Note: To read this webpage with mobile phones or tablets, please use them
in landscape format, i.e. the long screen axis should be horizontal.
The Alluna RC-20 Ritchey Chrétien telescope was installed in March, 2016.
The 20-inch telescope is able to locate and track any sky object (including Earth satellites and the International Space Station) with software called TheSkyX Professional, into which is embedded a unique T-Point model created for our site with the telescope itself.
Explanatory Notes:
Rise and
set times are given for the theoretical horizon, which is a flat horizon all the
way round the compass, with no mountains, hills, trees or buildings to obscure
the view. Observers will have to make allowance for their own actual horizon.
Transient phenomena are provided for the current month
and the next. Geocentric phenomena are calculated as if the Earth were fixed in
space as the ancient Greeks believed. This viewpoint is useful, as otherwise
rising and setting times would be meaningless. In the list of geocentric events,
the nearer object is given first.
When a planet is referred to as ‘stationary’, it means
that its movement across the stellar background appears to have ceased, not that
the planet itself has stopped. With inferior planets (those inside the Earth’s
orbit, Mercury and Venus), this is caused by the planet heading either directly
towards or directly away from the Earth. With superior planets (Mars out to
Pluto), this phenomenon is caused by the planet either beginning or ending its
retrograde loop due to the Earth’s overtaking it.
Apogee and perigee: Maximum and minimum distances of the
Moon or artificial satellite from the Earth.
Aphelion and perihelion: Maximum and minimum distances of a
planet, asteroid or comet from the Sun.
Eclipses always occur in pairs, a lunar and a solar but not necessarily in that
order, two weeks apart.
The meridian is a semicircle starting from a point on the horizon that is
exactly due north from the observer, and arching up into the sky to the zenith
and continuing down to a point on the horizon that is exactly due south. On the
way down it passes through the South Celestial Pole which is 26.6 degrees above
the horizon at Nambour. The elevation of the South Celestial Pole is exactly the
same as the observer's latitude, e.g. from Cairns it is 16.9 degrees above the
horizon, and from Melbourne it is 37.8 degrees. The Earth's axis points to this
point in the sky in the southern hemisphere, and to an equivalent point in the
northern hemisphere, near the star Polaris, which from Australia is always below
the northern horizon.
All astronomical objects rise until they reach the meridian, then they begin to
set. The act of crossing or 'transitting' the meridian is called 'culmination'.
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
Sagittarius, the Archer. It crosses into Capricornus, the Sea-Goat on
January 20.
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.
.gif)
First Quarter:
January 7
Last Quarter:
Moon at 8 days after
New, as on January 8.
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 Features 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 examine
more of the rugged area adjoining the Moon's North Pole. It extends west to longitude 50º west and south to latitude 65º north. It includes an ancient walled plain called Mouchez, and a much more recent crater called Philolaus. This is the first in a sequence of images to be presented this year, covering the zone of libration west from the North Pole around the Moon's north-western limb to the Equator. As the sequence progresses, the camera will start from the walled plain Hermite and then move west, following the Moon's north-western limb in a southerly direction until we reach the lunar equator in the vicinity of the crater Riccioli. Features recorded such as craters will move to the right as the sequence continues, as the image frame moves to the left. The average width of each image will be 265 kilometres.
This area adjoins that shown as image
# 99 in the first series of the Lunar Features of the Month Archive. This image was captured at 9:36 pm on 22 May 2024.The eastern walls of the 110 kilometre diameter walled plain Hermite are only 50 kilometres from the Moon's North Pole itself. Hermite is in the zone of libration, as is the nearby 58 kilometre walled plain Sylvester . Both of these features are about 3.8 billion years old, or middle-aged as lunar features go. South of these two, the lunar surface has been battered by eons of impacts, the walled plains Poncelet (70 kilometres), Mouchez (83 kilometres) and Anaximenes (80 kilometres) showing considerable degradation. On the other hand, the crater-plain Philolaus (71 kilometres) is much more recent, its age being less than 1.1 billion years. The rim of Philolaus remains sharply defined. A small part of the floor has been flooded with lava, but the rest of the floor is very rugged, with numerous hills and four clusters of mountains. The interior crater walls show many land slips which now appear as terraces.
Aristotle
tells us that this was the reason that Philolaus decided that there must be a
tenth planet, which he called the Antichthon or ‘counter-Earth’. He set
all of these bodies, including the Sun, in orbit around Hestia, the
central fire of Pythagoras, which people could never see because the side of the
Earth on which they lived was always facing away from it. He went on to say that
the counter-Earth was also never seen,
because it was on the opposite side of the central fire from the Earth, and so
was always hidden from view. This erroneous cosmology was based more on
philosophical principles and wishful thinking than on real astronomical
observations, but it had one remarkable feature. For the first time in history,
it described the Earth as one of a family of two planets moving in circular
orbits, though not around the Sun, but around Hestia, the central fire. It was
the first suggestion of a planetary system revolving around a central body, with
its various components also rotating upon their axes. Two thousand years later,
Nicolaus Copernicus would analyse the ideas of Philolaus when developing his own
truer conceptions
In this image, the craters Poncelet and Philolaus are now on the right-hand side, and Anaximenes is in the centre. The walled plains Pascal
(106 kilometres) and Poncelet C (67 kilometres) have now come into view, as has the more recent crater Carpenter (40 kilometres). A small craterlet called Anaximander H (9 kilometres) can be found near the lower left-hand corner.
A huge walled plain called Brianchon (146 kilometres), which lies in the zone of libration, appears in this image as a dark void, but
when it is illuminated one day later (see below), the far ramparts of Brianchon
are revealed, and the summits of small hills in the centre of the floor catch the light of
lunar dawn. This image also includes most of the ancient compound crater Desargues, which is an 86 kilometres diameter walled plain which has been struck by two later impactors. Desargues
will be shown in full next month.
However, Anaximander did place the floating Earth fixed at the centre of the universe. He claimed that the Earth was surrounded by an opaque crystalline black sphere that enclosed the air and weather. The celestial bodies moved on this sphere. Outside the sphere was a region of fire. Stars were tiny holes in the sphere through which the fire behind could be glimpsed.
