July
2020
Updated: 8 July 2020
Welcome to the night skies of Winter, featuring Virgo, Carina, Crux, Centaurus, Scorpius, Sagittarius, Jupiter, Saturn and Mars
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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.
The zenith is the point in the sky directly overhead from the observer.
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 exactly 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.
Solar System
Sun:
The Sun begins the month in the zodiacal constellation of Gemini, the Twins. It leaves Gemini and passes into Cancer, the Crab on July 20.
Penumbral
Moon at 8 days after New, as on July 28.
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.Click
here for a photographic animation showing the lunar phases. It also shows the Moon's wobble or libration, and how its apparent size changes as it moves from perigee to apogee each month. It takes a little while to load, but once running is very cool ! All these downloads are freeware, although the authors do accept donations if the user feels inclined to support their work.
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 a rugged area where the Apollo 14 astronauts landed on February 5, 1971.
There are four small craters on the floor of Fra Mauro, each about 3 kilometres
across. A fifth crater, slightly larger, is by chance near the exact centre of
Fra Mauro. It is called Fra Mauro E. These five craters are surrounded by
hundreds of smaller craterlets. The above image shows craterlets down to a
diameter of 800 metres.
South of Fra Mauro and deforming its southern wall are two smaller, flat-floored
crater plains. The larger of the two is called Bonpland, and is 60 kilometres
across. To its east (right) is Parry, 48 kilometres across. Although
these two crater plains are similar in structure to Fra Mauro and all three are
very ancient, Parry must be slightly younger as it overlaps the other two.
Crossing these three crater plains is a network of rilles or grabens, called the
Rimae Parry. Their combined lengths total 300 kilometres, and they traverse
the flat plains and climb over mountainous crater walls without deviating from
their path.
Fra Mauro (c.1400-1464) was a cartographer (map-maker) who lived in the Republic of Venice in what it now northern Italy.
He was a monk, hence his title 'Fra' or 'Father'. He was widely travelled, and was commissioned by King Alfonso V of Portugal (a nephew of Prince Henry the
Navigator) to create a 'mappa mundi' (world map) that would be much more accurate than existing maps. Venice was at the time the "crossroads of the
world", a hub where travellers, explorers, soldiers, sailors, merchants, traders and refugees all passed through as they moved between Europe, Africa and Asia,
following in the footsteps of Marco Polo, a Venetian who had found a land route to India, Cathay and Cipango (Japan) in the previous century. It was an ideal
place for a cartographer to gather information about the world, and Fra Mauro questioned hundreds, perhaps thousands of these people in his quest for
knowledge. Information he received was filed and then cross-checked at the extensive library in his monastery overlooking the Venetian lagoon. In modern
terms, he was using crowd-sourcing to create the Google Earth of his time.
The map he produced in 1450 was 2.4 metres square. It contained almost 3000 annotations of pertinent information.
Known as the greatest of all medieval maps, it remained in the Biblioteca
Nazionale Marciana in Venice for the last 550 years. It left Venice for the
first time in 2013, and made a visit
to the National Library of Australia in Canberra for an exhibition. A wonderful work of art, many of the countries in Europe are accurately shown, and are easily
recognisable to the viewer, except that south is at the top, as opposed to the modern convention. The further one
examines the map from the Mediterranean, the more noticeable are
inaccuracies, and imagination plays a part. Far Mauro's map shows a sea-route to India, which was not discovered until Vasco da Gama's voyage in 1495. As
Columbus would not sail across the Atlantic to the New World for another 42 years, the Americas (and Antarctica and Australia) are not shown at all. Click
here
Fra Mauro
Click 
The crater plains Fra Mauro,
Bonpland and Parry are located inside the rectangle above.
