To help you learn about the southern night sky, Sydney Observatory provides an audio guide, transcript of that audio, and a sky map or chart each month. This month’s guide is presented by Melissa Hulbert, Sydney Observatory’s Astronomy Programs Coordinator.
Mel suggests stars and constellations to look out for this month include Scorpius, with the red star at its heart, Antares; Sagittarius (which looks more like a teapot than a centaur); Crux – more commonly known as the Southern Cross; and Ophiuchus, the 13th sign of the zodiac! Mel also tells us the best times and dates to see the planets Venus, Saturn, Jupiter and Mercury.
See the Sky Chart
We provide an August 2017 night sky chart (PDF) which shows the stars, constellations and planets visible in the night sky from anywhere in Australia. To view PDF star charts you will need to download and install Adobe Acrobat Reader if it’s not on your computer already.
Read the Guide
Hello and welcome to the night sky guide for August. My name is Melissa Hulbert and I’m an Astronomy Programs Coordinator at Sydney Observatory.
Armed with your sky map and a small torch with some red cellophane covering it, find a nice dark place away from the glare of the street lights and make sure you know your cardinal directions – that’s north, south, east and west. Remember that the Sun rises in the east, moves through the northern sky during the day and sets in the west or a small compass will also point you in the right direction. Pick a comfortable spot either on a rug or a deck chair that you can lay back in. Wait about 5-10 minutes and allow your eyes to adapt to the darkness.
Look straight above you. High overhead is Scorpius the Scorpion. The Scorpion is one of the easiest constellations to pick out as it is one of the few that does look like what it’s supposed to represent. It covers about 30 degrees in the sky. Working out degrees in the sky is quite easy. Hold your arm out towards the sky and make a fist, from one side of your fist to the other, this is 10 degrees. Hold your other arm out and spread your fingers out as wide as you comfortably can (so the opposite of a fist), from your little finger to your thumb is 20 degrees. Put your hands side-by-side and you now have 30 degrees, the size the Scorpion covers in the sky. This does work for everyone, as your arm length is proportional to your hand size.
Now, look for the Scorpion’s Heart, Antares, a red supergiant star that is 400 times the diameter of our Sun. Antares means ‘rival of Mars’, and when they are close together in the sky they certainly do look very similar.
If you have a pair of binoculars, then near Antares is a small globular star cluster, M4, which is a group of old stars that lie about 7,000 light years away, making it one of the closest globular clusters to us. Below the sting of the Scorpion are two open star clusters, M7 and M6, which lie about 800 and 2,000 light years away respectively, and are both worth a look. See if you can see the butterfly in M6.
These names I’m giving the clusters are catalog names. ‘M’ stands for Messier and is named after Charles Messier, an 18th century French comet chaser. He made a catalog of 103 fuzzy objects that were not comets so that he didn’t waste his time looking at them. Other astronomers later added a few more objects to the catalog bringing the total to 110.
Follow the scorpion along from its head through its heart, Antares, and to the point where the body meets the tail. The star at this point is Zeta Scorpii and next to it sits NGC 6231, a bright open star cluster containing about 120 stars. This cluster is remarkable for its large population of high-luminosity supergiant stars, which includes two Wolf-Rayet stars. Wolf-Rayet stars are very luminous, very hot stars that have relatively short life times. It is a striking cluster that contains many white and yellow stars as well as many pairs and triplets and is about 6,500 light years away.
Just behind the sting of the scorpion is Sagittarius, which is depicted as a half-man, half-horse. Sagittarius was often confused with the other centaur, Centaurus however Sagittarius is different, in that he has a war-like posture with his arrow aimed at the heart of the Scorpion. It is thought that Sagittarius can be traced back to the Mesopotamian archer-god Nergal, who was associated with Irra, the wrathful god of war and fire. However, in our night sky Sagittarius looks more like a teapot than any of these mythical creatures.
Interestingly, the Sun lies in Sagittarius from mid-December until mid-January, meaning that it lies in this constellation at the time of Summer Solstice – its most distant point south of the equator.
The very centre of our galaxy is found in Sagittarius, along with many great binocular and telescopic objects. Alpha Sagittarii is one of several examples where the star labeled as alpha which represents the brightest star in a constellation, is not actually the brightest star. Epsilon Sagittarii has the honour of being the brightest. Sagittarius is known for its nebulae and clusters, 15 of which Messier cataloged – more than any other constellation.