2025
January 1: Limb of Moon 1 arcminute south of Pluto at 23:16 hrs
Mercury:
January 3: Moon occults the star Deneb Algedi (Delta Capricorni, mv = 2.85) between 14:34 and 15:06 hrs
January 3: Earth at perihelion at 21:52 hrs
January 4: Limb of Moon 25 arcminutes south-east of Venus at 2:17 hrs
January 5: Limb of Moon 56 arcminutes north of Saturn at 2:58 hrs
January 6: Moon 1.7º north of Neptune at 1:19 hrs
January 9: Jupiter 1.3º south of the star 94 Tau Tauri (mv = 4.24) at 00:39 hrs
January 10: Moon 4.9º north of Uranus at 1:43 hrs
January 10: Moon occults many bright stars in the Pleiades cluster between 9:45 am and 11:27 hrs
January 10: Venus at Greatest Elongation East (47º 10') at 17:33 hrs ( diameter = 24.5" )
January 11: Moon 5.3º north of Jupiter at 8:30 hrs
January 12: Moon occults the star Elnath (Beta Tauri, mv = 1.65) between 3:16 and 4:09 hrs
January 13: Mercury 1.7º north of the star Kaus Borealis (22 Lambda Sagittarii, mv = 2.82) at 10:16 hrs
January 14: Limb of Moon 4.6 arcminutes north of Mars at 13:21 hrs
January 16: Mars at opposition at 12:16 hrs ( diameter = 14.5" )
January 17: Mercury 2.5º north of the star Nunki (Sigma Sagittarii, mv = 2.02) at 13:16 hrs
January 19: Venus 2.2º north of Saturn at 14:17 hrs
January 19: Mercury at aphelion at 23:59 hrs ( diameter = 4.9" )
January 20: Mercury 2.6º south of the star Pi Sagittarii (mv = 1.75) at 2:16 hrs
January 21: Moon occults the star Spica (Alpha Virginis, mv = 0.98) between 15:24 and 15:42 hrs
January 21: Pluto in conjunction with the Sun at 22:21 hrs
January 23: Mars 2.4º south of the star Pollux (Beta Geminorum, mv = 1.1) at 5:35 hrs
January 24: Moon 1.6º north of the star Pi Scorpii (mv = 2.82) at 20:48 hrs
January 25: Limb of Moon 18 arcminutes south-west of the star Alniyat (Sigma Scorpii, mv = 2.9) at 4:57 hrs
January 25: Grazing occultation by the Moon of the star Antares (Alpha Scorpii, mv = 0.88) between 11:27 and 11:37 hrs
January 25: Moon 1.5º north of the star 23 Tau Scorpii (mv = 2.82) at 15:10 hrs
January 27: Moon 2.4º north of the star Alnasl (Gamma Sagittarii, mv = 2.98) at 1:28 hrs
January 27: Moon 1.3º north of the star Kaus Media (Delta Sagittarii, mv = 2.72) at 8:07 hrs
January 27: Moon 30 arcminutes south of the star Nunki (Sigma Sagittarii, mv = 2.02) at 22:40 hrs
January 29: Moon 2.3º south of Mercury at 6:01 hrs
January 29: Limb of Moon 43 arcminutes south of Pluto at 7:35 hrs
January 29: Mercury 1.4º north of Pluto at 18:35 hrs
January 30: Moon 1.7º north of the star Deneb Algedi (Delta Capricorni,(mv = 2.85) at 22:30 hrs
January 30: Uranus at eastern stationary point at 23:23 hrs ( diameter = 3.6" )
February 1: Saturn 55 arcminutes north of the star Situla (92 Chi Aquarii, mv = 4.93) at 1:53 hrs
February 1: Moon 1º north of Saturn at 13:52 hrs
February 1: Venus 3.3º north of Neptune at 7:27 hrs
February 1: Limb of crescent Moon 54 arcmiutes north of Saturn at 13:09 hrs
February 2: Moon 1º north of Neptune at 6:30 hrs
February 2: Moon 2º south of Venus at 6:36 hrs
February 2: Saturn 52 arcminutes south of the star 90 Phi Aquarii (mv = 4.22) at 21:56 hrs
February 4: Jupiter at eastern stationary point at 19:33 hrs ( diameter = 42.9" )
February 6: Moon 4.6º north of of Uranus at 5:40 hrs
February 6: Limb of First Quarter Moon 50 arcminutes north of the star Alcyone ((Eta Tauri, mv = 2.85) at 16:19 hrs
February 7: Moon 5.5º north of Jupiter at 10:54 hrs
February 8: Limb of waxing gibbous Moon 2 arcminutes south of the star Elnath (Beta Tauri, mv = 1.65) at 8:27 hrs, possible grazing occultation depending on the observing site.
February 9: Mercury at Superior Conjunction at 21:45 hrs ( diameter = 4.8" )
February 10: Limb of Moon 31 arcminutes north of Mars at 6:56 hrs
February 11: Mercury 36 arcminutes north of the star Deneb Algedi ((Delta Capricorni,(mv = 2.85) at 12:52 hrs
February 12: Uranus at eastern quadrature at 5:13 hrs ( diameter = 3.6" )
February 13: Moon 2.1º north of the star Regulus (Alpha Leonis, mv = 1.36) at 11:52 hrs
February 17: Limb of Moon 10 arcminutes north of the star Spica (Alpha Virginis, mv = 0.95) at 21:19 hrs
February 20: Venus at perihelion at 4:20 hrs ( diameter = 42.2" )
February 21: Neptune 1.9º north of the star 27 Piscium (mv = 4.9)at 6:19 hrs
February 21: Limb of Moon 29 arcminutes north of the star Pi Scorpii (mv = 2.82) at 5:29 hrs
February 21: Limb of Moon 4 arcminutes south of the star Alniyat (Sigma Scorpii, mv = 2.9) at 16:09 hrs
February 21: Limb of Moon 3 arcminutes north of the star Antares (Alpha Scorpii, mv = 0.88) at 18:51 hrs
February 21: Moon 1.8º north of the star 23 Tau Scorpii (mv = 2.82) at 21:15 hrs
February 23: Moon 2.1º north of the star Alnasl (Gamma Sagittarii, mv = 2.98) at 13:49 hrs
February 23: Moon 2º north of the star Kaus Media (Delta Sagittarii, mv = 2.72) at 18:46 hrs
February 24: Moon 1.6º south of the star Nunki (Sigma Sagittarii, mv = 2.02) at 8:48 hrs
February 24: Mars at eastern stationary point at 10:43 hrs ( diameter = 11.4" )
February 25: Moon occults Pluto between 20:27 and 20:57 hrs
February 25: Mercury 1.5º north of Saturn at 22:44 hrs
February 27: Limb of Moon 6 arcminutes south of the star Deneb Algedi (Delta Capricorni, mv = 2.85) at 7:13 hrs
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.)
