September 2024
September 2024
Updated: 1 September 2024
Welcome to the night skies of
Spring,
featuring
Carina, Crux, Centaurus, Virgo, Boötes, Hercules, Scorpius,
Ophiuchus, Sagittarius, Capricornus, Aquarius, Grus, Aquila, Lyra 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:
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'.
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 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 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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PARTIAL, SEPTEMBER 18: This partial eclipse will occur around midday, when the Moon will be below our horizon. No part of it will be visible from Australia. The next total lunar eclipses visible from the Sunshine Coast will occur on September 8, 2025 (in the hours before dawn), and on March 3, 2026 (perfectly timed to occur between 6:44 pm and 12:23 am).
ANNULAR, OCTOBER 3 (AEST): This total eclipse of the Sun will not be visible from Australia. It will begin in the northern Pacific Ocean and will head south-east towards Cape Horn in South America. The only places where the eclipse path will cross land are in the far south of Chile and Argentina, and the Falkland Islands.
The next total solar eclipse visible from parts of Australia will occur at 12:56 pm on July 22, 2028, the eclipse track running from Wyndham in Western Australia through Alice Springs to Birdsville and then Sydney, before crossing the Tasman Sea to Dunedin in New Zealand's South Island.
Lunar Phases:
New Moon:
October
3
04:50 hrs diameter =
29.4' Lunation #1259 begins - Annular Solar Eclipse
Last Quarter:
October
24
18:04 hrs
diameter = 30.6'
Lunar Orbital Elements:
September
October 2
Moon at 8 days after
New, as on September 12.
October 16
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 two battered crater plains near the Moon's western limb, Piazzi and Lagrange.
The western (upper) half of this image shows linear damage caused to the
moonscape by a surge of debris (rocks, boulders, flying mountains etc.) thrown
out radially when an asteroid struck the Moon between 3.2 and 3.85 billion years
ago. This created a huge basin known as the Mare Orientale (Eastern Sea),
which is 300 kilometres across its flat centre and 960 kilometres across from
rim to rim. This feature is on the far side of the Moon (just), but portions of
it are occasionally visible from Earth if the libration and phase are
favourable. It is described and pictured in item #6
This area is close to the Moon's south-west limb, and is centred near the 28
kilometre diameter crater Piazzi C which lies at latitude 37
Lagrange is a large walled plain with a diameter of 160 kilometres. The Mare Orientale impact produced great quantities of ejected debris in both molten and solid forms, which was deposited over the whole area, covering much of
Lagrange's floor. Some of this material gives the appearance of hills or even mountains. Where molten material has pooled on the floor, it is reasonably flat but the surface shows long, parallel, shallow grooves which are similar in cause
and appearance to the Imbrium Sculpture described in items
Lagrange
To find information about Joseph-Louis Lagrange, click on
September 1: Uranus at western
stationary point at 22:51 hrs (diameter = 3.6")
September 1: Moon 4.7º north of
Mercury at 22:49 hrs
September 5: Limb of Moon 45
arcminutes south of Venus at 21:09 hrs
September 5: Mercury at greatest
elongation west (18º 03') at 12:17 hrs (diameter = 7.2")
September 7: Limb of Moon 48
arcminutes north of the star Spica (Alpha Virginis, mv = 0.98) at 3:26
hrs
September 8: Saturn at opposition at
14:17 hrs (diameter = 19.1")
September 9: Mercury of the star
Regulus (Alpha Leonis, mv = 1.33) at 18:36 hrs
September 10: Mercury at perihelion at 1:07 hrs (diameter = 6.4")
