July 2024
Updated: 10 July 2024
Welcome to the night skies of Winter, featuring Hydra, Virgo, Boötes, Hercules, Carina, Crux, Centaurus, Scorpius and Sagittarius
Note: To read this webpage with mobile phones or tablets, please use them in landscape format, i.e. the long screen axis should be horizontal.
The Alluna RC-20 Ritchey Chrétien telescope was installed in March, 2016.
The 20-inch telescope is able to locate and track any sky object (including Earth satellites and the International Space Station) with software called TheSkyX Professional, into which is embedded a unique T-Point model created for our site with the telescope itself.
Explanatory Notes:
Rise and set times are given for the theoretical horizon, which is a flat horizon all the
way round the compass, with no mountains, hills, trees or buildings to obscure the view. Observers will have to make allowance for their own actual horizon.
Transient phenomena are provided for the current month and the next. Geocentric phenomena are calculated as if the Earth were fixed in space
as the ancient Greeks believed. This viewpoint is useful, as otherwise rising and setting times would be meaningless. In the list of geocentric events, the nearer object is
given first.
When a planet is referred to as ‘stationary’, it means that its movement across the stellar background appears to have ceased, not that
the planet itself has stopped. With inferior planets (those inside the Earth’s orbit, Mercury and Venus), this is caused by the planet heading either directly
towards or directly away from the Earth. With superior planets (Mars out to Pluto), this phenomenon is caused by the planet either beginning or ending its
retrograde loop due to the Earth’s overtaking it.
Apogee and perigee: Maximum and minimum distances of the Moon or artificial satellite from the Earth.
Aphelion and perihelion: Maximum and minimum distances of a planet, asteroid or comet from the Sun.
Eclipses always occur in pairs, a lunar and a solar but not necessarily in that
order, two weeks apart.
The
meridian is a semicircle starting from a point on the horizon that is exactly due north from the observer, and arching up into the sky to the zenith and
continuing down to a point on the horizon that is exactly due south. On the way down it passes through the South Celestial Pole which is 26.6 degrees above the
horizon at Nambour. The elevation of the South Celestial Pole is exactly the same as the observer's latitude, e.g. from Cairns it is 16.9 degrees above the
horizon, and from Melbourne it is 37.8 degrees. The Earth's axis points to this point in the sky in the southern hemisphere, and to an equivalent point in the
northern hemisphere, near the star Polaris, which from Australia is always below the northern horizon.
All astronomical objects rise until they reach the meridian, then they begin to set. The act of crossing or 'transitting' the meridian is called 'culmination'.
Objects closer to the South Celestial Pole than its altitude above the southern horizon do not rise or set, but are always above the horizon, constantly
circling once each sidereal day. They are called 'circumpolar'. A handspan at arm's length with fingers spread covers an angle of approximately 18 - 20 degrees.
Your closed fist at arm's length is 10 degrees across. The tip of your index finger at arm's length is 1 degree across. These figures are constant for most people, whatever
their age. The Southern Cross is 6 degrees high and 4 degrees wide, and Orion's Belt is 2.7 degrees long. The Sun and Moon average half-a-degree (30 arcminutes)
across.
mv = visual magnitude or brightness. Magnitude 1 stars are very bright, magnitude 2 less so, and
magnitude 6 stars are so faint that the unaided eye can only just detect them under good, dark conditions. Binoculars will allow us to see down to magnitude
8, and the Observatory telescope can reach visual magnitude 17 or 22 photographically. The world's biggest telescopes have detected stars and galaxies as faint as
magnitude 30. The sixteen very brightest stars are assigned magnitudes of 0 or even -1. The brightest star, Sirius, has a magnitude of -1.44. Jupiter can reach -2.4, and
Venus can be more than 6 times brighter at magnitude -4.7, bright enough to cast shadows. The Full Moon can reach magnitude -12 and the
overhead Sun is magnitude -26.5. Each
magnitude step is 2.51 times brighter or fainter than the next one, i.e. a magnitude 3.0 star is 2.51 times brighter than a magnitude 4.0. Magnitude 1.0
stars are 100 times brighter than magnitude 6.0 (5 steps each of 2.51 times, 2.51x2.51x2.51x2.51x2.51 = 2.515 =
The Four Minute Rule
How long does it take the Earth to complete one rotation? No, it's not 24 hours - that is the time taken for the Sun to cross
the meridian on successive days. This 24 hours is a little longer than one complete rotation, as the curve in the Earth's
orbit means that it needs to turn a fraction more (~1 degree of angle) in order for the Sun to cross the meridian again. It is called a 'solar day'. The stars,
clusters, nebulae and galaxies are so distant that most appear to have fixed positions in the night sky on a human time-scale, and for a star to return to
the same point in the sky relative to a fixed observer takes 23 hours 56 minutes 4.0916 seconds. This is the time taken for the Earth to complete exactly one
rotation, and is called a 'sidereal day'.
As our clocks and lives are organised to run on solar days of 24 hours, and the stars circulate in 23 hours 56 minutes approximately, there is a four minute
difference between the movement of the Sun and the movement of the stars. This causes the following phenomena:
1. The Sun slowly moves in the sky relative to the stars by four minutes of time or one degree of angle per day. Over the
course of a year it moves ~4 minutes X 365 days = 24 hours, and ~1 degree X 365 = 360 degrees or a complete circle. Together, both these facts mean that after the course of a
year the Sun returns to exactly the same position relative to the stars, ready for the whole process to begin again.
2.
For a given clock time, say 8:00 pm, the stars on consecutive evenings are ~4 minutes or ~1 degree further on than they were the previous night. This means
that the stars, as well as their nightly movement caused by the Earth's rotation, also drift further west for a given time as the weeks pass. The stars
of autumn, such as Orion, are lost below the western horizon by mid-June, and new constellations, such as Sagittarius, have appeared in the east. The stars
change with the seasons, and after a year, they are all back where they started, thanks to the Earth's having completed a revolution of the Sun and returned to
its theoretical starting point.
We can therefore say that the star patterns we see in the sky at 11:00 pm tonight will be identical to those
we see at 10:32 pm this day next week (4 minutes X 7 = 28 minutes earlier), and will be identical to those of 9:00 pm this date next month or 7:00 pm the month after. All
the above also includes the Moon and planets, but their movements are made more complicated, for as well as the Four Minute Drift with
the stars, they also drift at different rates against the starry background, the closest ones drifting the fastest (such as the Moon or Venus), and the most distant ones
(such as Saturn or Neptune) moving the slowest.