Near the top of the teapot’s lid is M22, a large globular cluster. It is visible to the naked eye in dark skies and appears as a fuzzy blob in binoculars. It takes a telescope with an aperture of 75mm or greater to reveal some of the outer stars, with some of the brightest appearing to have a reddish hue. Even small telescopes will reveal M22’s elliptical appearance and it lies about 10,000 light years away. M22 is considered to be one of the finest examples of a globular cluster in the sky, third only to Omega Centauri and 47 Tucanae.
Winter months remind us of the wonderful Milky Way sights of Scorpius and Sagittarius. But observers should not forget the 13th zodiac constellation – Ophiuchus which lies beside Scorpius and Sagittarius and is often overlooked with these two constellations overhead.
Ophiuchus is an ancient constellation, representing a snake coiled around a man. However, it is now often associated with Aesculapius, a mythical healer said to have the ability to raise the dead. Certainly in Greek mythology, it is Ophiuchus who raises Orion from the dead after he is bitten by the scorpion. Aesculapius is seen holding a snake, most likely as they were seen as a symbol of power.
Ophiuchus is seen as the 13th sign of the zodiac due to the Earth’s precession – meaning the wobble of Earth on its axis. The Sun, Moon and the planets now pass through this constellation – the Sun from 30 November to the 17 December.
Ophiuchus has many interesting and varied objects for the observer including the second-closest star to our Sun – Barnard’s Star, a red dwarf lying 5.9 light years away.
Lying just to the west of the scorpion is the seventh constellation of the zodiac, Libra, The Scales. To the ancient Greeks Libra was part of Scorpius representing the scorpion’s claws and this association is reflected in the names of the brightest stars in Libra – Zubenelgenubi and Zubeneschamali meaning ‘the southern claw’ and ‘the northern claw’ respectively. However, Latin writers considered Libra distinct from Scorpius, with the scales symbolising the equinoxes, the equal lengths of day and night. Two millennia ago, the Sun moving into Libra marked the September equinox (the point at which the Sun moves south of the celestial equator each year), but due to precession, at around 730BC, this point moved into the adjoining constellation Virgo. During the first century BC, in the reign of Julius Caesar, the Romans separated Scorpius into the two distinct constellations we are familiar with today and associated the scales with Astraeia, the goddess of justice who is also represented by the constellation Virgo.
Time to turn and look towards the south. Look slightly to the west of south. Here you will see the Pointers – Alpha and Beta Centauri. Follow the line of the Pointers down towards the southwest and there is the constellation Crux, better known to us as the Southern Cross. Crux is Latin for cross. The Southern Cross, like the Scorpion, is another constellation that actually does look like what it’s supposed to represent. The Pointers point to the Southern Cross and this is one way to check you have the right cross as there are many groups of stars in the southern sky that look like crosses.
The second brightest star in Crux is a marker for a wonderful binocular and telescope object. To find the 2nd brightest star, whose name is Mimosa, look for the star in Crux closest to the Pointers. Now just nearby – at about 10 o’clock, if you imagine a clock face over Mimosa, is a wonderful open star cluster called the Jewel Box. It looks like a sideways ‘A’ in telescopes and binoculars. In a telescope, wonderful colours can be seen with white stars and a red supergiant. Sometimes even green appears but of course there are no green stars – this is just an illusion. The famous 18th century astronomer John Herschel gave the cluster its name as he likened it to a piece of multi-coloured jewellery. It lies just under 5,000 light years from us.
Move back to the Pointers and look at the constellation of Centaurus, which surrounds Crux on three sides. It depicts a Centaur, a mythical beast – half man, half horse. The constellation was said to represent the scholarly Chiron, the centaur who was tutor to many of the Greek gods and heroes. He was put among the stars after accidentally being killed by a poisoned arrow from Hercules.
Close to the second Pointer, Beta Centauri lies Omega Centauri or NGC 5139, the brightest and largest globular cluster in the sky. In fact, it is so bright it was labeled as a star on early charts by Ptolemy and later recorded by Bayer as Omega Centauri. It was found to be a cluster in 1677 by Edmund Halley of Comet Halley fame, so Omega Centauri carries both a star designation and an object catalogue designation. It is easily seen with the naked eye, shining with the luminosity of a million Suns. Its brilliance and large aperture size is in part due to its relative closeness to us – only about 17,000 light years away, making it one of the closest globulars to us.