September 10: Moon 1.7º south of the star Dschubba
(Delta Scorpii, mv = 1.86) at 8:11 hrs
September 10: Limb of Moon 19 arcminutes south of
the star Alniyat (Sigma Scorpii, mv = 2.9) at 20:47 hrs
September 11: Moon occults the star Antares (Alpha
Scorpii, mv= 0.88) between 0:28 and 0:50 hrs
September 11: Moon 2º north of the star 23 Tau
Scorpii (mv= 2.82) at 3:21 hrs
September 12: Moon 1.9º north of the star Alnasl
(Gamma Sagittarii, mv = 2.98) at 14:03 hrs
September 12: Jupiter at western quadrature at
20:50 hrs (diameter = 39.8")
September 12: Moon 1.4º north of the star Kaus
Media (Delta Sagittarii, mv = 2.72) at 23:11 hrs
September 13: Moon 1.2º south of the star Nunki
(Sigma Sagittarii, mv = 2.02) at 10:31 hrs
September 13: Moon 2.3º north of the star Ascella
(Zeta Sagittarii, mv = 2.6) at 12:28 hrs
September 14: Mars 59 arcminutes north of the star
Tejat Posterior (Mu Geminorum, mv = 2.87) at 18:39 hrs
September 14: Moon 1.6º south of Pluto at 16:15
hrs
September 16: Limb of Moon 4 arcminutes south of
the star Deneb Algedi (Delta Capricorni, mv = 2.85) at 9:32 hrs
September 17: Neptune 1.8º north of the star 27
Piscium (mv = 4.9) at 17:34 hrs
September 17: Almost Full Moon occults Saturn and
its Rings between 18:19 and 18:56 hrs (Moonrise at Nambour is at 16:53 hrs)
September 18: Limb of Moon 22 arcminutes north of
Neptune at 15:46 hrs
September 18: Venus 2.4º north of the star Spica
(Alpha Virginis, mv = 0.98) at 18:42 hrs
September 21: Neptune at opposition at 9:54 hrs
(diameter = 2.3")
September 22: Moon 4.4º south of Uranus at 15:25
hrs
September 22: Moon occults stars in the Pleiades
star cluster between 18:28 and 20:20 hrs (see below)
September 23: Mars 1.7º south of the star Mebsuta
(27 Epsilon Geminorum, mv = 3.06) at 1:43 hrs
September 23: Jupiter 18 arcminutes north of the
star 109 Tauri (mv = 4.96) at 18:42 hrs
September 24: Moon 6.4º north of Jupiter at 9:42
hrs
September 24: Moon occults the star Elnath (Beta
Tauri, mv = 1.65) between 13:01 and 13:46 hrs
September 25: Moon 4.8º north of Mars at 22:10 hrs
September 26: Moon 1.5º south of the star Pollux
(Beta Geminorum, mv = 1.15) at 20:06 hrs
October 1:
Mercury in superior conjunction at 6:56 hrs (diameter = 4.8" )
October 3:
Annular Solar Eclipse at 4:45 am - not visible from Australia
October 3:
Limb of Moon 38 arcminutes south of Mercury at 6:59 hrs hrs
October 4:
Moon 1º north of the star Spica (Alpha Virginis, mv = 0.98) at 8:32
hrs
October 5: Venus 50 arcminutes south of the star Zuben Elgenubi (Alpha Librae, mv = 2.72) at 4:41 hrs
October 6:
Moon 2.1º south of Venus at 3:47 hrs
October 7:
Moon 1º north of the star Pi Scorpii (mv= 2.82) at 16:54 hrs
October 8:
Moon occults the star Alniyat (Sigma Scorpii, mv = 2.9) between 1:51
and 2:32 hrs
October 8:
Limb of Moon 20 arcminutes north of the star Antares (Alpha Scorpii, mv =
0.88) at 4:57 hrs
October 8:
Moon 2º north of the star 23 Tau Scorpii (mv= 2.82) at 7:01 hrs
October 9:
Jupiter at western stationary point at 16:53 hrs (diameter = 43.3" )
October 10: Moon 2.2º
north of the star Alnasl (Gamma Sagittarii, mv = 2.98) at 00:11 hrs
October 10: Moon 1.9º
north of the star Kaus Media (Delta Sagittarii, mv = 2.72) at 4:55 hrs
October 10: Mercury 2.4º
north of the star Spica (Alpha Virginis, mv = 0.98) at 15:31 hrs
October 10: Moon 1.6º
south of the star Nunki (Sigma Sagittarii, mv = 2.02) at 20:31 hrs
October 10: Moon 2.3º
north of the star Ascella (Zeta Sagittarii, mv = 2.6) at 23:10 hrs
October 12: Limb of Moon
36 arcminutes south of Pluto at 2:58 hrs
October 12: Pluto at
eastern stationary point at 6:41 hrs (diameter = 0.1" )
October 13: Limb of Moon
39 arcminutes south of the star Deneb Algedi (Delta Capricorni, mv =
2.85) at 19:24 hrs
October 14: Mars at
western quadrature at 17:53 hrs (diameter = 8.2" )