mid-January 2025
late February 2025
mid-April 2025
early June 2025
January 2026
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 January, the red planet is cruising in a westerly direction in the western part of the constellation Cancer. It rises in the north-east just before 8:15 pm. The Earth is rapidly catching up to Mars, and it is becoming bigger and brighter with each passing week. On January 1 its angular diameter will be 14 arcseconds and its brightness will be magnitude -1.2. On December 7 last, the fact that the Earth was catching up to Mars caused the red planet to stop and reverse its motion against the background stars. It began what is called a "retrograde loop". It will complete this loop on February 24, 2025, when it will stop and resume its easterly trajectory through the constellations. Mars will reach opposition exactly midway between these two dates, on January 16, when its diameter will be 14.5 arcseconds and its brightness will be magnitude -1.38. By then its retrograde motion will have returned it to the constellation Gemini. Completion of its retrograde loop will see Mars return to Cancer on April 13. It will pass through outlying stars in the Praesepe star cluster on May 5. Mars will cross into Leo on May 26, when it will again be small and faint. The Full Moon will be in the vicinity of Mars on January 14.
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 reached opposition with the Sun on December 8, and in January it is in the sky for most of the night. The waxing gibbous Moon will be to the left of Jupiter, high above the northern horizon, at 9 pm on January 10.
Jupiter as
photographed from Nambour on the evening of April 25, 2017. The images were
taken, from left to right, at 9:10, 9:23, 9:49, 10:06 and 10:37 pm. The rapid
rotation of this giant planet in a little under 10 hours is clearly seen. In the
southern hemisphere, the Great Red Spot (bigger than the Earth) is prominent,
sitting within a 'bay' in the South Tropical Belt. South of it is one of the
numerous White Spots. All of these are features in the cloud tops of Jupiter's
atmosphere.
Jupiter at opposition, May 9, 2018
Jupiter as it appeared at 7:29 pm on July 2, 2017. The Great Red Spot was in a
similar position near Jupiter's eastern limb (edge) as in the fourth picture in
the series above. It will be seen that in the past two months the position of
the Spot had drifted when compared with the festoons in the Equatorial Belt, so
must rotate around the planet at a slower rate. In fact, the Belt enclosing the
Great Red Spot rotates around the planet in 9 hours 55 minutes, and the
Equatorial Belt takes five minutes less. This high rate of rotation has made the
planet quite oblate. The prominent 'bay' around the Red Spot in the five earlier
images appeared to be disappearing, and a darker streak along the northern edge
of the South Tropical Belt was moving south. In June this year the Spot began to
shrink in size, losing about 20% of its diameter. Two new white spots have
developed in the South Temperate Belt, west of the Red Spot. The five upper
images were taken near opposition, when the Sun was directly behind the Earth
and illuminating all of Jupiter's disc evenly. The July 2 image was taken just
four days before Eastern Quadrature, when the angle from the Sun to Jupiter and
back to the Earth was at its maximum size. This angle means that we see a tiny
amount of Jupiter's dark side, the shadow being visible around the limb of the
planet on the left-hand side, whereas the right-hand limb is clear and sharp.
Three of Jupiter's Galilean satellites are visible, Ganymede to the left and
Europa to the right. The satellite Io can be detected in a transit of Jupiter,
sitting in front of the North Tropical Belt, just to the left of its centre.
Jupiter reached opposition on May 9, 2018 at 10:21 hrs, and the above photographs were taken that evening, some ten to twelve hours later. The first image above was taken at 9:03 pm, when the Great Red Spot was approaching Jupiter's central meridian and the satellite Europa was preparing to transit Jupiter's disc. Europa's transit began at 9:22 pm, one minute after its shadow had touched Jupiter's cloud tops. The second photograph was taken three minutes later at 9:25 pm, with the Great Red Spot very close to Jupiter's central meridian.
The third photograph was taken at 10:20 pm, when Europa was approaching Jupiter's central meridian. Its dark shadow is behind it, slightly below, on the clouds of the North Temperate Belt. The shadow is partially eclipsed by Europa itself. The fourth photograph at 10:34 pm shows Europa and its shadow well past the central meridian. Europa is the smallest of the Galilean satellites, and has a diameter of 3120 kilometres. It is ice-covered, which accounts for its brightness and whitish colour. Jupiter's elevation above the horizon for the four photographs in order was 50º, 55º, 66º and 71º. As the evening progressed, the air temperature dropped a little and the planet gained altitude. The 'seeing' improved slightly, from Antoniadi IV to Antoniadi III. At the time of the photographs, Europa's angular diameter was 1.57 arcseconds. Part of the final photograph is enlarged below.
Jupiter at 11:34 pm on May 18, nine days later. Changes in the rotating cloud
patterns are apparent, as some cloud bands rotate faster than others and
interact. Compare with the first photograph in the line of four taken on May 9.
The Great Red Spot is ploughing a furrow through the clouds of the South
Tropical Belt, and is pushing up a turbulent bow wave.
Jupiter at opposition, June 11, 2019
Jupiter reached opposition on June 11, 2019 at 01:20 hrs, and the above
photographs were taken that evening, some twenty to twenty-two hours later. The
first image above was taken at 10:01 pm, when the Great Red Spot was leaving
Jupiter's central meridian and the satellite Europa was preparing to transit
Jupiter's disc.
