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:
Times for transient sky phenomena are given using a 24 hour clock, i.e. 20:30 hrs = 8.30 pm. Times are in Australian Eastern Standard Time (AEST), which equals Universal Time (UT) + 10 hours. Daylight saving is not observed in
Queensland. Observers in other time zones will need to make their own corrections where appropriate. With conjunctions of the Moon, planets and stars, timings indicate the closest approach. Directions (north or south) are
approximate. The Moon’s diameter is given in arcminutes ( ’ ). The Moon is usually about 30’ or half a degree across. The 'limb' of the Moon is its edge as projected against the sky background.
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'. The best time to observe the Moon and planets is when they are culminating. 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 zodiacal constellation of Leo, the
Lion. It transits Leo
and crosses into Virgo, the Virgin on September 17.
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 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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Full Moon as seen from Earth Moon as seen from Artemis II, second day Moon as seen on fifth day
The craters marked with a capital letter are as follows: A =
Aristarchus; B = Byrgius A; C = Copernicus;
G = Grimaldi; H = Harding A; S = Schickard.
Earth by Moonlight:
A view of the North and South
Atlantic Oceans as photographed by Artemis
II Commander Reid Wiseman on April 3, 2026, after completion of the Trans-Lunar Injection burn. The continent of Africa is to the right, with the Sahara Desert showing up plainly as a cloud-free area. South America is to the left, under a
lot of cloud. The Mediterranean Sea is at 2 o’clock on the Earth’s limb, with Spain and Portugal clearly seen. The lights of Madrid, Barcelona, Lisbon and Porto are very bright. At the North Pole the Aurora Borealis can be seen as a
line of light above the horizon, with the Aurora Australis at the South Pole. The Sun is behind the Earth at 10 o’clock, as the side facing us is the night side, only illuminated by the
almost Full Moon. The day side of the Earth is hidden
from us, except for a thin sliver of light from 9 o’clock to 1 o’clock. Note the lights of towns. A few stars are showing. Near the upper-left corner is the bright planet Venus.
Third day of Artemis II:
NASA Astronaut and Artemis II Commander Reid Wiseman looks out of
one of Orion’s windows and sees all of Australia in one view. The spacecraft
was directly over Queensland that morning. The Gulf of
Carpentaria and Cape York Peninsula are near the centre of the image. The east
coasts of New Zealand’s North and South Islands are mostly clear of cloud, and
the east coast of China is at top left. The distant gibbous Moon can be seen in
the top right corner of the window. Commander Wiseman’s profile can be seen in
shadow at the top of the picture.
New Moon:
First Quarter:
Full Moon:
New Moon:
First Quarter:
Full Moon:
Lunar Orbital Elements:
Moon at 8 days after New, as on
September 20
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 an ancient walled plain called Vendelinus, which lies near the Moon's eastern limb.
Vendelinus (147 kms) is very old crater plain that was formed between
3.92 and 4.55 billion years ago – the Pre-Nectarian period. Image taken at 5:20
pm on 14 September 2018. 
The area containing the ancient crater Vendelinus is located inside the rectangle.