Observing astronomical objects depends on whether the sky is free of clouds. Not only that, but there are other factors such as wind,
presence of high-altitude jet streams, air temperature, humidity (affecting dew formation on equipment), transparency (clarity of the air), "seeing" (the amount of air
turbulence present), and air pressure. Even the finest optical telescope has its performance constrained by these factors. Fortunately, there is an Australian website that
predicts the presence and effects of these phenomena for a period up to five days ahead of the current date, which enables amateur and professional
astronomers to plan their observing sessions for the week ahead. It is called "SkippySky". The
writer has found its predictions to be quite reliable, and recommends the website as a practical resource. The website is at
http://skippysky.com.au and the detailed Australian data are at
http://skippysky.com.au/Australia/ .
Solar System
Sun:
The Sun begins the month in the zodiacal constellation of Gemini, the Twins. It crosses into Cancer, the Crab on July 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.
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Last Quarter:
July 28
12:52 hrs diameter =
32.1'
New Moon:
August 4
21:14 hrs
diameter = 30.1'
Lunation #1257 begins
Last
Quarter:
August
26
19:27 hrs diameter =
31.7'
Moon at 8 days after New, as on July 15.
The photograph above shows the Moon when approximately eight days after New, just after First Quarter. A rotatable view of the Moon, with ability to zoom in close to the surface (including the far side), and giving detailed information on each feature, may be downloaded
here. A professional version of this freeware with excellent pictures from the Lunar Reconnaissance Orbiter and the Chang orbiter (giving a resolution of 50 metres on the Moon's surface) and many other useful features is available on a DVD from the same website for 20 Euros (about AU $ 33) plus postage
Lunar Feature for this Month
Each month we describe a lunar crater, cluster of craters, valley, mountain range or other object, chosen at random, but one with interesting attributes. A recent photograph from our Alluna RC20 telescope will illustrate the object. As all large lunar objects are named, the origin of the name will be given if it is important. This month we will look at an area along the southern coast of the Mare Humorum (Sea of Moisture). There are numerous spectacular crater plains in the area, and some long clefts, e.g. Rimae Mersenius, Rimae De Gasparis and Rimae Doppelmayer. There is also a long escarpment called the Rupes Liebig.
Between Mersenius and the upper-left corner of this image is a wide rille heading south (to the right). This is 230 kilometres long and 2 kilometres wide, and is the main one of a network of rilles called the Rimae Mersenius. Another network of rilles 130 kilometres long runs south from Liebig to the floor of the 30 kilometre crater De Gasparis, where they intersect with a third network of rilles almost at right-angles. These rilles are called the Rimae De Gasparis and continue through that crater's walls on the far side of that crater and across the flat areas beyond. Running horizontally near the top of the image is a bright line 180 kilometres long. This is a long cliff or escarpment, formed when the level of the Mare Humorum dropped as the lava plain cooled and contracted. It is sometimes called Liebig's Scarp. When this image was taken, the Sun was rising in the Moon's east (above the top of this image), and so brightly illuminated the facing slopes of the fault scarp, the official name of which is the Rupes Liebig. Near the southern end of this cliff, the lava plain is fractured into more rilles, these being known as the Rimae Doppelmayer. More fractures in this network can be seen in the area near the image's top right-hand corner.
Although the two images above were taken thirty months apart, no changes in the lunar features have occurred as there have been no more impacts scarring the moonscape in historical times. The only meteoroids striking the moon in the present day are very small, and the craters they leave are not visible from Earth. The main difference between the two images is that the Sun was slightly higher in the lunar sky for the second one, causing the shadows inside craters to be smaller in extent. This is easily seen in the crater Cavendish. The second image shows an area further to the south-east than the first one. This brings into view another network of rilles, this time crossing the 41 kilometre crater Palmieri (near the top right-hand corner). A large patch of shadow on the lower margin at right of centre is part of the 88 kilometre crater plain Vieta. All of the crater plains in this area, previously overlooked by previous selenographers and therefore anonymous, were given names by the International Astronomical Union (IAU) in 1935.
This crater was named after the 17th century French physicist and philosopher
Marin Mersenne (1588-1648). He developed Mersenne's Laws, which describe the harmonics of vibrating strings as are found in guitars violins and pianos. He wrote a seminal work on music theory, Harmonie universelle, which entitles him
to be recognised as the 'father of acoustics'.
Annibale de Gasparis (1819-1892) was a 19th century Italian astronomer who discovered visually nine asteroids. He authored more than 200 scientific papers on mathematics, celestial
mechanics, astronomy and meteorology. He won the Gold Medal of the Royal Astronomical Society in 1851.
De Gasparis
Palmieri
Luigi Palmieri
(1807-1896) was a 19th century physicist and meteorlogist whose research was on telluric electric currents, terrestrial magnetism and atmospheric electricity. He became Director of the Vesuvius Observatory in 1850.
Cavendish
Hydrogen was first artificially produced in 1671 by Robert Boyle (1627-1691), who discovered and described the reaction between iron filings and dilute acids, which results in the production of a gas. In 1766, Henry Cavendish (1731-1810) was the first to recognise hydrogen as a discrete substance, by naming the gas from the metal-acid reaction "inflammable air", as it ignited explosively if a flame were introduced. He found in 1781 that his "inflammable air" produced water when it burned. He is usually given credit for the discovery of hydrogen as an element, but it remained without a name. In 1783 Antoine Lavoisier, in co-operation with the mathematician Pierre-Simon de Laplace, reproduced Cavendish's finding that water is created when "inflammable air" is burned. They synthesised water by burning jets of "inflammable air" and oxygen in a bell jar over mercury. This revealed that water also was not an element, but was a compound of two gases. Lavoisier gave the "inflammable air" the much more suitable name of hydrogen (hydro meaning water and genes meaning creator).