The third photograph was taken at 10:41 pm, when Europa was about a third of its way across Jupiter. Its dark shadow is trailing it, slightly below, on the clouds of the North Temperate Belt. The shadow is partially eclipsed by Europa itself. The fourth photograph at 10:54 pm shows Europa and its shadow about a quarter of the way across. This image is enlarged below. The fifth photograph shows Europa on Jupiter's central meridian at 11:24 pm, with the Great Red Spot on Jupiter's limb. The sixth photograph taken at 11:45 pm shows Europa about two-thirds of the way through its transit, and the Great Red Spot almost out of sight. In this image, the satellite Callisto may be seen to the lower right of its parent planet. Jupiter's elevation above the horizon for the six photographs in order was 66º, 70º, 75º, 78º, 84º and 86º. As the evening progressed, the 'seeing' proved quite variable.
There have been numerous alterations to Jupiter's belts and spots over the thirteen months since the 2018 opposition. In particular, there have been major disturbances affecting the Great Red Spot, which appears to be slowly changing in size or "unravelling". It was very fortuitous that, during the evenings of the days when the 2018 and 2019 oppositions occurred, there was a transit of one of the satellites as well as the appearance of the Great Red Spot. It was also interesting in that the same satellite, Europa, was involved both times.
Jupiter's moon Europa has an icy crust with very high reflectivity, which
accounts for its brightness in the images above. On the other hand, the largest
moon Ganymede (seen below) has a surface which is composed of two types of
terrain: very old, highly cratered dark regions, and somewhat younger (but still
ancient) lighter regions marked with an extensive array of grooves and ridges.
Although there is much ice covering the surface, the dark areas contain clays
and organic materials and cover about one third of the moon. Beneath the surface
of Ganymede is believed to be a saltwater ocean with two separate layers.
Jupiter is seen here on 17 November
2022 at 8:39 pm. To its far right is its largest satellite, Ganymede. This
"moon" is smaller than the Earth but is bigger than Earth's Moon. Its diameter
is 5268 kilometres, but at Jupiter's distance its angular diameter is only 1.67
arcseconds. Despite its small size, Ganymede is the biggest moon in the Solar
System. Jupiter is approaching eastern quadrature, which means that Ganymede's
shadow is not behind it as in the shadows of Europa in the two sequences taken
at opposition. In the instance above as seen from Earth (which is presently at a
large angle from a line joining the Sun to Ganymede), the circular shadow of
Ganymede is striking the southern hemisphere cloud tops of Jupiter itself. The
shadow is slightly distorted as it strikes the spherical globe of Jupiter. If
there were any inhabitants of Jupiter flying across the cloud bands above, and
passing through the black shadow, they would experience an eclipse of the
distant Sun by the moon Ganymede.
Above is a 7X enlargement of Ganymede, showing markings on its rugged, icy
surface. The dark area in its northern hemisphere is called Galileo Regio.
Saturn:
The ringed planet is located in the constellation of Aquarius, and will remain there until it crosses into Pisces on April 19, 2025. Saturn reached eastern quadrature (culminating at sunset) on December 5. On January 1, Saturn will be two handspans above the western horizon at 7:30 pm. During January, the angle of the plane of Saturn's Rings will reduce until they are almost edge-on. Saturn will reach conjunction with the Sun on March 12, and on March 24 the Rings will be exactly edge-on. Simultaneously, the various moons of Saturn will line 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.
The photograph above was taken at 8:33 pm on October 29, 2024, 51 days after opposition. The shadow of the planet once more falls across the far side of the rings, but in the intervening 11 months the angle of the rings as seen from
Earth has lessened considerably. The ring system will be difficult to observe during 2025. The light-coloured equatorial zone on Saturn is prominent, and is crossed by the black shadow of the rings on the planet. 
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 (March 24, 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-January, at
8 pm: Venus and Saturn are above the western horizon and preparing
to set. (The waxing crescent Moon passed by them on January 3 and 4.) 11.8º (about two-thirds of a handspan)
north-east of Saturn is Neptune, which will require the use of a small telescope to find. Uranus
is nearing the meridian and is 44º above the northern horizon (about two-and-a-half handspand). Jupiter has an altitude of 37 (two handspans) above the north-north-eastern horizon, and Mars is
just rising above the east-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 Meteor
Showers:
On January 1 there will be a fine grouping of Mars, Jupiter and Uranus in the northern sky
after 11 pm. 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
bright stars Sirius, Rigel,
Betelgeuse and Procyon,
and the less bright Twins Pollux and Castor. Mars has been moving slowly eastwards during
December, cruising through Cancer. As it approached the Beehive Star Cluster
(called 'Praesepe' or M44), this movement stopped on December 7, and
Mars reversed direction, heading westwards, back towards Gemini. It is performing what is
called a "retrograde loop". This movement is actually caused by the fact that
the Earth is catching up and overtaking the red planet. When Mars reaches the
centre of this loop it is said to be at "opposition", when it is at its largest
and brightest. This will occur on January 16. The giant planet Jupiter will move away from the clusters in Taurus, also heading towards Gemini,
but it won't reach that constellation until June 12 next year. The waning crescent
Moon will pass through this grouping from December 13 to 18.
Radiant: Near the star Vega. Named after the obsolete constellation Quadrans Muralis (Mural Quadrant).
Us
ZHR
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:
At long last, observers in the Southern Hemisphere are favoured with a comet in their western twilight skies this month. Look for the constellation Capricornus above the western horizon shortly after sunset. Your ability to see the comet
with your naked eye will depend on your eyesight and where you live. A pair of binoculars or a spotting scope will be useful. Find a location with a good view down to the horizon, preferably away from buildings, hills and city lights. In
early January the comet will be close to the horizon, but each evening it will appear higher up and slightly fainter, as it is travelling away from the Sun and us. Comet C.2024 G3 (ATLAS) gets its name from how it was discovered. The "2024 G3" shows that it was the third comet discovered last year, while the facility that found it is called the "Asteroid Terrestrial-impact Last
Alert System" or "ATLAS". It's hard to predict how a comet will develop in advance. Originally, some experts thought that C/2024 G3 would break apart when it reached the point in its orbit where it is at its closest to the Sun, or 'perihelion".
The latest information hints that Comet C/2024 G3 may be a periodic comet, i.e. its orbit around the Sun is elliptical, and it takes roughly 160 000 years to complete an orbit. In other words, this comet has already made one or more passages around
the Sun and may have been observed by prehistoric cavemen. This means that it has already survived close passages by the Sun, and will probably survive this one, too. If so, it may be the brightest comet of 2025.