Click
September 2: Venus 1.5º south of
the star Spica (Alpha Virginis, mv = 0.9) at 22:21 hrs
September 3: Moon 1.2º north of the star Alcyone (Eta Tauri, mv = 2.85) at 20:58 hrs
September 4: Moon 6.1º north of Uranus at 8:23 hrs
September 4: Venus at aphelion at 22:22 hrs ( diameter - 31.6" )
September 5: Limb of Moon 1 arcminute south of the star Elnath (Beta Tauri, mv = 1.65) at 13.53 hrs (possible grazing occultation)
September 6: Moon 1.9º north of the star Mebsuta (Epsilon Geminorum, mv = 3.06) at 18:15 hrs
September 7: Moon 3.7º north of Mars at 4:16 hrs
September 9: Moon 1.4º north of Jupiter at 3:25 hrs
September 10: Limb of Moon 1 arcminute north of the star Regulus (Alpha Leonis, mv = 1.33) at 4:49 hrs (possible grazing occultation)
September 11: Uranus at western stationary point at 2:50 hrs ( diameter = 3.6" )
September 12: Moon 3.3º south of Mercury at 15:49 hrs
September 14: Limb of Moon 30 arcminutes north of Venus at 22:56 hrs
September 17: Limb of Moon 40 arcminutes north of the star 6 Pi Scorpii (mv = 2.89) at 8:07 hrs
September 17: Limb of Moon 48 arcminutes south of the star Alnyat (Sigma Scorpii, mv = 2.9) at 20:35 hrs
September 17: Moon occults the star Antares (Alpha Scorpii, mv = 0.88) between 00:09 and 00:50 hrs
September 18: Moon 1.4º north of the star 23 Tau Scorpii (mv = 2.82) at 3:05 hrs
September 20: Limb of Moon 5 arcminutes south of the of the star Nunki (Sigma Sagittarii, mv = 2.02) at 15:30 hrs
September 22: Moon 2.5º north of Pluto at 8:56 hrs
September 26: Neptune at opposition at 11:23 hrs ( diameter = 2.3" )
September 26: Mercury 53 arcminutes north of Spica (Alpha Virginis, mv = 0.9) at 17:27 hrs
September 27: Moon 5º north of Neptune at 2:24 hrs
September 27: Moon 6.2º north of Saturn at 16:35 hrs
September 27: Mercury at aphelion at 18:49 hrs ( diameter = 5.5" )
October 1: Moon of the star Alcyone (Eta Tauri, mv = 2.85) at 5:27 hrs
The Planets for this month:
Mercury:
Venus:
This, the brightest planet, was a 'Morning Star' in the eastern sky until January 7 (eight months ago), and then returned to the evening sky. It became noticeable as an 'Evening Star' in the western twilight sky in April, and has
been a dominant feature ever since then. Venus is currently in the constellation Virgo, and can be found in the north-western sky after sunset. It will pass through inferior conjunction (between the Earth and the Sun) on October 24. At that time, Venus will
leave our western twilight sky and will reappear in the pre-dawn eastern sky as a 'Morning Star'. It will cross into Libra on December 14.
(The coloured fringes to the second, third and fifth 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 first and fourth photographs, which were taken when Venus was at its greatest elongation from the Sun.)
Click
here
for a photographic animation showing the Venusian phases. Venus is always
far brighter than anything else in the sky except for the Sun and Moon. For most of 2023, Venus appeared as an 'Evening Star' in the western twilight sky, but in August 2023 it moved to the pre-dawn eastern sky to be a 'Morning Star'. It returned to the
western evening sky as an 'Evening Star' in June 2024, and went back to the morning sky in March 2025. It is now approaching superior conjunction on January 7 and is too
close to the Sun to be observed. It will reappear in the western twilight sky as an 'Evening Star' again in March. 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.
Mars: When Mars was at opposition on January 16, 2025 (last year), its diameter was at a maximum of 14.5 arcseconds and its brightness was also a maximum of magnitude -1.38. This was because Mars was at
its closest approach to Earth, and so looked big and bright. Our speeding Earth, travelling much faster than Mars, has left the red planet far behind, and it now appears small and faint. By
Christmas 2025, its diameter
had diminished to 4 arcseconds, and its magnitude had fallen to 1.41. When it
reached conjunction (behind the Sun) last January, its diameter had shrunk to 3.9 arcseconds and its magnitude to 1.1. 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.
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 %.
Jupiter:
Jupiter moves much slower than Mercury, Venus and Mars, which have much smaller orbits. It entered the constellation Gemini on June 12 last year, and dawdled its way through Gemini until it crossed into Cancer on June 22 last. Jupiter was in conjunction with the Sun on July 29, and is now in the pre-dawn eastern sky. This month it can be glimpsed close to the east-north-eastern horizon, where it rises at 4:48 am on September 1. Observers will need a clear horizon, e.g. the Pacific Ocean. By the end of September, Jupiter will be rising at 3:13 am.