Jean-Baptiste Joseph Fourier
Click
here
for the
Lunar Features of the Month Archive
July 2: Moon 3.8º north of Mars at 1:03 hrs
Mercury: 
It should be remembered that close approaches of Moon, planets and stars are only perspective effects as seen from the Earth - that is why they are called
'geocentric or Earth-centred phenomena'. The Moon, planets and stars do not really approach and dance around each other as it appears to us from the vantage
point of our speeding planet.
July 2: Neptune at western stationary point at 14:49 hrs (diameter = 2.3")
July 2: Moon 3.9º north of Uranus at 19:16 hrs
July 2: Jupiter 1.3º south of the star 65-Kappa1 Tauri (mv= 4.22) at 21:25 hrs
July 3: Moon occults the star Alcyone (Eta Tauri, mv= 2.85) between 23:57 and 1:14 hrs
July 3: Moon 5.1º north of Jupiter at 18:15 hrs
July 4: Jupiter 1.8º south of the star 69-Upsilon Tauri (mv= 4.28) at 3:25 hrs
July 4: Limb of Moon 13 arcminutes south of the star Elnath (Beta Tauri, mv= 1.65) at 19:16 hrs
July 5: Earth at aphelion at 18:08 hrs
July 7: Moon 1.7º south of the star Pollux (Beta Geminorum, mv= 1.15) at 1:30 hrs
July 7: Moon 3.9º north of Venus at 2:01 hrs
July 8: Moon 3.4º north of Mercury at 4:48 hrs
July 10: Venus at perihelion at 11:31 hrs (diameter = 9.8")
July 14 Moon 1.5º north of the star Spica (Alpha Virginis, mv=0.98) at 12:09 hrs
July 16: Mars 32 arcminutes south of Uranus at 2:13 hrs
July 17: Moon 1.6º south of the star Dschubba (Delta Scorpii, mv= 1.86) at 15.36 hrs
July 17: Moon 1.7º north of the star Pi Scorpii (mv= 2.89) at 16:35 hrs
July 18: Moon occults the star Alniyat (Sigma Scorpii, mv=2.9) between 3:51 and 4:35 hrs
July 18: Limb of Moon 34 arcminutes north of the star Antares (Alpha Scorpii, mv= 0.88) at 6:57 hrs
July 18: Moon 2.4º north of the star 23-Tau Scorpii (mv= 2.82) at 9:31 hrs
July 20: Moon 1.9º north of the star Kaus Media (Delta Sagittarii, mv= 2.72) at 4:28 hrs
July 21: Moon 1.5º south of Pluto at 22:52 hrs
July 22: Jupiter 1.4º south of the star 94-Tau Tauri (mv=4.26) at 3:06 hrs
July 22: Mercury at greatest elongation east (26º 55') at 13:32 hrs (diameter = 7.8")
July 23: Limb of Moon 3 arcminutes south of the star Deneb Algedi (Delta Capricorni, mv= 2.85) at 13.06 hrs
July 23: Pluto at opposition at 15:13 hrs
July 25: Moon 1.1º north of Saturn at 7:29 hrs
July 25: Mercury of the star Regulus (Alpha Leonis, mv=2.9)
July 25: Limb of Moon 14 arcminutes north of Neptune at 22:56 hrs
July 28: Mercury at aphelion at 1:27 hrs (diameter = 8.6")
July 30: Moon 4.1º north of Uranus at 0:22 hrs
July 30: Moon 4.9º north of Mars at 19:26 hrs
July 31: Moon 6.1º north of Jupiter at 8:38 hrs
August 3: Moon 1º south of the star Pollux (Beta Geminorum, mv= 1.15) at 8:16 hrs
August 5: Mercury at eastern stationary point at 14:44 hrs (diameter = 10.0")
August 5: Venus 1º north of the star Regulus (Alpha Leonis, mv= 1.33) at 16:05 hrs
August 6: Moon 3.3º north of the star Regulus (Alpha Leonis, mv= 1.33) at 6:22 hrs
August 6: Moon 2.3º north of Venus st 8:16 hrs
August 6: Moon 7º north of Mercury at 17:08 hrs
August 10: Limb of Moon 21 arcminutes north of the star Spica (Alpha Virginis, mv=0.98) at 22:36 hrs
August 12: Neptune 1.6º north of the star 29 Piscium (mv=5.11) at 12:54 hrs
August 14: Moon 1.8º south of the star Dschubba (Delta Scorpii, mv= 1.86) at 3:09 hrs
August 14: Grazing occultation by the Moon of the star Alniyat (Sigma Scorpii, mv=2.9), centred on 10:31 hrs
August 14: Limb of Moon 9 arcminutes north of the star Antares (Alpha Scorpii, mv= 0.88) at 14:01 hrs
August 14: Moon 1.5º north of the star 23 Tau Scorpii (mv= 2.82) at 18:19 hrs
August 15: Mars 18 arcminutes north of Jupiter at 01:10 hrs
August 16: Moon 2.6º north of the star Alnasl (Gamma Sagittarii, mv= 2.98) at 7:13 hrs
August 16: Moon 1.9º north of the star Kaus Media (Delta Sagittarii, mv= 2.72) at 11:37 hrs
August 17: Moon 1.1º south of the star Nunki (Sigma Sagittarii, mv= 2.02) at 3:50 hrs
August 17: Moon 2.7º north of the star Ascella (Zeta Sagittarii, mv= 2.6) at 5.32 hrs
August 17: Jupiter 30 arcminutes north of the star 102 Iota Tauri (mv=4.62) at 23:10 hrs
August 18: Limb of Moon 27 arcminutes south of Pluto at 8:34 hrs
August 19: Mercury in inferior conjunction with the Sun at 11:51 hrs (diameter = 10.9")
August 19: Limb of Moon 39 arcminutes south of the star Deneb Algedi (Delta Capricorni, mv= 2.85) at 20:29 hrs
August 20: Saturn 1.1º south of the star 90 Phi Aquarii (mv=4.22) at 1:08 hrs
August 20: Uranus at western quadrature at 2:28 hrs (diameter = 3.6")
August 21: Saturn 41 arcminutes north of the star Situla (92 Chi Aquarii, (mv=4.75) at 0:39 hrs
August 21: Limb of Moon 41 arcminutes north of Saturn at 12:34 hrs
August 22: Moon 2.1º north of Neptune at 8:49 hrs
August 26: Moon 5º north of Uranus at 9:18 hrs
August 26: Limb of Moon 9 arcminutes north of the star Alcyone (Eta Tauri, mv= 2.85) at 14:36 hrs
August 27: Moon 5.6º north of Jupiter at 20:54 hrs
August 28: Limb of Moon 8 arcminutes north of the star Elnath (Beta Tauri, mv= 1.65) at 5:09 hrs
August 28: Mars 2º north of the star Alheka (Zeta Tauri, mv= 2.97) at 8:04 hrs
August 28: Moon 5.8º north of Mars at 11:31 hrs
August 29: Mercury at western stationary point at 7:07 hrs (diameter = 9.0")
August 30: Moon 1.6º south of the star Pollux (Beta Geminorum, mv= 1.15) at 16:22 hrs
August 31: Saturn 1.8º north of the star 91 Psi Aquarii (mv= 4.24) at 19:31 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.)