This comet will reach perihelion on January 13, when it will also be at its closest approach to the Earth. It will then be at its brightest, almost as bright as Venus, but it will be too close to the Sun for safe observations. On
that evening the comet's head or coma will be near the magnitude 3.9 star 44 Sagittarii.
By January 28 the comet will be close to the magnitude 4 star 16 Psi Capricorni. By February 14 it will be near the magnitude 4.5 star 14 Mu
Piscis Austrini and fading rapidly. By March 1 the comet will be mid-way between magnitude 4.3 Beta Sculptoris and magnitude 2.4 Ankaa (Alpha Phoenicis).
You can track how bright Comet C/2024 G3 is by checking the
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).
Comet 12P Pons-Brooks, photographed from Nambour at 6:40 pm on June 7, 2024. Comet C/2023 A3 (Tsuchinshan-ATLAS)
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 was not be very bright, and in the first weeks of February it was only faintly visible to the unaided eye from sites far from the light pollution of cities and towns.
Comet 12P Pons-Brooks
Green Comet ZTF (C/2022 E3)
Both of these comets appeared recently in orbits that caused them to dive towards the Sun's surface before swinging around the Sun and heading back towards the far reaches of the Solar System. Such comets are called 'Sun grazers', and their close approach to the Sun takes them through its immensely powerful gravitational field and the hot outer atmosphere called the 'corona'. They brighten considerably during their approach, but most do not survive and disintegrate as the ice which holds them together melts. While expectations were high that these two would emerge from their encounter and put on a display as bright comets with long tails when they left the Sun, as they came close to the Sun they both broke up into small fragments of rock and ice and ceased to exist.
Comet 46P/Wirtanen
In December 2018, Comet 46P/Wirtanen swept past Earth, making one of the ten closest approaches of a comet to our planet since 1960. It was faintly visible to the naked eye for two weeks. Although Wirtanen's nucleus is only 1.2 kilometres across, its green atmosphere became larger than the Full Moon, and was an easy target for binoculars and small telescopes. It reached its closest to the Sun (perihelion) on December 12, and then headed in our direction. It passed the Earth at a distance of 11.5 million kilometres (30 times as far away as the Moon) on December 16, 2018.
Comet Lulin
This comet, (C/2007 N3), discovered
Comet Lulin at 11:25 pm on February 28, 2009, in Leo. The brightest star is Nu Leonis, magnitude 5.26.
The
LINEARNearly all of these programs are based in the northern hemisphere, leaving gaps in the coverage of the southern sky. These gaps are the areas of sky where amateur astronomers look for comets from their backyard observatories.
To find out more about current comets, including finder charts showing exact positions and magnitudes, click
here. To see pictures of these comets, click here.
The 3.9 metre Anglo-Australian Telescope (AAT) at the Australian Astronomical Observatory near Coonabarabran, NSW.
Deep Space
Sky Charts and Maps available on-line:
There are some useful representations of the sky available here. The sky charts linked
below show the sky as it appears to the unaided eye. Stars rise four minutes earlier each night, so at the end of a week the stars have gained about half an
hour. After a month they have gained two hours. In other words, the stars that were positioned in the sky at 8 pm at the beginning of a month will have the
same positions at 6 pm by the end of that month. After 12 months the stars have gained 12 x 2 hours = 24 hours = 1 day, so after a year the stars have returned
to their original positions for the chosen time. This accounts for the slow changing of the starry sky as the seasons progress.
The following interactive sky charts are courtesy of Sky and Telescope magazine. They can simulate a view of the sky from any location on Earth at any time of day or night between the years 1600 and 2400. You can also print an all-sky map. A Java-enabled web browser is required. You will need to specify the location, date and time before the charts are generated. The accuracy of the charts will depend on your computer’s clock being set to the correct time and date.
To produce a real-time sky chart (i.e. a chart showing the sky at the instant the chart is generated), enter the name of your nearest city and the country. You will also need to enter the approximate latitude and longitude of your observing site. For the Sunshine Coast, these are:
latitude: 26.6o South longitude: 153o East
Then enter your time, by scrolling down through the list of cities to "Brisbane: UT + 10 hours". Enter this one if you are located near this city, as Nambour is. The code means that Brisbane is ten hours ahead of Universal Time (UT), which is related to Greenwich Mean Time (GMT), the time observed at longitude 0o, which passes through London, England. Click here to generate these charts._____________________________________
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 poorly placed for viewing in the evening, but can be observed above the south-eastern horizon from 9 pm onwards. It will culminate at 3 am and is visible for the rest of the night.
These descriptions of the night sky are for 10 pm on January 1 and 8 pm on January 31. Broadly speaking, the following description starts low in the south-west and follows the
horizon to the right, heading round to the east, then overhead, then south.
The Stars and Constellations for this month:
Low in the south-west, the flattened triangle of Grus is setting. It is nearly upside-down at this time. To its right, the star Fomalhaut is curving down to the west-south-western horizon, and in the west-south-west, Aquarius is about to set. There are no stars brighter than third magnitude in that constellation, but it does contain many interesting objects, including a group of four stars known as the 'Water Jar'. A little to the north of due west, the faint constellation of Pisces, the Fishes, is setting. A well-known asterism in Pisces is the Circlet, a faint circle of seven fourth and fifth magnitude stars. East of Pisces are the zodiacal constellations of Aries and then Taurus.
In the north-west, the Great Square of Pegasus and Andromeda are also dipping below the horizon. These constellations contain the well-known spiral galaxies M31 (in Andromeda) and M33 (in Triangulum - see below). These large spirals are members of the Local Group of galaxies (our Milky Way is a third member), and can be easily seen with binoculars. They are the nearest galaxies that can be seen from the large observatories in the Northern Hemisphere. This month, they are best seen as soon as darkness falls, for they soon head towards the horizon. (Southern Hemisphere observers can see two closer galaxies, the Clouds of Magellan.)
The Great Spiral M33 in Triangulum.
The Great Galaxy in Andromeda, M31, photographed at Starfield Observatory with an off-the-shelf
digital camera on 16 November 2007.
The three main stars in Andromeda are, from left to right, Alpheratz, Mirach, and
Almach, and above them is the faint constellation of Triangulum, a narrow triangle of stars. M31 lies about eight degrees (one third of a handspan) below
Mirach tonight, while M33 is a similar distance above Mirach.