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 period of only 1 hour 27 minutes 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 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. Europa's transit began at 10:11 pm, and its shadow touched Jupiter's cloud tops almost simultaneously. Europa was fully in transit by 10:15 pm. The second photograph was taken two minutes later at 10:17 pm, with the Great Red Spot heading towards Jupiter's western limb.
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 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.
Saturn:
The ringed planet was in conjunction with the Sun on March 25 last. On July 6 it passed through western quadrature (rising at midnight) and is now approaching opposition on October 4, when it will rise above the eastern horizon as the Sun sets below the western horizon. From now until next February, unique Saturn will be available to amaze any observer with a telescope. The fabulous ring system is visible with magnifications above 30x. It begins the month in the constellation of Pisces the Fishes, but as it is retrograding westwards, it crosses into Cetus, the Whale on September 7. The retrograde motion continues until December 11, when Saturn reaches a stationary point and then heads back east, when it will return to Pisces, the Fishes on February 24, 2027. On September 1, Saturn will be found at 9:30 pm, one handspan above the east-north-eastern horizon. The almost Full Moon will be one third of a handspan (6 degrees) to the left of Saturn on September 27.Last year, the angle of Saturn's Rings reduced until they were almost edge-on. They were at their thinnest in last March and November. From now on, they will become easier to observe until they are wide-open in 2032. For the past 14 years, our view of Saturn has been mainly of that planet's northern hemisphere. For the next 14 years, the main hemisphere we will be observing will be the southern. At present, with the rings edge-on, we can see both hemispheres at once. Simultaneously, the various moons of Saturn are seen lining up as in the image below.
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.
Saturn with its Rings wide open on July 2, 2017. The shadow of its globe can just be seen on the far side of the Ring system. There are three main concentric rings: Ring A is the outermost, and is separated from the brighter Ring B by a dark gap known as the Cassini Division, which is 4800 kilometres wide, enough to drop Australia through. Ring A also has a gap inside it, but it is much thinner. Called the 'Encke Gap', it is only 325 kilometres wide and can be seen in the image above. The innermost parts of Ring B are not as bright as its outermost parts. Inside Ring B is the faint Ring C, almost invisible but noticeable where it passes in front of the bright planet as a dusky band. Spacecraft visiting Saturn have shown that there are at least four more Rings, too faint and tenuous to be observable from Earth, and some Ringlets. Some of these extend from the inner edge of Ring C to Saturn's cloudtops. The Rings are not solid, but are made up of countless small particles, 99.9% water ice with some rocky material, all orbiting Saturn at different distances and speeds. The bulk of the particles range in size from dust grains to car-sized chunks. At bottom centre, the southern hemisphere of the planet can be seen showing through the gap of the Cassini Division. The ring system extends from 7000 to 80 000 kilometres above Saturn's equator, but its thickness varies from only 10 metres to 1 kilometre. The globe of Saturn has a diameter at its equator of 120 536 kilometres. Being made up of 96% hydrogen and 3% helium, it is a gas giant, although it has a small, rocky core. There are numerous cloud bands visible.
The photograph above was taken when Saturn was close to opposition, with the Earth between Saturn and the Sun. At that time, the shadow of Saturn's globe upon the Ring system was directly behind the planet and hardly visible. The photograph below was taken at 7:14 pm on September 09, 2018, when Saturn was near eastern quadrature. At such a time, the angle from the Sun to Saturn and back to the Earth is near its maximum, making the shadow fall at an angle across the Rings as seen from Earth. It may be seen falling across the far side of the Ring to the left side of the globe.