Very close to Omega Centauri is NGC 5128, one of the strongest radio sources in the sky and is known to astronomers as Centaurus A. Optically in long exposure photographs it appears as a giant elliptical galaxy, split in half by a dust band. It is thought that Centaurus A is the result of a merger between two galaxies – one elliptical and the other spiral. In good skies, Centaurus A can be seen in binoculars, but a telescope is required to see the dust lanes (of the spiral galaxy) intersecting the bright elliptical halves. Centaurus A lies about 13 million light years from us.
Crux sits within one of the arms of our Milky Way and if you are away from the city lights you will see this arm and notice a dark patch between the brightest and second brightest stars of this constellation. This dark patch is called the Coalsack and is a dark nebula – lots of gas and dust that are blocking out the background stars. In the Indigenous Dreamtime the Coalsack formed the head of the Emu and if you follow the dark dust lanes of the Milky Way towards the Scorpion, you will see the Emu’s body and legs. This time of year is perfect for seeing the Emu stretching across the sky.
So what else can we look forward to seeing in the sky in August 2017?
Jupiter, the largest planet in our Solar System is in the western sky after sunset in the constellation of Virgo. During the month, it makes its way towards the brightest star in the constellation, the 1st magnitude Spica. By the end of the month, Jupiter and Spica will be less than four degrees apart. On the 25th, the 4-day waxing crescent Moon is below Jupiter, with Spica above and slightly to the west (left) of the gas-giant. This will be a wonderful sight in the western sky and well worth the effort to find a location with a clear view to the west on this night.
High in the northern sky in the constellation of Ophiuchus is the beautiful ringed-planet Saturn. Saturn’s impressive ring system can be seen in even small aperture telescopes and depending on the telescope you are using you may even catch a glimpse of a few of Saturn’s moons including the second largest in our solar system, Titan. On the 26th, Saturn reaches the end of its retrograde loop and is stationary before moving towards the constellation Sagittarius (retrograde is the apparent westward motion of a planet against the background stars). The ringed planet has a double encounter this month with the waxing gibbous Moon on the 3rd and the 30th, with the Moon below and to the northeast of Saturn on both dates.
After sunset look towards the western horizon where Mercury will be making its final appearance during the first half of the month before fading into the twilight and becoming lost in the Sun’s glare by the end of the month. However it will return to the morning sky in September.
Venus is the planet of choice for early-birds. It is visible in the eastern sky before dawn and on the 19th the 26-day old waning crescent Moon is above and to the northeast (left) of the brilliantly shining planet.
Early-birds can also enjoy a partial lunar eclipse on the morning of 8th August.
Lunar eclipses are wonderful events to watch as the only equipment you require are your eyes.
Lunar eclipses occur when our closest celestial neighbour in space, the Moon passes through the Earth’s shadow and can only occur at a full moon. So why don’t we see a lunar eclipse once a month at every full moon? The Moon’s orbit is tilted by 5 degrees to the Earth’s orbit around the Sun, so the Moon actually spends most of its time above or below the plane of Earth’s orbit. Thus, most full moons do not pass through the Earth’s shadow and no eclipse occurs. However, about two to four times a year some part of the Moon does pass through the shadow and an eclipse occurs.
The Earth’s shadow consists of two parts – the penumbra and the umbra. The penumbra is the outer part of the shadow and is where the Earth blocks part but not all of the Sun’s direct light from reaching the Moon. The umbra or inner part of the shadow is the area where Earth blocks all of the Sun’s direct light from reaching the Moon. About 35% of lunar eclipses are total eclipses and during this type of eclipse, the entire Moon passes through the umbra part of the shadow.
The Moon is still illuminated in the umbra due to indirect sunlight that has been refracted in, and emerges from, the Earth’s atmosphere. The dust and air in Earth’s atmosphere scatters the blue coloured light and allows red light to pass by. The remaining light is a deep red or coppery colour. So, if the air is clear we see a coppery coloured Moon. However if there is extra dust etc. in the atmosphere (maybe due to a volcanic eruption) the eclipsed Moon can turn darker shades of red, sometimes even completely black as was the case on October 4th, 1884. This was due to the eruption of Krakatoa. It is also interesting to consider that if the Earth had no atmosphere then the Moon would be completely black during totality.
Unlike solar eclipses, lunar eclipses are safe to watch with your eyes, you don’t need any special protective filters or even binoculars or a telescope, though these do help make the coppery colour stand out.