Geocentric Events:
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 1: Jupiter 41 arcminutes north of Pluto at 00:00 hrs
August 1: Moon 2.1º north of the star Kaus Borealis (Lambda Sagittarii, mv= 2.82) at 09:41 hrs
M
July 1: Mercury at inferior conjunction at 12:50 hrs (diameter = 11.9")
July 2: Moon 1.8º north of the star Graffias (Beta-1 Scorpii, mv= 2.56) at 16:05 hrs
July 2: Mercury 2.2º north of the star Alhena (Beta-1 Scorpii, mv= 2.56) at 02:05 hrs
July 5: Moon 1.7º north of the star Kaus Borealis (Lambda Sagittarii, mv= 2.82) at 03:43 hrs
July 5: Earth at aphelion at 05:55 hrs
July 6: Moon 1.2º south of Jupiter at 09:14 hrs
July 6: Limb of Moon 17 arcminutes south of Pluto at 09:57 hrs
July 6: Moon 2.1º south of Saturn at 18:02 hrs
July 8: Limb of Moon 52 arcminutes south of the star Nashira (Gamma Capricorni, mv= 3.69)
at 13:11 hrs
July 8: Moon 1.1º south of the star Deneb Algiedi (Alpha Capricorni, mv= 2.85) at 15:35 hrs
July 10: Moon 3.9º south of Neptune at 19:32 hrs
July 10: Venus at aphelion at 22:07 hrs
July 12: Moon 1.1º south of Mars at 08:05 hrs
July 12: Venus 57 arcminutes north of the star Aldebaran (Alpha Tauri, mv= 0.87) at 09:29 hrs
July 12: Mercury at western stationary point at 18:16 hrs (diameter = 10.1")
July 14: Jupiter at opposition at 17:33 hrs (diameter = 47.6")
July 14: Moon 3.4º south of Uranus at 22:48 hrs
July 16: Pluto at opposition at 04:48 hrs (diameter = 0.1")
July 17: Moon 3.3º north of Venus at 17:48 hrs
July 18: Moon 2.2º north of the star Alheka (Zeta Tauri, mv= 2.97) at 16:42 hrs
July 19: Moon 1.7º north of the star Propus (Eta Geminorum (mv= 3.31) at 05:50 hrs
July 19: Moon 2.1º north of the star Tejat Posterior (Mu Geminorum, mv= 2.87) at 10:46 hrs
July 19: Moon 4.3º north of Mercury at 16:01 hrs
July 19: Moon 41 arcminutes south of the star Mebsuta (Epsilon Geminorum (mv= 3.06) at 19:59 hrs
July 21: Saturn at opposition at 08:15 hrs (diameter = 18.4")
July 23: Mercury at greatest elongation west (20º 01') at 08:22 hrs (diameter = 7.7")
July 30: Moon 1.3º north of the star Graffias (Beta-1 Scorpii, mv= 2.56) at 01:14 hrs
August 2: Limb of Moon 33 arcminutes south of Jupiter at 10:03 hrs
August 2: Venus 1.8º south of the star Alheka (Zeta Tauri, mv= 2.97) at 14:42 hrs
August 2: Limb of Moon 23 arcminutes south of Pluto at 11:45 hrs
August 2: Uranus at western quadrature at 21:07 hrs (diameter = 3.5")
August 3: Moon 2.1º south of Saturn at 00:51 hrs
August 3: Mars at perihelion at 21:36 hrs (diameter - 14.9")
August 4: Moon 1.5º south of the star Nashira (Gamma Capricorni, mv= 3.69) at 19:48 hrs
August 5: Moon 1.5º south of the star Deneb Algiedi (Alpha Capricorni, mv= 2.85) at 00:41 hrs
August 6: Mercury at perihelion at 13:35 hrs
August 7: Moon 3.5º north of Neptune at 04:58 hrs
August 9: Limb of Moon 15 arcminutes south of Mars at 17:42 hrs
August 11: Moon 2.6º south of Uranus at 11:07 hrs
August 14: Venus at greatest elongation west (43º 17') at 00:01 hrs (diameter = 23.4")
August 15: Moon 1.6º north of the star Propus (Eta Geminorum (mv= 3.31) at 17:39 hrs
August 15: Moon 1.7º north of the star Tejat Posterior (Mu Geminorum, mv= 2.87) at 20:06 hrs
August 15: Uranus at western stationary point at 20:33 hrs (diameter = 3.6")
August 15: Moon 4.1º north of Venus at 22:37 hrs
August 16: Limb of Moon 19 arcminutes south of the star Mebsuta (Epsilon Geminorum (mv= 3.06) at 03:02 hrs
August 18: Mercury in superior conjunction at 00:55 hrs (diameter = 5.0")
August 19: Moon 2.8º north of Mercury at 17:38 hrs
August 20: Mercury 1.3º north of the star Regulus (Alpha Leonis, mv= 1.4) at 13:19 hrs
August 26: Moon 1.6º north of the star Graffias (Beta-1 Scorpii, mv= 2.56) at 05:16 hrs
August 28: Moon 1.6º north of the star Kaus Borealis (Lambda Sagittarii, mv= 2.82) at 14:00 hrs
August 29: Limb of Moon 32 arcminutes south of Jupiter at 11:02 hrs
August 29: Moon 1.1º south of Pluto at 21:45 hrs
August 30: Moon 1.6º south of Saturn at 04:42 hrs
The Planets for this month:
Venus: This, the brightest planet, was an 'evening star' in the first five months of 2020. It passed through inferior conjunction (when it overtook the Earth by passing between us and the Sun) on June 4, after which it moved to the eastern pre-dawn sky. It is now a 'morning star', and may be spotted in the east just before sunrise, in the constellation Taurus. On July 1 it rises at about 4:15 am, and on July 31 at about 3:30 am. For the first two weeks of July, Venus will be passing through the Hyades star cluster. The waning crescent Moon will be near Venus on July 17. It will pass out of Taurus and into Orion on August 5, and remain as a 'morning star' until early next year.