This photograph was taken on September 9, 2018 at 7:14 pm, when Saturn was just past eastern quadrature. A newer camera was used. On that night, the Sun was near its maximum angle from a direct line between Saturn and the Earth. This meant that the shadow of the planet fell on the Rings, not directly behind the planet, but offset to the left, where it is easily seen. The southern hemisphere of Saturn can be faintly seen through the gap in the near side of the Ring System (the Cassini Division).
The photograph above was taken at 8:17 pm on November 03, 2022, when Saturn was again near eastern quadrature. The shadow of the planet once again falls across the far side of the rings, but in the intervening four years the angle of the rings as seen from Earth has been greatly reduced. The shadow of Ring B across the globe of Saturn is much darker from this angle.
This photograph was taken at 8:45 pm on November 4, 2024, 57 days after opposition. Although the seeing was only 3 on the Antoniadi Scale, this image was secured as it shows Saturn's largest Moon, Titan, in the same field of view as its parent planet. Such a picture is only possible when the Rings are close to edge-on. Titan has a diameter of 5150 kilometres, and is the second largest moon in the Solar System (Jupiter's Ganymede is the largest, and our Moon is the third largest). Titan has an atmosphere of nitrogen, methane and hydrogen, and has lakes and seas of liquid methane.
Uranus:
This ice giant planet shines at about magnitude 5.6, so a pair of binoculars or a small telescope is required to observe it. Uranus is currently in the constellation of Taurus, and will pass through opposition with the Sun on November 26. On May 23 Uranus passed through conjunction with the Sun and has now moved to the pre-dawn morning sky.
Neptune:
The icy blue planet reached conjunction with the Sun on March 22. It is now in the eastern pre-dawn sky, in the constellation Pisces. It reached opposition (rising at sunset) on September 26. The Full
Neptune, photographed from Nambour on October 31, 2008
Pluto:
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.
Planetary Alignments:
The movements of planets including their alignments and close-up images can be watched using the freeware Stellarium .
Meteor Showers:
Use
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.
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:
Comet 12P Pons-Brooks, photographed from Nambour at 6:40 pm on June 7, 2024.
The same comet, photographed the following night, showing its rapid movement
through the constellation Taurus.
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.
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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, the Eta Carinae Nebula is not well-placed for viewing, as at 6:30 pm on September 1 it is only a handspan above the south-south-western horizon.
The Stars and Constellations for this month:
These descriptions of the night sky are for 9 pm on September 1 and 7 pm on September 30. They start at the western horizon.
Only Venus will be visible at these times, although Saturn will be observable later.
The Ring Nebula was ejected from the central Sun-sized star towards the end of its life, as powerful stellar winds blew away the outer layers of its atmosphere to form a ring.
Due north is the constellation of Cygnus, the Swan. Cygnus is also known as the Northern Cross, but to us in the southern hemisphere it appears this month upside-down and tilted to the left. The star at the bottom of the Cross, or the
highest one above the horizon as we see it tonight, is a beautiful double star or binary, called Albireo (see below). Whereas most binaries are a pair of
similar stars, there are many in which the two stars are very different, such as brilliant Sirius the Dog Star with its tiny white dwarf companion known as 'The Pup'. Albireo's two components have a marked colour contrast, the brighter star being a golden
yellow, and the fainter companion being a vivid electric blue. It is a wonderful object to view with a small telescope. At the top of the Northern
Cross (which is the star closest to the horizon as we see it) is the bright first magnitude star Deneb, or Alpha Cygni. Deneb is a white star, and is the nineteenth brightest in the sky. It will be due north at about
8:50 pm at mid-month. Its name means 'The Tail', as it marks the tail of the Swan. Deneb, although bright, is one of the most distant individual stars we can see with the unaided eye. It lies at a distance of about 2600 light years. This means
that the light entering your eye tonight actually left Deneb in the 6th century BC, about the time when the world's first 'scientist', Thales of Miletus, was studying the
night sky from ancient Ionia, part of today's Turkey. Everything you see in the night sky is a view into the past, due to the finite speed of light.
