Tropic of Capricorn.
* M7: This number means that this galactic cluster in Scorpius is No. 7 in a list of 103 astronomical objects compiled and published in 1784 by Charles Messier. Charles was interested in the discovery of new comets, and his aim was to provide a list for observers of fuzzy nebulae and clusters which could easily be reported as comets by mistake. Messier's search for comets is now just a footnote to history, but his list of 103 objects is well known to all astronomers today, and has even been extended to 110 objects.
** NGC 5139: This number means that Omega Centauri is No. 5139 in the New General Catalogue of Non-stellar Astronomical Objects. This catalogue was first published in 1888 by J. L. E. Dreyer under the auspices of the Royal Astronomical Society, as his New General Catalogue of Nebulae and Clusters of Stars. As larger telescopes built early in the 20th century discovered fainter objects in space, and also dark, obscuring nebulae and dust clouds, the NGC was supplemented with the addition of the Index Catalogue (IC). Many non-stellar objects in the sky have therefore NGC numbers or IC numbers. For example, the famous Horsehead Nebula in Orion is catalogued as IC 434. The NGC was revised in 1973, and lists 7840 objects.
The recent explosion of discovery in astronomy has meant that more and more catalogues are being produced, but they tend to specialise in particular types of objects, rather than being all-encompassing, as the NGC / IC try to be. Some examples are the Planetary Nebulae Catalogue (PK) which lists 1455 nebulae, the Washington Catalogue of Double Stars (WDS) which lists 12 000 binaries, the General Catalogue of Variable Stars (GCVS) which lists 28 000 variables, and the Principal Galaxy Catalogue (PGC) which lists 73 000 galaxies. The largest modern catalogue is the Hubble Guide Star Catalogue (GSC) which was assembled to support the Hubble Space Telescope's need for guide stars when photographing sky objects. The GSC contains nearly 19 million stars brighter than magnitude 15.
Two close galaxies:
At 10 pm on December 1, two faint smudges of light may be seen high in the south. These are the two Clouds of Magellan, known to astronomers as the LMC (Large Magellanic Cloud) and the SMC (Small Magellanic Cloud). The LMC is to the left of the SMC, and is noticeably larger. They lie at distances of 190 000 light years for the LMC, and 200 000 light years for the SMC. They are about 60 000 light years apart. These dwarf galaxies circle our own much larger galaxy, the Milky Way. The LMC is slightly closer, but this does not account for its larger appearance. It really is larger than the SMC, and has developed as an under-sized barred spiral galaxy.
The Large Magellanic Cloud - the bright knot of gas to left of centre is the famous Tarantula Nebula (below)
These two Clouds are the closest galaxies to our own, but lie too far south to be seen by the large telescopes in Hawaii, California and Arizona. They are 15 times closer than the famous Andromeda and Triangulum galaxies in the northern half of the sky, and so can be observed in much clearer detail. Our great observatories in Australia, both radio and optical, have for many years been engaged in important research involving these, our nearest inter-galactic neighbours.
The Andromeda Galaxy and the President
of the United States:
In 1901, U.S.A. President William McKinley was assassinated and his Vice-President,
Teddy made the acquaintance of noted American naturalist, scientist, explorer and author
It is said that after dinner, Roosevelt, Beebe and their friends would step outside for cigars and lengthy discussions about world affairs. At the conclusion, they would look up at the starry sky. Roosevelt or Beebe would point out a small, faint smudge of light close to the Great Square of Pegasus and they would both recite, almost as a litany, something similar to the following:
"That is the Spiral Galaxy in Andromeda. It is as large as our Milky Way. It is one of a hundred million galaxies. It consists of one hundred million suns, many larger than our sun." The President would then turn to the others. "Now I think we are small enough," he would say. "Let's go to bed."
Whereas from the latitude of Washington D.C. the Andromeda Galaxy is visible for most of the year, from Australia it is so far north (41 degrees north declination) that it is only visible in the evenings during spring and early summer. For us, this magnificent galaxy is due north at 8 pm on December 1, about one handspan above the horizon.
The Great Galaxy in Andromeda, M31, photographed at Starfield Observatory with
an off-the-shelf digital camera on 16 November 2007.
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