Click
here
for a photographic animation showing the Venusian phases. Venus is always far brighter than anything else in the sky except for the Sun and Moon. For most of 2023, Venus appeared as an 'Evening Star' in
the western twilight sky, but last August it moved to the pre-dawn eastern sky to be a 'Morning Star'. It is now very hard to find, low to the north-western horizon at sunset, as last month it passed behind the Sun
(superior conjunction). It will reappear as an 'Evening Star' in the west at sunset in August. Because Venus is visible as the 'Evening Star' and as the 'Morning Star', astronomers
of ancient times believed that it was two different objects. They called it Hesperus when it appeared in the evening sky and Phosphorus when it was seen
before dawn. They also realised that these objects moved with respect to the so-called 'fixed stars' and so were not really stars themselves, but planets (from the Greek
word for 'wanderers'). When it was finally realised that the two objects were one and the same, the two names were dropped and the Greeks applied a new name Aphrodite
(Goddess of Love) to the planet, to counter Ares (God of War). We use the Roman versions of these names, Venus and Mars, for these two planets.
Mars: At the beginning of July, the red planet is cruising through the constellation Aries, and rises in the east-north-east at about 2:35 am. It is about 57
The waning crescent Moon will be close by Mars on July 2 and July 30.
In this image, the south polar cap of Mars is easily seen. Above it is a dark triangular
area known as Syrtis Major. Dark Sinus Sabaeus runs off to the left, just south of the equator. Between the south polar cap and the equator is a large desert
called Hellas. The desert to upper left is known as Aeria, and that to the north-east of Syrtis Major is called Isidis Regio. Photograph taken in 1971.
Brilliant
Mars at left, shining at magnitude 0.9, passes in front of the dark molecular clouds in Sagittarius on October 15, 2014. At the top margin is the white fourth magnitude
star 44 Ophiuchi. Its type is A3 IV:m. Below it and to the left is another star, less bright and orange in colour. This is the sixth magnitude star SAO 185374,
and its type is K0 III. To the right (north) of this star is a dark molecular cloud named B74. A line of more dark clouds wends its way down through the image
to a small, extremely dense cloud, B68, just right of centre at the bottom margin. In the lower right-hand corner is a long dark cloud shaped like a figure
5. This is the Snake Nebula, B72. Above the Snake is a larger cloud, B77. These dark clouds were discovered by Edward Emerson Barnard at Mount Wilson in 1905.
He catalogued 370 of them, hence the initial 'B'. The bright centre of our Galaxy is behind these dark clouds, and is hidden from view. If the clouds were
not there, the galactic centre would be so bright that it would turn night into day.
Venus at 6.55 pm on September 7, 2018. The phase is 36 % and the angular diameter is 32 arcseconds.
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.
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 north-eastern pre-dawn sky, and can be found around 4:30 am in the first week of July. The waning crescent Moon will be just to the left of Jupiter, close to the north-eastern horizon at 4:45 am on July 3 and July 31.
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 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 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 at opposition, June 11, 2019
Jupiter reached opposition on June 11, 2019 at 01:20 hrs, and the above photographs were taken that evening, some
twenty to twenty-two hours later. The first image above was
taken at 10:01 pm, when the Great Red Spot was leaving Jupiter's central meridian and the satellite Europa was preparing to transit Jupiter's disc.
Europa's transit began at 10:11 pm, and its shadow touched Jupiter's cloud tops almost simultaneously. Europa was fully in transit by 10:15 pm. The second photograph was
taken two minutes later at 10:17 pm, with the Great Red Spot heading towards Jupiter's western limb.
The third photograph was taken at 10:41 pm, when Europa was
about a third of its way across Jupiter. Its dark shadow is trailing it, slightly below, on the clouds of the North Temperate Belt. The shadow is partially eclipsed by Europa itself. The fourth
photograph at 10:54 pm shows Europa and its shadow about a quarter of the way across. This image is enlarged below. The fifth photograph shows Europa on Jupiter's central meridian at 11:24
pm, with the Great Red Spot on Jupiter's limb. The sixth photograph taken at 11:45 pm shows Europa about two-thirds of the way through its transit, and the
Great Red Spot almost out of sight. In this image, the satellite Callisto may be seen to the lower right of its parent planet. Jupiter's elevation above the horizon for
the six photographs in order was 66º, 70º, 75º, 78º, 84º and 86º. As the evening progressed, the 'seeing' proved quite variable.
There have been numerous alterations to Jupiter's belts and spots over the thirteen months since the 2018 opposition. In particular, there have
been major disturbances affecting the Great Red Spot, which appears to be slowly changing in size or "unravelling".
It was very fortuitous that, during the evenings of the days when the 2018 and 2019 oppositions occurred, there was a transit of one of the satellites as well as the
appearance of the Great Red Spot. It was also interesting in that the same satellite, Europa, was involved both times.
Jupiter'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 July 1 may be found about
Left: Saturn showing the Rings when edge-on. Right: Over-exposed Saturn surrounded by its satellites Rhea, Enceladus, Dione, Tethys and Titan - February 23/24, 2009.