Vega is the brightest star at centre left, with the stars of Lyra to its right. Deneb is the brightest star near the bottom edge. Cygnus, or the 'Northern Cross', stretches up vertically from Deneb to Albireo, above centre. We see the Northern Cross upside-down. The three bright stars of Aquila form a line at upper right, with Altair, the brightest, being the middle one. The small diamond-shaped constellation of Delphinus, the Dolphin, is above centre-right.
In the north-east, the Great Square of Pegasus has cleared the horizon, and is standing on one corner. It is very large, each side being around 15 degrees long. It is about as large as a fist held at arm's length. The Great Square is remarkable for having few naked-eye stars within it.
The names of the four stars marking the corners of the Square (starting at the uppermost one and moving in a clockwise direction) are Markab,
Algenib, Alpheratz and Scheat. Although these four stars are known as the Great Square of Pegasus, only three are actually in the constellation of
Pegasus, the Winged Horse. In point of fact, Alpheratz is the brightest star of the constellation
Andromeda, the Chained Maiden. A little less than a handspan to the south (right) of the star Algenib in Pegasus is the ringed planet Saturn. It is the brightest object in this eastern part of the sky. Its ring system is almost edge-on this
year, so its family of satellites tend to line up as mentioned above. At the beginning of September, Saturn will rise above the eastern horizon at close to 8 pm. If your horizon is not flat but obscured by hills or buildings, you may have to wait for
half-an-hour or so before Saturn comes into view. Although the ringed planet is much fainter than normal this year, due to the Ring System being extremely thin and faint, Saturn is still the brightest object in that part of the sky.
In the east, a mv 2.2 star is about two handspans above the horizon. This is Beta Ceti, the brightest ordinary star in the constellation of Cetus, the Whale. Its common name is Diphda, and it has a yellowish-orange colour. By rights, the star Menkar or Alpha Ceti should be brighter, but Menkar is actually more than half a magnitude fainter than Diphda. Menkar does not rise at the beginning of September until about 10:30 pm.
Cetus is a large constellation, running vertically above the eastern horizon tonight, and to the unaided eye it appears unremarkable. But it does contain a most interesting star, which was noticed in medieval times. Johannes Hevelius named it Mira, the Wonderful (see below). Between Cetus and Pegasus is the faint zodiacal constellation of Pisces, the Fishes, which contains a faint ring of five stars known as the 'Circlet'.
One and a half handspans above (west of) Diphda is a bright, white first magnitude star in the faint constellation Piscis Austrinus, the Southern Fish, called Fomalhaut. The first planet to be photographed circling another star is embedded in the dust ring surrounding Fomalhaut. Above Fomalhaut and to the right is a large, flattened triangle of stars, Grus, the Crane.
About 27 degrees (one and a half handspans) above the south-eastern horizon is Achernar, which is the ninth brightest star. From locations south of Newcastle, Achernar is circumpolar, i.e. it never dips below the horizon but is
always in the sky. A hot blue-white star, Achernar is the main star in the constellation Eridanus the River, which winds its way from Achernar all the way to Orion (which won't rise tonight until 1:30 am).
Achernar's visual magnitude ( mv) is 0.45, and it is a hot blue-white star of B3 spectral type. The width of the field is 24 arcminutes and the faintest stars are mv 15.
To the left and higher than Achernar, the faint constellation of Phoenix may be seen. Its brightest star is Ankaa or Alpha Phoenicis, a mv 2.39 star which is halfway between Diphda and Achernar, but slightly above. One-and-a-half degrees to the right of Ankaa is a fourth magnitude star, Kappa Phoenicis.
The Large Magellanic Cloud (LMC) is a faint, diffuse glowing patch low to the south-south-eastern horizon, only 11 degrees up from the latitude of Nambour. The Small Magellanic Cloud (SMC) is about a handspan above it, and to the right of Achernar. Both of these Clouds appear as faint smudges of light, but in reality they are dwarf galaxies containing millions of stars.