October 15: Limb of Moon
40 arcminutes north of Saturn at 5:06 hrs
October 16: Moon 2.3º
north of Neptune at 1:19 hrs
October 19: Moon 4.8º
north of Uranus at 23:10 hrs
October 20: Limb of Moon
32 arcminutes north of the star Alcyone (Eta Tauri, mv = 2.85) at 8:00
hrs
October 20: Venus 45
arcminutes north of the star Dschubba (Delta Scorpii, mv = 1.86) at
15:10 hrs
October 21: Venus 3º south
of the star Graffias (Beta1 Scorpii, mv = 2.56) at 3:10 hrs
October 21: Moon 5.7º
north of Jupiter at 16:42 hrs
October 21: Moon occults
the star Elnath (Beta Tauri, mv = 1.65) between 18:58 and 19:35 hrs
October 24: Mercury at
aphelion at 00:43 hrs ( diameter = 5.0" )
October 24: Mercury 1.5º
south of the star Zuben Elgenubi (Alpha Librae, mv = 2.72) at 3:28 hrs
October 24: Limb of Moon
38 arcminutes south of the star Pollux (Beta Geminorum, mv = 1.1) at
2:35 hrs
October 24: Moon 4.2º
north of Mars at 8:52 hrs
October 24: Venus 2.6º
north of the star Alniyat (Sigma Scorpii, mv = 2.9) at 22:45 hrs
October 25: Jupiter 17
arcminutes north of the star 109 Xi Tauri (mv = 4.96) at 11:28 hrs
October 26: Venus 3.1º north of the star Antares (Alpha Scorpii, mv = 0.88) at 14:36 hrs
October 27: Venus 4.5º north of the star 23 Tau Scorpii (mv= 2.82) at 22:36 hrs
October 30: Venus at
aphelion at 19:56 hrs ( diameter = 14.1" )
October 31: Limb of Moon
15 arcminutes north of the star Spica (Alpha Virginis, mv = 0.98) at
17:30 hrs
Mercury:
Venus:
(The coloured fringes to the second, fourth 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 third photographs, which were 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:
At the beginning of September, the red planet is cruising through the constellation Taurus, and rises in the east-north-east at about 1:43 am. It is near the star Elnath (Beta Tauri). Mars will cross into Gemini on September 6, and will pass by the star Mebsuta (Epsilon Geminorum) on September 24. The Earth is rapidly catching up to Mars, and it is becoming bigger and brighter with each passing month. On September 1 its angular diameter will be 7 arcseconds and its brightness will be magnitude 0.7. By the end of September its magnitude will have increased to 0.5 and its angular diameter to 7.5 arcseconds. On that date, it will be rising earlier, at 1:01 am.
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 passed though conjunction with the Sun on May 19 and then left the evening sky. It has now reappeared in the pre-dawn sky, and can be found rising in the north-east around 1:30 am in the first week of September. The waning gibbous Moon will be just to the left of Jupiter, close to the north-eastern horizon soon after midnight on September 24. Jupiter will reach opposition with the Sun (when it is at its largest and brightest) on December 8.
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 conjunction with the Sun on February 29 and on September 15 may be found about 26
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.
The photograph above was taken at 7:41 pm on December 07, 2023, 14 days after
eastern quadrature. The shadow of the planet once more falls across the far side
of the rings, but in the intervening 13 months the angle of the rings as seen
from Earth has lessened considerably. The light-coloured equatorial zone on
Saturn shows through the gap known as the Cassini Division.
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)"
takes 29.457 years. The angle of the rings will continue to reduce until they
are edge-on again in 2025. They will appear so thin that it will seem that
Saturn has no rings at all.
Uranus:
Neptune:
T
Neptune, photographed from Nambour on October 31, 2008
Pluto: The
erstwhile ninth and most distant planet passed through
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.