Saturn with its Rings wide open on
July 2, 2017. The shadow of its globe can just be seen on the far side of the Ring system. There are three main concentric rings: Ring A is the outermost, and is separated
from the brighter Ring B by a dark gap known as the Cassini Division, which is 4800 kilometres wide, enough to drop Australia through. Ring A also has a gap inside it, but
it is much thinner. Called the 'Encke Gap', it is only 325 kilometres wide and can be seen in the image above. The innermost parts of Ring B are not as bright as its
outermost parts. Inside Ring B is the faint Ring C, almost invisible but noticeable where it passes in front of the bright planet as a dusky band. Spacecraft visiting
Saturn have shown that there are at least four more Rings, too faint and tenuous to be observable from Earth, and some Ringlets. Some of these extend from the inner edge of
Ring C to Saturn's cloudtops. The Rings are not solid, but are made up of countless small particles, 99.9% water ice with some rocky material, all orbiting Saturn at
different distances and speeds. The bulk of the particles range in size from dust grains to car-sized chunks. At bottom centre, the southern hemisphere of the planet can be
seen showing through the gap of the Cassini Division. The ring system extends from 7000 to 80 000 kilometres above Saturn's equator, but its thickness varies from only 10
metres to 1 kilometre. The globe of Saturn has a diameter at its equator of 120 536 kilometres. Being made up of 96% hydrogen and 3% helium, it is a gas giant,
although it has a small, rocky core. There are numerous cloud bands visible.
Neptune:
The photograph above was taken when Saturn was close to opposition, with the Earth between Saturn and the Sun. At that time, the shadow of Saturn's globe upon the
Ring system was directly behind the planet and hardly visible. The photograph below was taken at 7:14 pm on September 09, 2018, when Saturn was near eastern quadrature.
At such a time, the angle from the Sun to Saturn and back to the Earth is near its maximum, making the shadow fall at an angle across the Rings as seen from
Earth. It may be seen falling across the far side of the Ring to the left side of the globe.
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 March 2025. They will appear so thin that it will seem that
Saturn has no rings at all. This time, the rings will disappear from sight in March 2025. They’ll gradually come back into view as seen through large telescopes, before sliding out of view again in November 2025.
After that, the rings will gradually become more and more open. The rings will be wide-open again in 2032.
Uranus:
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 east-north-eastern horizon in mid-July, at 5 am:
The highest is Saturn, which is at an altitude of 62º (three and a half handspans) above the north-north-western horizon. 11º (a little more than half a handspan) east of Saturn is Neptune, which will require the use of a small telescope to find. 56º (three handspans) east of Neptune is Mars, which has an obvious orange-red colour. 45 arcminutes (three quarters of a degree) north-east of Mars is Uranus, which also requires the use of a telescope. 15 degrees below Mars, and 15 degrees above the theoretical horizon, bright Jupiter can be seen. The waning crescent Moon will pass through this group between June 28 and July 3, and between July 25 and 31.
Meteor Showers:
Radiant: Near the star Markab
S Delta Aquarids
July 29
Waning crescent Moon, 43%
sunlit ZHR = 20
Radiant:
Between the stars Skat and Deneb Algedi
Alpha Capricornids July 30
Waning crescent Moon, 34% sunlit ZHR =
8
Radiant: Near the star Algedi
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.
A potentially great comet is currently moving sunward, promising a spectacular show later this year. That comet, C/2023 A3 (Tsuchinshan-ATLAS), was discovered in
January 2023, and astronomers soon realised it has the potential to become truly dazzling. Current predictions suggest Tsuchinshan-ATLAS will be at least as bright as the brightest stars in late September and early October this year. During that time, it will
pass almost directly between Earth and the Sun. It might even briefly become visible in broad daylight at that time. In the days following that chance alignment, the comet will gradually become visible in the evening sky and could be an incredible sight, up to
a hundred times brighter than Pons–Brooks at its best.
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 in 2007 at Lulin Observatory by a collaborative team of Taiwanese and Chinese astronomers, is now in the outer Solar System, and has faded below magnitude 15.
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 Eta Carinae Nebula is not well placed for viewing, being high in the south-south-west as darkness falls. It culminates at 3 pm at mid-month, and is visible until 9 pm.
The Stars and Constellations for this
In the north-west, Leo the Lion is setting. It will have completely disappeared by 10.30 pm. The bright star
Regulus (Alpha Leonis) marks the Lion’s heart. A handspan to the right and above Regulus is Denebola, a white star marking the lion's tail. It is about 30 degrees
above the north-western horizon. We see the lion upside-down from the Southern Hemisphere. Regulus is the western-most star in a pattern called 'The Sickle' (or
reaping-hook). It marks the end of the Sickle's handle, with the other end of the handle, the star
Eta Leonis, below and to the right. The blade of the Sickle
curves around clockwise from Eta Leonis to the horizon. The Sickle forms the mane and head of the lion, when observed right-way-up. The Sickle is just touching the
theoretical horizon at this time tonight.
The constellation Leo, as we see it from Australia. Regulus is above centre left, and Denebola above centre right. The Sickle curves down from Regulus.
High in the north, (about 43 degrees above the horizon, and about 10 degrees west of the meridian or north-south line) we can find the third brightest star in the night sky,
Arcturus. It is outshone only by Sirius and Canopus. Arcturus differs from those just named, for it is an obvious orange colour, a K2 star of zero magnitude.
It is a particularly beautiful star, and, as it is the brightest in the constellation of Boötes, the Herdsman, it has the alternative name of Alpha Boötis
. (Boötes is pronounced 'Bo-oh-tees). Boötes is due north (culminating) at this time of night.
Arcturus
East of Boötes and above the north-north-eastern horizon is a fainter circle of fourth magnitude stars, Corona Borealis, the Northern Crown. The
brightest star in the Crown is named Alphecca, and shines at magnitude 2.3.
East of the Northern Crown is Hercules, stretching from the north-north-eastern horizon upwards. Rising in the north-east is a bright white A0 star,
Vega, which is the fifth brightest star, after Sirius, Canopus, Arcturus and Alpha Centauri. Vega is the main star in the small constellation of
Lyra the Lyre, which contains the famous Ring Nebula, M 57.
The Ring Nebula was driven away from the central star by powerful stellar winds.
About fifteen degrees (a little less than a handspan) to the right of Vega can be seen Albireo, a beautiful double star with contrasting colours. It is the highest
star of the Northern Cross, Cygnus.