From Nambour's latitude, these two clouds never set. Each day they circle the South Celestial Pole, which is a point in our sky 26.6 degrees above the horizon's due south point. Objects in the sky that never set are called 'circumpolar'. The LMC and SMC are described below.
The Southern Cross (Crux) is setting low in the south-south-west, with the two Pointers Alpha and Beta Centauri vertically above it. Alternative names for these two Pointers are Rigel Kentaurus and Hadar.
The two pointers are eight degrees apart. Alpha is the one further away from Crux. Whereas Alpha Centauri (see below) is the nearest star system to our Sun, only 4.2 light years distant, Beta Centauri is eighty times further away. Beta Centauri must have an
absolute magnitude much greater than Alpha, in order to appear nearly as bright.
At top - the two Pointers, Alpha and Beta Centauri. Centre - Crux (Southern Cross) with the
dark cloud of dust known as the Coalsack above Alpha Crucis. Bottom - star clusters in the Milky Way with the Eta Carinae nebula near the lower edge.
Just above the second brightest star in the Cross (Beta Crucis) is a brilliant small
star cluster known as 'Herschel's Jewel Box'. In the centre of the cluster is a red supergiant star, which is just passing through.
Beta Crucis (left) and the Jewel Box cluster
Adjoining Crux on its left-hand side is a small, fainter quadrilateral of stars, Musca, the Fly. Out of all the 88 constellations, it is the only insect. To the upper left of Alpha Centauri is a (roughly) equilateral triangle of 4th magnitude stars. This is the constellation Triangulum Australe, the Southern Triangle.
Six of the zodiacal constellations are overhead tonight. The faint constellation of Libra is low in the west. Starting from Libra and looking above it, we see the bright constellation of Scorpius, the Scorpion, with the red-supergiant star Antares marking the Scorpion's heart. This famous zodiacal constellation is like a large letter 'S', and, unlike most constellations, is easy to recognise as the shape of a scorpion. Antares is the brightest star in Scorpius, and is a red type M supergiant of magnitude 0.9. Antares is the fifteenth brightest star.
Another very distant star that is easily seen with the unaided eye is Zeta Scorpii, which, like Deneb, also lies at a distance of 2600 light years. It can be found by following the line of the tail of Scorpius, and is the fourth star from Antares, heading south. It lies at the point where the tail takes a sharp turn east. Actually, there are two stars there, Zeta 1 and Zeta 2. Zeta 1, the distant star, is a blue-white B1 type, while Zeta 2 is an orange K type, and at a distance of only 151 light years is 17 times closer.
Arching across the sky from the Southern Cross (low in the south-south-west) to Cygnus (low in the north) is a faint band of light called the Milky Way. Once thought to be a river of glowing gas, Galileo looked at it through his first telescope in 1609 and reported, "It is nothing but innumerable stars!" We now know that it is our own galaxy seen edge-on, for we are inside it. As well as billions of stars, it does contain millions of glowing gas clouds and dark clouds too. The Milky Way arches directly overhead at 8:40 pm on September 1, with the centre of the galaxy near our zenith. It runs north from the Southern Cross through Centaurus, Ara, Scorpius, Sagittarius, Scutum and Aquila to Cygnus (the Northern Cross).
About a handspan south-west of the zenith is the brightest patch in the Milky Way, which is the centre of our Galaxy seen behind the foreground stars of the constellation of Sagittarius
the Archer. Sagittarius teems with stars, glowing nebulae and dust clouds. East of Sagittarius is the faint constellation
Capricornus, the Sea-Goat (near the zenith). This month Capricornus is host to the very faint planet Pluto, which may be found in its western end. Adjoining Sagittarius to the south,
there is a beautiful curve of very faint stars. This is Corona Australis, the Southern Crown, and it is very elegant and delicate. The brightest star in this
constellation has a magnitude of only 4.1.
Zeta 1 Scorpii is the upper star in the bright group of three at centre right. Zeta 2 is below it.