Here are the positions of the planets above the horizon in mid-September, at 4 am: Saturn is at an altitude of 20º (about a handspan) above the western horizon. 13º (about two-thirds of a handspan) east of Saturn is Neptune, which will require the use of a small telescope to find. Uranus is close to culmination (crossing the
meridian or north-south line), and is about 43º above the northern horizon. Jupiter has an altitude of two handspans above the north-north-western horizon, and Mars is one-and-a-half handspans above the north-eastern horizon (Jupiter is much brighter than Mars).
The movements of planets including their alignments and close-up images can be watched using the freeware
On September 1 there will be a fine grouping of Mars, Jupiter and Uranus in the north-eastern sky before dawn. It will be quite spectacular, as the star clusters the Pleiades (Seven Sisters) and the Hyades will be nearby, as well as the bright stars
Alderbaran, Betelgeuse and Capella. Not far away will be the stars Sirius, Rigel and Procyon. Mars is moving eastwards during September, from Taurus into Gemini. Jupiter will move away from the clusters in Taurus, also heading towards Gemini. The waning crescent
Moon will pass through this grouping from September 22 to 26.
Meteor
Showers:
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 periodic comet returns every 71 years, and is in our western twilight sky at present. Look due west, close to the horizon as soon as the sky darkens. On 16 July, the comet was near the magnitude 2.2 star Suhail (Lambda Velorum).
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.
Green Comet ZTF (C/2022 E3)
Comet SWAN (C/2020 F8) and Comet ATLAS (2019 Y4)
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,
The Stars and Constellations for this
These descriptions of the night sky are for 9 pm on September 1 and 7 pm on September 30. Broadly speaking, the following description starts low in the west and
follows the horizon to the right, heading round to the east, then south, then west, then overhead.
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.
Almost due north is the constellation 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 9.00pm 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.
In the east, a mv 2.2 star is about a handspan above the horizon. This is Beta Ceti, the brightest ordinary star in the constellation 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 11 pm.
Cetus is a large constellation, running around 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 stars known as the 'Circlet'. Dominating Pisces this month is the planet Jupiter.
Above Diphda is Fomalhaut, a bright, white first magnitude star in the faint constellation Piscis Austrinus, the Southern Fish. 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. A little more than a handspan to the north of Fomalhaut is the ringed planet Saturn, brighter than any star in that part of the sky.
A little more than a handspan 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 towards the south-eastern horizon and then turns along the horizon to the east towards
Cetus. It then continues below the horizon all the way to Orion, which this month will not rise above the eastern horizon until a little after midnight.
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.
Due south, the Large Magellanic Cloud (LMC) is a faint, diffuse glowing patch low to the horizon, only eight 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-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 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
If the night is dark and the skies are clear, a black dust cloud known as the Coalsack can be seen just above Acrux, the left-most and brightest star of the
Cross. Surrounding Crux on three sides is the large constellation Centaurus, its two brightest stars being the brilliant Alpha and Beta Centauri. The rest of the
constellation of Centaurus arches over Crux from above it, to its right-hand side, and then underneath it, where it adjoins Carina and Vela.
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 7:30 pm on September 1, with the centre of the galaxy at our zenith. It runs north from the Southern Cross through Centaurus, Ara, Scorpius, Sagittarius, Scutum and Aquila to Cygnus (the Northern Cross).
About three-and-a-half handspans 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. This month Sagittarius is host to the very faint planet Pluto, which may be found in its eastern end. East of Sagittarius is the faint constellation
Capricornus, the Sea-Goat (near the zenith). 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 Snake, 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. 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 Coalsack forms its 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 star clouds of Sagittarius. 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. Now it has 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. Mira is now fading and will reach a minimum around this coming Christmas. It will then brighten again and will reach its next maximum on April 6, 2025. On that date it will rise at 7:03 am and set at 7:17 pm.
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).
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.
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. Binary stars may be widely spaced, as the two examples just mentioned, or so close that a telescope is struggling to separate them (Acrux, 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.
The Milky Way
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.
Alpha Centauri (also known as Rigil Kentaurus, Rigil Kent or Toliman) is a binary easily
seen with a small 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
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.
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 September. This glow is the light of millions of faint stars combined with that coming from
glowing gas clouds called emission nebulae. It is concentrated along the plane of our galaxy, and this month it is seen crossing the sky, starting from the
south-south-west and passing through Crux to Centaurus, Scorpius, Sagittarius and Aquila to Cygnus in the north-east.