There are three
main stars in the small constellation of Aries, the Ram: Hamal
(Alpha Arietis, magnitude 2), Sheratan (Beta Arietis, magnitude 2.6), and
Mesarthim (Gamma Arietis, magnitude 4.5).
A large but faint constellation, Cetus, the Whale, lies along
the north-western horizon at an altitude of about 45 degrees. Though this part of the sky has no really bright stars,
about 30 degrees above the western horizon is a mv 2.2 star. This is Beta Ceti, the brightest ordinary star in Cetus. Its common name is
Diphda, and it has a yellowish-orange colour. It appears all by itself in a large area of sky deficient in bright stars,
and marks the whale's tail. By rights, we would expect the star
Menkar or Alpha Ceti to be brighter, but Menkar is actually more than half a magnitude fainter than Diphda. Menkar may be seen
50 degrees above the north-western horizon, and marks the whale's head. It is 30
degrees (1.6 handspans) south-west of Jupiter.
Cetus is a large constellation, and to the unaided eye it appears unremarkable. But it does contain a most interesting star, which
even medieval people noticed. Hevelius named it Mira, the Wonderful (see below). Between Cetus and Pegasus is the zodiacal constellation of Pisces, the
Fishes, described above. In January, the spectacular constellations
are in the eastern half of the evening sky. Taurus, with its two star clusters the
Pleiades (or
Subaru) and the Hyades, is high in the north-north-west (see below). The brightest
star in Taurus is an
orange star dominating the Hyades cluster, but not a member of it. This is Aldebaran
(Alpha Tauri), a K5 star with a visual magnitude of 0.87.
The Pleiades is a small group like a question mark, and is often called the
Seven Sisters, although excellent eyes are needed to detect the seventh star
without optical aid. The group is also known as ‘Santa’s Sleigh’, as it appears
around Christmas time. All the stars in this cluster are hot and blue. They are
also the same age, as they formed as a group out of a gas cloud or nebula. There
are actually more than 250 stars in the Pleiades.
The Pleiades is the small cluster at centre left, while the Hyades is the much larger grouping at centre right.
Wisps of the nebula which formed the Pleiades can be seen around the brighter stars in the cluster.
The Hyades cluster is closer and appears much larger than the Pleiades. It has the appearance of a capital A or inverted V, with Aldebaran lying at the foot of its right leg. The V shape looked to the ancients like the face of a bull, with Aldebaran as his angry orange eye. Being in the southern hemisphere, we see it upside down. The Pleiades form the bull’s shoulder. If we project the two sides of the Bull's face V to the north-east, they show us the two long horns of the Bull. The tip at the end of each horn is marked by a star. The brighter star is Elnath (Beta Tauri, magnitude 1.65), and the other is Alheka (magnitude 2.95).
Below the Pleiades we can see part of the far-northern constellation of Perseus. The two brightest stars are Mirphak (Alpha Persei) and Algol (Beta Persei). Algol is the higher above the northern horizon of the two. Since ancient times, Algol has shown regular variations in brightness. It usually shines at magnitude 2.1, but every 2 days 20 hours 49 minutes it dims to magnitude 3.4 for 10 hours before recovering its original brightness. Because of this clock-like change, early astronomers called it the 'Demon Star'. It marks the head of Medusa, the Gorgon, which is carried in the hand of Perseus.
The Dutch-born Englishman John Goodricke (1764-1786) was a young amateur astronomer. He was profoundly deaf and also without much speech after suffering from scarlet fever as an infant. Goodricke is best known for his explanation of the variations in the star Algol in 1782. Although several stars were already known to vary in apparent magnitude, Goodricke was the first to propose a mechanism to account for this. He suggested that Algol is actually a pair of stars circling each other, and the variation in brightness is caused when one passes behind the other. Such a star system is now known as an eclipsing binary. He presented his findings to the Royal Society in May 1783, and for this work, the Society awarded him the Copley Medal for that year.
Goodricke is also credited with discovering the periodic variation of the namesake, Delta Cephei, of the type of variable stars called Cepheids, in 1784. It was the second Cepheid found, the first, Eta Aquilae, being found by Goodricke’s friend Edward Pigott earlier the same year. Goodricke proposed that Cepheids were unstable stars that regularly swell up and then fall back - 'pulsating variables' (see
Mira, the Wonderful below).
He was elected a
Fellow of the Royal Society on April 16, 1786, but never learned of this honour, as he was very ill and died four days
later, probably from pneumonia, aged only twenty-one years and seven months.
Between the Hyades and the northern horizon is a large constellation shaped roughly like a tall pentagon. This is Auriga the Charioteer, its brightest star being Capella, at the left side of the base of the pentagon. Above Capella and slightly to the left is a small triangle of stars known as 'The Kids'. At the top of Auriga's pentagon is a bright star that is actually the second-brightest star in Taurus. Its name is Elnath or Beta Tauri, and it marks the tip of the Bull's western horn. Coincidentally, Jupiter is this month sitting on this horn, close to the star Aldebaran.
To the east of Auriga, Gemini is quite high, with its brightest two twin stars at its eastern end, Pollux and Castor. Pollux is the brighter one. To the right of Pollux is the planet Mars, much brighter than either of the twins. Mars is this month circling around the twins in its retrograde loop. Rising in the north-east is the head of Leo, its brightest star Regulus (Alpha Leonis) marking the Lion’s heart. The whole constellation will have risen by midnight on January 1.
Between Gemini and Leo is a fainter constellation, Cancer the Crab. Though a fairly unremarkable constellation in other ways, Cancer does contain a large star cluster called Praesepe or the Beehive, which is a splendid sight in binoculars. The red planet Mars will be in Cancer for the first 13 days of January, after which it will return to Gemini as part of its retrograde loop. It will reverse direction again on February 24 and cross back into Cancer of April 13.
In January, the constellation of Orion is as high as he can ever appear from our latitude. He is about a handspan north of the zenith, and is about to cross the meridian (the line that runs from due south to due north and passing through the zenith, directly overhead). When a sky object crosses the meridian, it is said to be culminating. At that point, it ceases rising and begins setting. The brilliant white star Rigel (Beta Orionis) is approaching the zenith at 10 pm on January 1. Orion appears upside-down to us, but to an observer in the northern hemisphere, e.g. London or New York, he appears right-way-up, striding along the southern horizon this month.