Between Scorpius and Centaurus is an interesting constellation composed of mainly third magnitude stars, Lupus, the Wolf. Midway between Triangulum Australe and
Scorpius is an asterism like a small, elongated triangle. This is Ara, the Altar. High in the north-west, between Scorpius and Hercules, are two large
but faint constellations, Serpens, the Serpent, and Ophiuchus, the Serpent Bearer. They have no stars brighter than magnitude 2. Ophiuchus is over two
handspans across in all directions, and may be found at 7 pm at mid-month about two handspans north-west of the zenith.
The constellations surrounding the Southern Cross
The body of Scorpius is at right, with the two stars in the Sting to the left, at centre right of picture. The red supergiant star Antares appears close to the top right corner. The stars in the left half of the picture are in Sagittarius. The well-known 'Teapot' shape may be seen. Near the lower left margin is a graceful curve of fourth magnitude stars, Corona Australis, the Southern Crown. To see this view, stand facing south and look directly overhead at 7.00 pm at the beginning of the month. Later in the month, the stars will be further to the west at the same time.
Antares, a red supergiant star
The star which we call Antares is a binary system. It is dominated by the great red supergiant Antares A which, if it swapped places with our Sun, would enclose all the planets out to Jupiter inside itself. Antares A is accompanied by the much smaller Antares B at a distance of between 224 and 529 AU - the estimates vary. (One AU or Astronomical Unit is the distance of the Earth from the Sun, or about 150 million kilometres or 8.3 light minutes.) Antares B is a bluish-white companion, which, although it is dwarfed by its huge primary, is actually a main sequence star of type B2.5V, itself substantially larger and hotter than our Sun. Antares B is difficult to observe as it is less than three arcseconds from Antares A and is swamped in the glare of its brilliant neighbour. It can be seen in the picture above, at position angle 277 degrees (almost due west or to the left) of Antares A. Seeing at the time was about IV on the Antoniadi Scale, or in other words below fair. Image acquired at Starfield Observatory in Nambour on July 1, 2017.
The Trifid Nebula, M20, in Sagittarius, is composed of a reflection nebula (blue), an emission nebula (pink), and dark lanes of dust.
The centre of our galaxy is teeming with stars, and would be bright enough to turn night into day, were it not for intervening dust and molecular clouds. This dark cloud is known as 'The Snake'. A satellite passed through the field of view at right.
Culminating high in the north, between
Capricornus and Albireo, is the constellation of Aquila, the Eagle (see picture). The centre of this constellation is marked by a short line of three stars, of
which the central star is the brightest. These stars, from north to south, are Tarazed, Altair and Alshain, and they indicate the Eagle's body
and wingspan. A handspan to the east of bright, first magnitude Altair is a faint but easily recognised diamond-shaped group of stars, Delphinus the Dolphin. The line of the ecliptic along which the Sun, Moon and planets travel, passes through the following constellations this month: Libra, Scorpius, Sagittarius, Capricornus, Aquarius and Pisces. The aborigines had a large constellation which is visible tonight, the Emu. The Emu is not defined by stars, but by dark dust clouds along the Milky Way. The Coalsack forms the Emu's head, with the faint sixth magnitude star in
the Coalsack, its eye. The Emu's neck is a dark lane of dust running east through the two Pointers, to Scorpius. The whole constellation Scorpius forms the Emu's
body. The Emu is sitting, waiting for its eggs to hatch. The eggs are the large glowing star clouds in Sagittarius, the centre of our galaxy. More photographs of the amazing sights visible in our sky this month are found in our
Gallery. 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
Mira, the Wonderful
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. In the mid-17th century, the astronomer Johannes Hevelius watched the star fade away
during the year until it disappeared, and then it slowly reappeared again. Its not surprising that he 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 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
of 2023. It then brightened rapidly, and reached that year's maximum on June 13. Then it faded again, and reached its minimum brightness early in 2024. A small telescope was needed in order to find it. By Easter it brightened again
and reached maximum brightness on May 10, 2024. It was then a morning sky object, rising on that day at 4:48 am. Each of these cycles lasts 332 days. This year, it reached a maximum on March 3 last. Mira is now fading and will reach a minimum around
this coming November. It will then brighten again and will reach its next maximum on January 29, 2027. On that date it will rise at 11:26 am and set at 23:44 pm. At 8 pm it will be two-and-a-half handspans above the west-north-western horizon.