(The coloured fringes to the first and third images below are due to refractive effects in our own atmosphere, and are not intrinsic to Venus itself. The planet was closer to the horizon when these images were taken than it was for the second photograph, which was taken when Venus was at its greatest elongation from the Sun).
March 2020 mid-May 2020 early July 2020 mid-August 2020 December 2020
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 all of 2019 up to July, Venus appeared as an 'Morning Star' in the eastern pre-dawn sky, but in August last year it moved to the evening sky to be an 'Evening Star'. It is now quite easy to find in the west as twilight begins to fade.
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: The red planet is becoming a better object for viewing this month, as the Earth is catching up to it. Mars is slowly increasing in angular size, and by July 1 it reached 12 arcseconds in diameter, with a brightness of -0.5 (nearly as bright as the second-brightest star, Canopus). It passed through western quadrature (rising at midnight) on June 7. Mars began the month of June in the constellation of Pisces and entered Aries on June 25. It will pass into a corner of the non-zodiacal constellation Cetus, the Whale, on July 8, and cross back into Pisces on July 27. At the beginning of July, Mars could be found midway between the stars Diphda (Beta Ceti) and Algenib (Gamma Pegasi, the south-eastern-most star of the Great Square of Pegasus). It is easy to locate as it is by far the brightest object in that part of the sky. Mars will perform its retrograde S-bend in Pisces between September 10 and November 14. During this period it will be close to Earth and quite large and bright. At the midpoint between these two dates, Mars will be at its closest distance to Earth, an event called "opposition", as it will be directly opposite the Sun in the sky, rising in the east as the Sun sets in the west. The actual date of opposition will be October 14, when the angular diameter of Mars will be 22.3 arcseconds. The Last Quarter Moon will be just above and to the right of Mars after midnight on June 13 and July 12.
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: This gas giant planet will reach opposition on July 14, and now is suitable for viewing for most of the night. On July 1 it will rise at 6:01 pm in the constellation of Sagittarius. The Full Moon was one-third of a handspan above Jupiter on July 5, with Saturn a little more than that below Jupiter.
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 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
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. This run of good fortune will continue for the 2020 opposition, except that the satellite in
transit will be Io.
This year, opposition will occur at 5:33 pm on July 14. On that date, the Sun will set at 5:13 pm, just eight minutes after Jupiter rises. The satellite Io
will commence its transit across the face of Jupiter at 5:31 pm, with its shadow almost exactly behind it. The Great Red Spot will be well on its way across
Jupiter's disc. Io will move off the face of Jupiter at 7:49 pm, when Jupiter will be at an elevation of 34.6 degrees. In 2021 the Great Red Spot will be
present, but there will be no transit of any satellite.
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 fifth 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.
Saturn:
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.
Uranus:
Neptune, photographed from Nambour on October 31, 2008
Pluto: The erstwhile ninth and most distant planet
will reach opposition on July 16. Rising at Sunset, it will be visible in the
constellation Sagittarius all night. The best time to search for Pluto this
month will be in the hours around midnight. Pluto's angular diameter is 0.13 arcseconds, less than one twentieth that of Neptune. Located
just south-east of Jupiter this month in the eastern end of Sagittarius, it is a faint 14.1 magnitude
object. On July 1, Jupiter and Pluto will be only 41 arcminutes apart, but
this separation will increase to 3.2º by the end of the month. A telescope with an aperture of 25 cm is capable of locating Pluto.