About a hand-span to the south-east of Rigel is the brightest star in the night sky, Sirius, also known
as Alpha Canis Majoris. Sirius will culminate at 11.00 pm in mid-January.
Skirting the horizon from north-east to south-east is a long, faint constellation, Hydra, the Water Snake. It is the constellation with the largest area and is very long, stretching half-way across the sky. Well up in the east is Hydra’s brightest star, Alphard, mv= 2.2. This orange star was known by Arabs in ancient times as ‘The Solitary One’, as it lies in an area of sky with no bright stars nearby.
Coming up in the south-south-east, Crux (Southern Cross) is lying at an angle. Crux is the smallest of the 88 constellations. If you have a low, clear horizon in that direction, you will be able to see below it and to the right the two bright Pointers, Alpha and Beta Centauri. To the right of Crux is a small, fainter quadrilateral of stars, Musca, the Fly. In mid-month Crux will be horizontal by midnight, and vertical just before sunrise.
Between Crux and Sirius is a very large area of sky filled with interesting objects. This was once the constellation Argo Navis, named for Jason’s famous ship used by the Argonauts in their search for the Golden Fleece. The constellation Argo Navis was found to be too large, so in 1755 Nicholas Louis de la Caille divided it into three sections - Carina (the Keel), Vela (the Sails) and Puppis (the Stern).
A handspan south of the zenith and two handspans south of Sirius is the second brightest star in the night sky, Canopus (Alpha Carinae). On the border of Carina and Vela is the False Cross, larger and more lopsided than the Southern Cross. Both of these Crosses are actually more like kites in shape, for, unlike Cygnus (the Northern Cross), they have no star at the intersection of the two cross arms.
Two handspans south-west of Canopus is Achernar, Alpha Eridani. It is the brightest star in Eridanus the River, which winds its way with faint stars from Achernar in a northerly direction to Cursa, a mv = 2.9 star close to brilliant Rigel in Orion. Achernar is midway between Canopus and Fomalhaut, which is setting low in the south-west.
High in the south, about 45 degrees above the horizon, the Large Magellanic Cloud (LMC) is faintly visible as a diffuse glowing patch. To its right and below is the Small Magellanic Cloud (SMC), a smaller glowing patch. The LMC and SMC are described below.
The zodiacal constellations visible tonight, starting from the south-western horizon and heading overhead to the north-east horizon, are Pisces, Aries, Taurus, Gemini, Cancer and Leo.
The amazing thing about the star Mira or Omicron Ceti is that it varies dramatically in brightness, rising to magnitude 2 (brighter than any other star in Cetus), and then dropping to magnitude 10 (requiring a telescope to detect it), over a period of 332 days.
This drop of eight magnitudes means that its brightness diminishes over a period of five and a half months to one six-hundredth of what it had been, and then over the next five and a half months it regains its original brightness. To the ancients, they saw the familiar star fade away during the year until it disappeared, and then it slowly reappeared again. Its not surprising that it became known as 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.
I This month, keen observers with telescopes will be able to
detect Mira 56º (3 handspans) above the north-western horizon at 8 pm at mid-month,
but it is quite faint at tenth magnitude.
Mira near minimum, 26 September 2008 Mira near maximum, 22 December 2008
Astronomers using a NASA space telescope, 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.
The Hunter and his Dogs:
Two of the most spectacular constellations in the sky may be seen high above the eastern horizon soon after sunset in January. These are Orion the Hunter, and his greater dog, Canis Major. Orion straddles the celestial equator, midway between the south celestial pole and its northern equivalent. This means that the centre of the constellation, the three stars known as Orion's Belt, rise due east and set due west. A smaller constellation, the lesser dog Canis Minor, accompanies them.
This is one of the most easily recognised constellations, as it really does give a very good impression of a human figure. From the northern hemisphere he appears to stand upright when he is high in the sky, but from our location ‘down under’ he appears lying down when rising and setting, and upside down when high in the sky. You can, though, make him appear upright when high in the sky (near the meridian), by observing him from a reclining chair, with your feet pointing to the south and your head tilted back.
Orion rising as darkness falls in January
Orion has two bright stars marking his shoulders, the red supergiant Betelgeuse and Bellatrix. A little north of a line joining these stars is a tiny triangle of stars marking Orion’s head. The three stars forming his Belt are, from west to east, Mintaka, Alnilam and Alnitak. These three stars are related, and all lie at a distance of 1300 light years. They are members of a group of hot blue-white stars called the Orion Association.
To the south of the Belt, at a distance of about one Belt-length, we see another faint group of stars in a line, fainter and closer together than those in the Belt. This is the Sword of Orion. Orion’s two knees are marked by brilliant Rigel and fainter Saiph. Both of these stars are also members of the Orion Association.
The Saucepan, with Belt at left, M42 at lower right
Orion is quite a symmetrical constellation, with the Belt at its centre and the two shoulder stars off to the north and the two knee stars to the south. It is quite a large star group, the Hunter being over twenty degrees (a little more than a handspan) tall.
The stars forming the Belt and Sword are popularly known in Australia as ‘The Saucepan’, with the Sword forming the Saucepan’s handle. This asterism appears upside-down tonight, as in the photographs above. The faint, fuzzy star in the centre of the Sword, or the Saucepan's handle, is a great gas cloud or nebula where stars are being created. It is called the ‘Great Nebula in Orion’ or ‘M42’ (number 42 in Messier’s list of nebulae). A photograph of it appears below:
The Sword of Orion, with the Great Nebula, M42, at centre
The central section of the Great Nebula in Orion
New stars are forming in the nebula. At the brightest spot is a famous multiple star system, the Trapezium, illustrated below
Canis Major:
To the right of Orion as twilight ends, a brilliant white star will be seen about one handspan away. This is Sirius, or Alpha Canis Majoris, and it is the brightest star in the night sky with a visual magnitude of -1.43. It marks the heart of the hunter's dog, and has been known for centuries as the Dog Star. As he rises, the dog is on his back with his feet in the air. The star at the end of his front foot is called Mirzam. It is also known as Beta Canis Majoris, which tells us that it is the second-brightest star in the constellation. Mirzam is about one-third of a handspan above Sirius.