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.
Mira near minimum, 26 September 2008 Mira near maximum, 22 December 2008
Astronomers using a NASA space telescope, the Galaxy Evolution Explorer, have spotted an amazingly long comet-like tail behind
Mira as the star streaks through space. 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.
Double and multiple 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).
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 Centauri, 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
shorter its life.
Close-up of the star field around Proxima Centauri
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. Albireo rises in the north-east at about 8 pm on July 1.
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.
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.)
A glowing band of light crossing the sky is especially noticeable during the winter months. This glow is the light of millions of faint stars combined with that coming from glowing gas clouds called nebulae. It is concentrated along the plane of our galaxy,
and this month it is seen crossing the sky, starting from the south-west and passing through Carina to Crux, Centaurus, Scorpius and Sagittarius to Cygnus in the north-east.
The plane of our galaxy from Scutum (at left) through Sagittarius and
Scorpius (centre) to Centaurus and Crux (right). The Eta Carinae nebula is at the right margin, below centre. The Coalsack is clearly visible, and the dark dust lanes can
be seen. Taken with an ultra-wide-angle lens.
It is rewarding to scan along this band with a pair of binoculars, looking for star clusters and emission nebulae. Dust lanes along the plane of the Milky Way appear to split it in
two in some parts of the sky. One of these lanes can be easily seen, starting near Alpha Centauri and heading towards Antares. At 8.00 pm in mid-August, the Milky Way crosses the zenith, almost dividing the sky in two. It runs from south-west
to north-east, and the very centre of our galaxy passes directly overhead.
The centre of our galaxy. The constellations partly visible here are Sagittarius (left), Ophiuchus (above centre) and Scorpius (at right). The planet Jupiter is the bright object below centre left. This is a normal unaided-eye view.
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 top of the Cross down through its base and continue straight on towards the south 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 high in the south-west to Achernar low in the south-east. Both stars will be at about the same elevation above the horizon at 10.00 pm in the middle of August. Find the midpoint of this line to locate the pole. To find due south on the ground, first find the South Celestial Pole as described above, and from that point drop vertically to the horizon.
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 move during the exposure, being recorded on the film as short arcs of a circle. The arcs will be different
colours, like the stars are. All the arcs will have a common centre of curvature, which is the south celestial pole.
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 below the scorpion's sting, 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 well-placed for viewing 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
The globular cluster Omega Centauri
The central core of Omega Centauri
There is another remarkable globular, second only to Omega Centauri. Close to the SMC (see 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, both Omega Centauri and 47 Tucanae are not at their best positions for viewing, but are observable.
The globular cluster 47 Tucanae or NGC 104.
The globular cluster NGC 6752 in the constellation Pavo. * M42: This number means that the Great Nebula in Orion is No. 42 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
The Large Magellanic Cloud - the
bright knot of gas to left of centre is the famous Tarantula Nebula.
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.
Very low in the south, two faint smudges of light may be seen. 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 above 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.
Astronomers have recently reported the largest star yet found, claimed to have 300 times the mass of the Sun, located in a cluster of stars embedded in the Tarantula Nebula (above). Such a huge star would be close to the Eddington Limit, and would have a short lifespan measured in only a couple of million years.
This month, the SMC can be observed from a dark site from about 8 pm, but the LMC will reach a similar position in the sky five hours later. Both of these Clouds are about two handspans above the southern horizon at 4:30 am. They 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, 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.
Astronomy Picture of the Day
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