Rising above the eastern horizon is the great main-sequence star Altair. This A7 white star is the eleventh brightest in the heavens. Altair is also known as Alpha Aquilae, as it is the brightest star in the constellation of Aquila, the Eagle. It marks the heart of the eagle, and is flanked by two lesser stars marking each wing-tip, Gamma Aquilae and Beta Aquilae. This threesome, making a short horizontal line in the east, is easy to find.
Just to the west of the zenith is the next zodiacal constellation after Leo, Virgo, the Virgin. It is a large but
fairly inconspicuous constellation, but it does have one bright star, Spica, which is an ellipsoidal variable star whose brightness averages magnitude 1. This star,
also known as Alpha Virginis, is a hot, blue-white star of spectral type B2. It is the sixteenth brightest star, and the rest
of the constellation Virgo lies to the north-west of it. Tonight, Spica is at an altitude of 65 degrees, between the zenith
and Corvus. It is roughly halfway between Arcturus and the Southern Cross.
The Quasi-stellar Object 3C-273 is an extremely remote but powerful energy source in Virgo. It shines at magnitude 13 and looks like a faint blue star. Actually it is a violently exploding galaxy, about 1000 times as far away
as the Great Galaxy in Andromeda. It lies about a quarter of the distance from Porrima to Denebola.
The Quasi-Stellar Object 3C-273 lies at a distance of 2440 million light years, over one sixth of the way to the edge of the universe. Click here to find the story about how it was identified by the Parkes Radio Telescope in 1962.
Directly overhead is the faint constellation of Libra, the Scales, the brightest stars of which are two of magnitude 2.7 with exotic names, Zuben Elgenubi and Zuben Eschamali. Two handspans west of the zenith is the constellation of
Corvus the Crow. Corvus is a lopsided quadrilateral of four third magnitude stars. It is about three handspans above the western horizon.
Approaching the zenith is the spectacular constellation of Scorpius, the Scorpion (see below), which is very rich in objects to
find with a small telescope or binoculars. This famous zodiacal constellation is like a large letter 'S', and, unlike most constellations, is easy to recognise as the shape
of a scorpion. At this time of year, he has his tail down and claws raised. The brightest star in Scorpius is Antares, a red type M supergiant of magnitude 0.9.
Antares is the fifteenth brightest star, and will be almost exactly overhead at 9:40 pm on July 1 (4 minutes earlier per night for succeeding nights).
Adjoining Sagittarius to the south (right), there is a beautiful curve of 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. Below Sagittarius and above the eastern horizon is a large constellation known as
Capricornus, the Sea-Goat. This constellation is lacking in any bright stars, as is the next zodiacal constellation of Aquarius the Water-bearer. This
latter constellation will not be fully risen at mid-month until 9 pm, but if we wait until then, we will see that it presently contains the ringed planet Saturn, a
wonderful sight in a small telescope and brighter than any neighbouring stars. Saturn presently rises almost due east, and at present is located at Aquarius' hips.
The Trifid Nebula, M20, in Sagittarius, is composed of a reflection nebula (blue), an emission nebula (pink), and dark lanes of dust.
The body of Scorpius is at top, with the two stars in the Sting underneath, just
above the centre of the picture. The red supergiant star Antares appears close to the top left corner. The stars in the lower
half of the picture are in Sagittarius. Near the lower right margin is a graceful curve of fourth magnitude stars, Corona
Australis, the Southern Crown.
High in the south-south-west, Crux (Southern Cross) is at an altitude of 50 degrees. Crux was in a vertical position about two hours ago (6.00
pm on July 1), but now it is has tilted over to the west so that it leans at an angle of 30 degrees from the vertical. The two Pointers, Alpha and Beta
Centauri, lie to its left and form a horizontal line. The two pointers are 8 degrees apart. Alpha is the one further away from Crux. Whereas Alpha Centauri is the
nearest star system to our Sun, only 4.37 light-years distant, Beta is 390 light-years away. Alpha is composed of two Sun-like stars, but Beta Centauri is
a supergiant, which accounts for its appearing almost as bright despite being nearly 90 times further away. If the night is dark and the skies are clear, a black dust
cloud known as the Coalsack can be seen just to the left of Acrux, the bottom 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 to its
right-hand side, where it adjoins Carina and Vela.
At left - the two Pointers, Alpha and Beta Centauri. Centre - Crux (Southern Cross) with the dark cloud of dust known as the Coalsack at its lower left. Right - star clusters in the Milky Way and the Eta Carinae nebula.
Just to the left of 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
>
Herschel's Jewel Box
Below Crux and to its left is a small, fainter quadrilateral of stars, Musca, the Fly. Out of all the 88 constellations, it is the only insect.
Below and to the left of Alpha Centauri is a (roughly) equilateral triangle of 4th magnitude stars. This is the constellation Triangulum Australe, the Southern
Triangle. 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.
Between Crux and the south-western horizon is a very large area of sky filled with interesting objects. This was once the constellation Argo, named by ancient
Greeks for Jason’s famous ship used by the Argonauts in their quest for the Golden Fleece. The constellation Argo was found to be too large, so modern star atlases divide
it into three sections - Carina (the Keel), Vela (the Sails) and Puppis (the Stern).
The constellations surrounding the Southern Cross
Very close to the south-south-western horizon, the star Canopus may be glimpsed soon after darkness falls.
It will have set by 7:30 pm on July 16. You will need a flat horizon in this direction. Canopus is the second-brightest star in the sky after Sirius, the Dog Star. The path of the Milky Way between Aquila and Canopus is filled with clusters, dark clouds, glowing nebulae, multiple stars and other
interesting objects. Check it out with binoculars or a telescope. Halfway between Crux and the southern horizon is a white star of magnitude 1.7, Miaplacidus. It is the second-brightest star in the constellation
Carina, after Canopus, so it has the alternative name of Beta Carinae. Half a handspan to the right of Miaplacidus is the False Cross, larger and more
lopsided than the Southern Cross. The False Cross is two handspans below Crux, and is also tilted in the same way. It is about a handspan above the south-western
horizon, and will have completely set by midnight. Both of these Crosses are actually more like kites in shape, for, unlike Cygnus (the Northern Cross, rising in
the north-north-east just before midnight on July 1), they have no star at the intersection of the two cross arms. Between the Southern Cross and the False Cross may be seen a glowing patch of light. This is
the famous Eta Carinae Nebula, which is a remarkable sight through binoculars or a small telescope working at low magnification. A photograph of this emission
nebula with dark lanes appears below. The brightest star in the nebula, Eta Carinae itself, is a peculiar unstable star which has been known to explode, becoming
very bright. It last did this in 1842, and is called a 'cataclysmic variable star', or 'recurrent nova'. It is also an extremely large and massive star, and is a possible
candidate for the next supernova in our Galaxy.