The eclipse on 8th August will be a partial lunar eclipse with only the southern part of the Moon immersed in the Earth’s shadow – a quarter of the linear diameter. The full Moon will look like a bite has been taken out of it.
The eclipse (penumbral) starts at 1:48am EST (note that all times I mention here are in EST) with the umbral (the part we can see with our eyes) starting at 3:22am. Mid-eclipse occurs at 4:21am with the umbral eclipse ending at 5:19am and the penumbral eclipse over at 6:53am.
A solar eclipse will occur across the USA on the 21st August (22nd August in Australia). Billed as the ‘Great American Eclipse’, numerous eclipse chasers will be out on mass to watch the eclipse as it travels across twelve states in the USA.
A solar eclipse is when the Moon passes directly between the Earth and the Sun. A little quirk of nature means that the diameter of the Sun is 400 times greater than that of the Moon, but the Sun is on average 400 times farther away from the Earth, so both appear to have the same angular diameter (0.5) in the sky. It’s this combination that allows us to see solar eclipses.
When the Moon is directly between the Earth and the Sun, it casts a conical shadow on the Earth. The darkest part of the shadow is called the umbra and no part of the Sun can be observed at totality from Earth. The lighter part of the shadow is known as the penumbra and the Moon will only partially cover the Sun. What we see will depend on where we are in the Moon’s shadow.
Standing at the point on the Earth where the umbra falls, results in observers seeing the disk of the Moon completely covering the disk of the Sun. This is called a Total Solar Eclipse. Observers only slightly removed from the umbra shadow would be in the penumbral shadow and see only part of the Sun’s disk covered by the Moon. This is called a Partial Solar Eclipse.
If the Moon’s orbit was exactly in the plane of the Earth’s orbit, then we would see an eclipse of the Sun every month. However, this is not the case with the Moon’s orbit inclined by five degrees. Thus, solar eclipses occur two to five times a year, with one particular locality seeing a total eclipse about once every 350 years.
The Moon’s shadow sweeps across Earth at speeds of about 1700Km/h, due to a combination of the Earth’s rotation and the Moon’s orbital motion, so a solar eclipse occurs over a short period of time.
On the 21st August the total solar eclipse will be visible along a narrow track that’s just 112 kilometres wide. Across the rest of the USA, a partial solar eclipse will be seen. How much of the eclipse you will see, will depend on how close to the track you are – the closer you are, the larger the partial eclipse will be. For observers in the rest of the world (including Australia) no eclipse will be visible.
I must emphasise caution if you plan to watch a solar eclipse. It is dangerous to watch the eclipse while the Moon is moving across the Sun. You must use a special solar filter on your telescope or binoculars or for a few dollars you can purchase eclipse glasses which are just like a pair of sunglasses fitted with special film to protect your eyes. If you are under the narrow path that allows you to see totality then there is a small part of the eclipse that is safe to watch with just your eye-totality itself. Once totality occurs it is safe to look directly at the eclipse, without eye protection – but as totality will be short in this eclipse, about two and a half minutes, observers must be extremely careful!
A total solar eclipse is an awe-inspiring event to witness and something that should not be missed! However if you can’t make it to the USA this year don’t despair! The next mainland Australian total solar eclipse occurs on 22 July 2028 with the path crossing Sydney.
Let us know if you observe any of the wonderful planetary events or either of the eclipses this month via our Blog, Facebook page and Twitter account.
I will also be uploading posts to our blog while I am in the USA for this month’s eclipse.
I leave you now with this quote from Mark Twain: “Herschel removed the speckled tent-roof from the world and exposed the immeasurable deeps of space, dim-flecked with fleets of colossal suns sailing their billion-leagued remoteness.”
Wishing you clear skies and see you next month under the stars!
If you have enjoyed this August sky guide and think you might want to regularly check out what’s in the night sky, why not purchase a copy of Sydney Observatory’s book the ‘Australasian Sky Guide’. It not only contains detailed monthly sky guides, but is packed with astronomical information including rise and set times for the Sun, Moon and planets, tides and a detailed look at our Solar System and upcoming astronomical events. Only $17.00 from Sydney Observatory and Powerhouse Museum shops or you can purchase it online (additional costs apply). The ‘Australasian Sky Guide’ for 2018 will be available in October.
This has been Melissa Hulbert from Sydney Observatory with the August sky guide.