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 Season of the Scorpion
The spectacular constellation of Scorpius is about a handspan west of the zenith in mid-September. Three bright stars in a
gentle curve mark his head, and another three mark his body. Of this second group of three, the centre one is a bright, red
supergiant, Antares. It marks the red heart of the scorpion. This star is so large that, if it swapped places with
our Sun, it would engulf the Earth and extend to the orbit of Mars. It is 604 light years away and shines at magnitude 1.06.
Antares, an M type star, has a faint companion which can be seen in a good amateur telescope if the seeing is excellent.
The rest of the stars run around the scorpion's tail, ending with two blue-white B type stars, Shaula (the brighter of the two) and Lesath,
at the tip of the scorpion's sting. These two stars are at the eastern end of the constellation, and are near the bottom of the picture below. West of
Lesath in the body of the scorpion is an optical double star, which can be seen as two with the unaided eye.
Scorpius, with its head at top left and tail (with sting) at lower right.
Probably the two constellations most easily recognisable (apart from Crux, the Southern Cross) are Orion the Hunter and Scorpius the Scorpion. Both are
large constellations containing numerous bright stars, and are very obvious 'pictures in the sky'. Both also contain a very bright red supergiant
star, Betelgeuse in Orion and Antares in Scorpius.
The Lagoon Nebula, M8, in Sagittarius, adjacent to Scorpius
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The centre of the Lagoon Nebula
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 northern window is between the constellations Virgo and Coma
Berenices, roughly between the stars Denebola and Arcturus. It is out of sight this month.
The southern window is in the constellation Sculptor, just south of the star Fomalhaut. This window is low in the south-east in the early evening, but
later in the night it will rise high enough for distant galaxies to be observed. 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.
The Milky Way is a barred spiral galaxy some 100000 – 120000 light-years in
diameter which contains 100 – 400 billion stars. It may contain at least as many planets as well. Our galaxy is shaped like a flattened disc with a central bulge.
The Solar System is located within the disc, about 27000 light-years from the Galactic Centre, on the inner edge of one of the spiral-shaped concentrations of gas
and dust called the Orion Arm. When we look along the plane of the galaxy, either in towards the centre or out towards the edge, we are looking along the disc
through the teeming hordes of stars, clusters, dust clouds and nebulae. In the sky, the galactic plane gives the appearance which we call the Milky Way, a brighter
band of light crossing the sky. This part of the sky is very interesting to observe with binoculars or telescope. The brightest and most spectacular constellations,
such as Crux, Canis Major, Orion and Scorpius are located close to the Milky Way.
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 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 8 pm in the middle of September. Find the midpoint of this line to locate the pole.
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 photograph below shows a typical open cluster,
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 the disc of 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.
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 almost a solid mass of faint stars. Most other galaxies also have a halo of globular clusters circling around them.
The largest and brightest globular cluster in the sky is
NGC 5139**, also known as Omega Centauri. It has a slightly oval shape. It is an outstanding winter object, but this month it is low and only observable before 8 pm. Shining at fourth magnitude, it is faintly visible to the unaided eye, but is easily seen with binoculars, like a light in a fog. A telescope of 20 cm aperture or better will reveal its true nature, with millions of faint stars giving the impression of diamond dust on a black satin background. It lies at a distance of 5 kiloparsecs, or 16 300 light years. Omega Centauri is not a typical globular cluster, being much larger than any other in our Galaxy, with millions of stars. It is also slightly oval in shape.
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, neither Omega Centauri nor 47 Tucanae is at its best position for viewing, but both are observable.
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 Ptolemy's 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. As Messier had quite a small telescope, his list is an excellent guide to the brightest and most spectacular objects in the sky, although he did not include any of our excellent southern objects such as 47 Tucanae, Omega Centauri and Eta Carinae, as they were never visible from his home in Paris.
** 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. It contained only objects that needed a telescope to be seen. It was an updated version of the
previous General Catalogue
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 H
I Parkes All-Sky Survey (HIPASS: HI = clouds of neutral hydrogen in space), 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 Galaxies 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
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.
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, 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.
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