The movement of the dwarf planet Pluto in two days, between 13 and 15 September, 2008. Pluto is the one object that has moved.
Width of field: 200 arcseconds
This is a stack of four images, showing the movement of Pluto over the period October 22 to 25, 2014. Pluto's image for each date appears as a star-like point at the upper right corner of the numerals. The four are equidistant points on an almost-straight line. Four eleventh magnitude field stars are identified. A is GSC 6292:20, mv = 11.6. B is GSC 6288:1587, mv = 11.9. C is GSC 6292:171, mv = 11.2.
D is GSC 6292:36, mv = 11.5. (GSC = Guide Star Catalogue). The position of Pluto on October 24 (centre of image) was at Right Ascension = 18 hours 48 minutes 13 seconds, Declination = -20º 39' 11". The planet moved 2' 51" with respect to the stellar background during the three days between the first and last images, or 57 arcseconds per day, or 1 arcsecond every 25¼ minutes.
For the rest of this year, there will be a fine conjunction of the planets Jupiter and Saturn in the
night sky. On July 6, the Full Moon
passed through the grouping, adding additional interest.
In March, Mars approached Jupiter and Saturn, and on March 4
and 5 Jupiter was almost exactly between Mars and Saturn. By March 18, Mars
and Jupiter were together, with the crescent Moon just above them. The
following night, the Moon was below Saturn. On March 20, Mars and Jupiter
were at their closest, only 42 arcminutes apart. On March 21, Saturn
crossed into Capricornus, and on March 30 Mars did likewise. On the
morning of April 1, Mars reached Saturn, and the two planets were
less than a degree apart, with Jupiter 6.3
Meteor Showers:
Pegasids
July 10
Waning gibbous Moon, 83%
sunlit ZHR = 8
Radiant: Near the star Markab
S Delta Aquarids
July 29
Waxing gibbous Moon, 63%
sunlit ZHR = 20
Radiant:
Between the stars Skat and Deneb Algedi
Alpha Capricornids
July 30
Waxing gibbous Moon, 72% sunlit ZHR =
8
Radiant: Near the star Algedi
Use this
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.
Note: NEOWISE refers to the
Near-Earth Object Wide-field Infrared Survey Explorer satellite which
discovered this comet. This satellite was placed in orbit in December 2009
as the WISE (W
Comet
Comet
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. In the week preceding it was at its brightest at magnitude 4, but this was a cloudy week at Nambour. It passed between the Pleiades and Hyades star clusters on the night of December 19-20, but the light of the almost Full Moon made it difficult to see. It then headed towards the star Capella in Auriga, which it passed on December 24-25. It is currently 470 million kilometres away in the constellation Virgo, between Spica and Arcturus, and close to the star Heze (Zeta Virginis), but at 18th magnitude it is too faint for most amateur telescopes. Click here for more information and chart
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.
On July 1, the Eta Carinae Nebula can be found about a handspan to the
west-south-west of the Southern Cross at 6 pm.
The Stars and Constellations for this
month:
In the north-west, Leo the Lion is preparing to set. 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.
These descriptions of the night sky are for 8 pm on July 1 and 6 pm on July 31. They start at the western horizon.
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.
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).
Below or east of Scorpius is Sagittarius the Archer, through which the Milky Way
passes. Sagittarius teems with stars, glowing nebulae and dust clouds, as it is
in line with the centre of our galaxy.
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,
but is this year
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.
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 8:30 pm on July 1. 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
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 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 ?
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 Estimates vary that between 15% and 50% of stars are single bodies like our Sun, although the latest view is that less than 25% of stars are solitary. At least 30% of stars and possibly as much as 60% of stars are in double systems, where the two stars are gravitationally linked and orbit their mutual centre of gravity. Such double stars are called binaries. The remaining 20%+ of stars are in multiple systems of three stars or more. Binaries and multiple stars are formed when a condensing Bok globule or protostar splits into two or more parts.
Binary stars may have similar components (Alpha Centauri A and B are both stars like our Sun), or they may be completely dissimilar, as with Albireo (Beta Cygni, where a bright golden giant star is paired with a smaller bluish main sequence star).
The binary stars Rigil Kentaurus (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.
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