The central part of the Eta Carinae nebula, showing dark lanes, molecular clouds, and glowing clouds of fluorescing hydrogen.
The Keyhole, a dark cloud obscuring part of the Eta Carinae Nebula.
The Homunculus, a tiny planetary nebula ejected by the eruptive variable star, Eta Carinae.
Low in the south-south-west, about 10 degrees above the horizon, the Large Magellanic
Cloud (LMC) is faintly visible as a diffuse glowing patch. It is about a handspan to the left (south) of Canopus. About a handspan to the left of the LMC is the
Small Magellanic Cloud (SMC), a smaller glowing patch, also close to the southern horizon. 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. Between Arcturus and Denebola, and 30 degrees above the north-western horizon, is a faint Y-shaped cluster of stars called Coma Berenices, or
Berenice's Hair. Most of the stars in this group have a visual magnitude of about 4.5. The area of sky between Spica and Coma Berenices is called a 'galactic window'. Being well away from the plane of the Milky Way (which we can see
passing from the west-south-western horizon through the star clusters and nebulae of Carina to Crux and Centaurus high in the south, and then through Scorpius and
Sagittarius high in the east to Aquila on the east-north-eastern horizon), there are fewer stars and dust clouds to obscure our view, and we can see right out of our galaxy
into the depths of inter-galactic space. A 20 cm (eight inch) telescope can see numerous galaxies in this region, nearly thirty being brighter than twelfth magnitude. A
larger amateur telescope can detect hundreds more. Large telescopes equipped with sensitive cameras can detect millions of galaxies in this part of the sky. The line of the ecliptic along which the Sun, Moon and planets travel, passes through the following constellations this month: Leo, Virgo, Libra,
Scorpius, Sagittarius and Capricornus. 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 of Scorpius forms the Emu's
body. The Emu is sitting, waiting for its eggs to hatch. The eggs are the large and bright star clouds of Sagittarius. If you would like to become familiar with the constellations, we suggest that you access one of the world's best collections of constellation
pictures by clicking
The Milky Way
A glowing band of light crossing the sky is especially noticeable during the winter months. This glow is the light of millions of faint stars combined with that coming from
glowing gas clouds called nebulae. It is concentrated along the plane of our galaxy, and this month it is seen crossing the sky, starting from the south-west in the constellation Carina, and passing through Crux, Centaurus,
Lupus, Scorpius, Sagittarius and Scutum to Aquila and Lyra in the north-east.
The plane of our galaxy from Scutum (at left) through Sagittarius and Scorpius (centre) to Centaurus and Crux (right). The Eta Carinae nebula is at the right margin, below centre. The Coalsack is clearly visible, and the dark dust lanes can be seen. Taken with an ultra-wide-angle lens.
It is rewarding to scan along this band with a pair of binoculars, looking for star clusters and
emission nebulae. Dust lanes along the plane of the Milky Way appear to split it in two in some parts of the sky. One of these lanes can be easily seen, starting
near Alpha Centauri and heading towards Antares. At 10.00 pm in mid-July, the Milky Way crosses the zenith, almost dividing the sky in two. It runs from
south-west to north-east, and the very centre of our galaxy passes directly overhead.
The centre of our galaxy. The constellations partly visible here are Sagittarius (left), Ophiuchus (above centre) and Scorpius (at right). The planet Jupiter is the bright object below centre left. This is a normal unaided-eye view, except that it reaches fainter stars than the eye can see.
The Season of the Scorpion
The spectacular constellation of Scorpius is about 45 degrees above the eastern horizon at sundown in July. 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.
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 root of the scorpion's sting. These two stars are closest to the eastern horizon tonight, and are near the bottom of the picture below. Above 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 red supergiant star Antares.
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.) 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 III on the Antoniadi Scale, or in other words about fair. Image acquired at Starfield Observatory in Nambour on July 1, 2017.
Some fainter constellations
Between Regulus and Alphard is the inconspicuous constellation of Sextans, the Sextant. Between Sextans and the quadrilateral of Corvus, the Crow is another faint star group, Crater, the Cup.
Between the Milky Way and the southern horizon may be found the lesser-known constellations of Apus the Bird of Paradise, Chamaeleon, Pavo the Peacock, Octans the Octant, Mensa the Table Mountain, Dorado the Goldfish, Indus the Indian, Hydrus the Southern Water Snake, Pictor the Painter's Easel and Telescopium the Telescope.
Why are some constellations bright, while others are faint ?
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 northern window is between the constellations Virgo and Coma Berenices, roughly between the stars Denebola and Arcturus.
This window is 50 degrees above the north-western horizon early in the evenings this month, so it's a good time for observing galaxies in the Virgo cluster. The southern window is in the constellation Sculptor, not far from the star Fomalhaut. This window will be rising well before midnight. 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.
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 southern horizon for another 4 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 find the South Celestial Pole from southern Queensland is to choose a time when the Southern Cross is vertical (6.00 pm on July 1), and simply locate that spot in the sky which is midway between the bottom star of the Cross (Alpha Crucis), and the theoretical southern horizon.
Interesting photographs of this area can be taken by using a camera on time exposure. Set the camera on a tripod pointing due south, and open
the shutter for thirty minutes or more. The Earth's rotation will cause the stars to appear to 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.
A wide-angle view of trails around the South Celestial Pole, with Scorpius and Sagittarius at left, Crux and Centaurus at top, and Carina and False Cross at right.
Star trails between the South Celestial Pole and the southern horizon. All stars that do not pass below the horizon are circumpolar.
Double and multiple stars
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 (Alpha Centauri) at left, and Albireo (Beta Cygni) at right.