The hindquarters of the Dog are indicated by a large right-angled triangle of stars located to the right of Sirius and tilted. The end of his tail is the lower-right corner of the triangle, about one handspan south (to the right) of Sirius.
Both Sirius and Rigel are bright white stars and each has a tiny, faint companion. Whereas a small telescope can reveal the companion to Rigel quite easily, the companion to Sirius the Dog Star, (called ‘the Pup’), can only be observed by using a powerful telescope with excellent optics, as it is very close to brilliant Sirius and is usually lost in the glare (see above).
Canis Major as it appears high in the east soon after sunset in January
Canis Minor:By 10.00 pm this small constellation is about 40 degrees up in the east-north-east. It contains only two main stars, the brighter of which is Procyon (Alpha Canis Minoris). This yellow-white star of mv= 0.5 forms one corner of a large equilateral triangle, the other two corners being the orange Betelgeuse and white Sirius. Beta Canis Minoris is also known as Gomeisa, a blue-white star of mv= 3.1.
Between the two Dogs is the constellation Monoceros the Unicorn, undistinguished except for the presence of the remarkable Rosette Nebula. South of Orion is a small constellation, Lepus the Hare. Between Lepus and the star Canopus is the star group Columba the Dove. Eridanus the River winds its way from near Orion west of the zenith to Achernar, high in the south-west. Between Achernar and the western horizon is the star Fomalhaut, a white star of first magnitude in the small constellation of Piscis Austrinus (the Southern Fish). To the left of Fomalhaut is the triangular constellation of Grus, the Crane. Between the zenith and the south-western horizon are a number of small, faint constellations, Horologium, Pictor, Caelum, Mensa, Tucana, Phoenix, Hydrus and Reticulum. The LMC lies in the constellation Dorado, and the South Celestial Pole is in the very faint constellation Octans.
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 , takes about one 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, at 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.
Alpha Centauri, with Proxima
Knowing the orbital period of the two brightest stars A and B, we can apply Kepler’s Third Law to find the distance they are apart. This tells us that Alpha Centauri A and B are about 2700 million kilometres apart or about 2.5 light hours. This makes them a little less than the distance apart of the Sun and Uranus (the orbital period of Uranus is 84 years, that of Alpha Centauri A and B is 80 years.)
Albireo (Beta Cygni) is sometimes described poetically as a large golden topaz with a small blue sapphire. It is one of the sky’s most beautiful objects. The stars are of classes G and B, making a wonderful colour contrast. It lies at a distance of 410 light years, 95 times further away than Alpha Centauri.
Binary stars may be widely spaced, as the two examples just mentioned, or so close that a telescope is struggling to separated them (Acrux, 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; the two stars take only 17½ minutes to complete an orbit.
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 South Celestial Pole is that point in the southern sky around which the stars appear to 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 at 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 (the star Gacrux) through its base (the star Acrux) and continue straight on 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 (near the south-south-eastern horizon) to Achernar (a little less than two handspans above the south-western horizon at 10 pm at mid-month). Both of these stars will be at about the same altitude, on either side of the South Celestial Pole, at about 11:50 pm. Bisect this line to find 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 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
Galactic Cluster M7 in Scorpiuss
Outside the plane of our galaxy, there is a halo of Globular Clusters. These are very old, dense clusters, sometimes containing over a million stars. These stars are closer to each other than is usual, and because of its great distance from us, a globular cluster gives the impression of a solid mass of faint stars. Many other galaxies also have a halo of globular clusters circling around them.
The largest and brightest globular cluster in the sky is NGC 5139
Globular Cluster NGC 104 in Tucana
Another remarkable globular, second only to Omega Centauri, is well positioned for viewing in the evenings this month. About two degrees to the right of the SMC (see "Two close galaxies
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 appears to lie above 47 Tucanae as we see it in
mid-evening this month. It 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.
* 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 two Index Catalogues (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.
High in the south, to the left of Achernar, two large 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 to the left and slightly above the SMC, and is noticeably larger. They lie at a distance of about 200 000 light years, and are about 60 000 light years apart. They are dwarf galaxies, and they circle our own much larger galaxy, the Milky Way. They are linked to our Galaxy by a long arc of hydrogen, the Magellanic Stream. The LMC is a little 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.
From our latitude both Magellanic Clouds are circumpolar. This means that they are closer to the South Celestial Pole than that Pole's altitude above the horizon, so they never dip below the horizon. They never rise nor set, but are always in our sky. Of course, they are not visible in daylight, but they are there, all the same.
The Large Magellanic Cloud - the bright knot of gas to left of centre is the famous Tarantula Nebula
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 referred to above in the northern half of the sky, and so can be observed in much clearer detail. Our great observatories in Australia, both radio and optical, have for many years been engaged in important research involving these, our nearest inter-galactic neighbours.
The LMC is less than a handspan above the horizon, and the SMC is a little more than a handspan above and slightly to the right of the LMC.
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.
If we look at ninety degrees to the plane, either straight up and out of the galaxy or straight down, we are looking through comparatively few stars and gas clouds and so can see out into deep space. These are the directions of the north and south galactic poles, and because we have a clear view in these directions to distant galaxies, these parts of the sky are called the intergalactic windows. The southern window is in the constellation Sculptor, not far from the star Fomalhaut. This window is well-placed for viewing this month, and many distant galaxies can be observed in this area of the sky. The northern window is between the constellations Virgo and Coma Berenices, roughly between the stars Denebola and Arcturus. It is below the horizon in the evenings this month.
Some of the fainter and apparently insignificant constellations are found around these windows, and their lack of bright stars, clusters and gas clouds presents us with the opportunity to look across the millions of light years of space to thousands of distant galaxies.
Astronomy Picture of the Day
Click here to access a new spectacular picture every day - this link will also provide you with access to a wonderful library of astronomical photographs from telescopes, spacecraft and manned lunar missions.
Virtual Moon freeware>
Study the Moon in close-up, spin it around to see the far side, find the names and physical attributes of craters, seas, ranges and other features, by clicking
Calsky software
Stellarium freeware
New version. Check out where the stars and constellations are, as well as most other sky objects, by clicking
here.
Observatory Home Page and Index