Rigel (Beta Orionis, left) is a binary star which is the seventh brightest star in the night sky. Rigel A is a large white supergiant which is 500 times brighter than its small companion, Rigel B, yet Rigel B is itself composed or a very close pair of Sun-type stars that orbit each other in less than 10 days. Each of the two stars comprising Rigel B is brighter in absolute terms than Sirius (see above). The Rigel B pair orbit Rigel A at the immense distance of 2200 Astronomical Units, equal to 12 light-days. (An Astronomical Unit or AU is the distance from the Earth to the Sun.) In the centre of the Great Nebula in Orion (M42) is a multiple star known as the Trapezium (right). This star system has four bright white stars, two of which are binary stars with fainter red companions, giving a total of six. The hazy background is caused by the cloud of fluorescing hydrogen comprising the nebula.
Acrux, the brightest star in the Southern Cross, is also known as Alpha Crucis. It is a close binary, circled by a third dwarf companion.
Alpha Centauri (also known as Rigil Kentaurus, Rigil Kent or Toliman) is a binary easily seen with the
smallest telescope. The components are both solar-type main sequence stars, one of type G and the other, slightly cooler and fainter, of type K. Through a small
telescope this star system looks like a pair of distant but bright car headlights. Alpha Centauri A and B take 80 years to complete an orbit, but a tiny third component,
the 11th magnitude red dwarf Proxima Centauri , takes about 1 million years to orbit the other two. It is about one tenth of a light year from the bright pair and a little
closer to us, hence its name. This makes it our nearest interstellar neighbour, with a distance of 4.3 light years. Red dwarfs are by far the most common type of star, but,
being so small and faint, none is visible to the unaided eye. Because they use up so little of their energy, they are also the longest-lived of stars. The bigger a
star is, the shorter its life.
Close-up of the star field around Proxima Centauri
Knowing the orbital period of the two brightest stars A and B, we can apply Kepler’s Third Law to find the distance they are apart. This tells us
that Alpha Centauri A and B are about 2700 million kilometres apart or about 2.5 light hours. This makes them a little less than the distance apart of the Sun and Uranus
(the orbital period of Uranus is 84 years, that of Alpha Centauri A and B is 80 years.)
Albireo (Beta Cygni) is sometimes described poetically as a large topaz with a small blue sapphire. It is one of the sky’s most beautiful objects. The stars are of classes G and B, making a wonderful colour contrast. It lies at a distance of 410 light years, 95 times further away than Alpha Centauri.
Binary stars may be widely spaced, as the two examples just mentioned, or so close that a telescope is struggling to separated them (Acrux, Castor, Antares, Sirius). Even closer double stars cannot be split by the telescope, but the spectroscope can disclose their true nature by revealing clues in the absorption lines in their spectra. These examples are called spectroscopic binaries. In a binary system, closer stars will have shorter periods for the stars to complete an orbit. Eta Cassiopeiae takes 480 years for the stars to circle each other. The binary with the shortest period is AM Canum Venaticorum, which takes only 17½ minutes.
Sometimes one star in a binary system will pass in front of the other one, partially blocking off its light. The total light output of the pair will be seen to vary, as regular as clockwork. These are called eclipsing binaries, and are a type of variable star, although the stars themselves usually do not vary.
Star Clusters
The two clusters in Taurus, the Pleiades and the Hyades, are known as Open Clusters or
Galactic Clusters. The name 'open cluster' refers to the fact that the stars in the cluster are grouped together, but not as tightly as in globular
clusters (see below). The stars appear to be loosely arranged, and this is partly due to the fact that the cluster is relatively close to us, i.e. within
our galaxy, hence the alternate name, 'galactic cluster'. These clusters are generally formed from the condensation of gas in a nebula into stars, and some
are relatively young.
The photograph below shows a typical open cluster, M7
*. It lies in the constellation Scorpius, just below the scorpion's sting. It lies in the direction of our galaxy's centre. The cluster itself is the group of white stars in the centre of the field. Its distance is about 380 parsecs or 1240 light years. M7 is visible tonight.
Galactic Cluster M7 in Scorpius
Outside the plane of our galaxy, there is a halo of Globular Clusters. These are very old, dense clusters, containing perhaps several hundred thousand stars. These stars are closer to each other than is usual, and because of its great distance from us, a globular
cluster gives the impression of a solid mass of faint stars. Many other galaxies also have a halo of globular clusters circling around them.
The largest and brightest globular cluster in the sky is NGC 5139
**, also known as Omega Centauri. It has a slightly oval shape. It is an outstanding winter object, and this month it is observable all night. 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 hundreds 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.
The globular cluster
The central core of Omega Centauri
There is another remarkable globular, second only to Omega Centauri. About two degrees below 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
Globular Cluster NGC 104 in Tucana
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 the Index Catalogue (IC). Many non-stellar objects in the sky have therefore NGC numbers or IC numbers. For example, the famous Horsehead Nebula in Orion is catalogued as IC 434. The NGC was revised in 1973, and lists 7840 objects.
The recent explosion of discovery in astronomy has meant that more and more catalogues are being produced, but they tend to specialise in particular types of objects, rather than being all-encompassing, as the NGC / IC try to be. Some examples are the Planetary Nebulae Catalogue (PK) which lists 1455 nebulae, the Washington Catalogue of Double Stars (WDS) which lists 12 000 binaries, the General Catalogue of Variable Stars (GCVS) which lists 28 000 variables, and the Principal Galaxy Catalogue (PGC) which lists 73 000 galaxies. The largest modern catalogue is the Hubble Guide Star Catalogue (GSC) which was assembled to support the Hubble Space Telescope's need for guide stars when photographing sky objects. The GSC contains nearly 19 million stars brighter than magnitude 15.
Two close naked-eye galaxies
Close to the southern horizon, 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 to the right of the SMC, and is noticeably larger. They lie at distances of 190 000
light years for the LMC, and 200 000 light years for the SMC. They are about 60 000 light years apart. These dwarf galaxies circle our own much larger galaxy,
the Milky Way. The LMC is slightly closer, but this does not account for its larger appearance. It really is larger than the SMC, and has developed as an
under-sized barred spiral galaxy.
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 (below).
These two Clouds are the closest galaxies to our own, but lie too far south to be seen by the large telescopes in Hawaii, California and
Arizona. They are 15 times closer than the famous Andromeda and Triangulum galaxies, and so can be observed in much
clearer detail. Our great observatories in Australia, both radio and optical, have for many years been engaged in important research involving these, our
nearest inter-galactic neighbours.
Astronomy