Observations

September 2018 night sky guide

To help you learn about the southern night sky, Sydney Observatory provides an audio guide/podcast, transcript of that audio, and a sky map or chart each month. This month’s guide is presented by Geoffrey Wyatt, Sydney Observatory’s Astronomy Programs Producer.

Hear the Audio
You can subscribe to the podcast on iTunes or directly download this month’s guide to your favourite audio listening device.

See the Sky Chart
We provide an September 2018 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.

Hello there. I’m Geoffrey Wyatt, one of the team here at Sydney Observatory, part of the Museum of Applied Arts and Sciences. I’m going to talk to you about what’s visible in the sky for the month of September.

What we’re going to do is a tour the night sky so of course you’ll need some provisions. Somewhere warm and comfortable to sit, a blanket, a cup of coffee or Milo, or if you’re old enough perhaps a glass of red wine to assist your imagination. One of the most important things that you can have with you is your printable map that you can download from our website at maas.museum/observations

If possible I want you to find a high position so that you can see as much of the horizon as you possibly can in all four cardinal directions. That’s right: North, East, South, and West. If you’re up against your neighbour’s tree or house, then of course you’re going to lose some of the view, but some of us just can’t help that. So make do with what you’ve got, but if you can, a clear view in all directions will make all the difference.

Wait for Sun to set and then a little more until it’s nice and dark perhaps up to an hour or so after sunset. Look up and what do you see? Clouds? Possibly. Stars? Hopefully. ‘For some, who are travelers, the stars are guides. For others they are no more than little lights in the sky.’ As said in The Little Prince by Antoine de Saint-Exupéry. I would like us all to be travelers together and we shall begin by looking towards the western horizon.

From there, go up to about 60 degrees or three outstretched hand spans from pinkie to thumb tip.

What you’ll see is the 15th brightest star in the night sky. It’s a red supergiant, which means, of course, it’s a pretty big star. It’s about 15 times the mass of the Sun, 700 times the diameter of the Sun and 600 light years away. It is, Antares. It’s the brightest star in the group of stars, or constellation, that we call Scorpius.

A light year is the distance that light travels in a year in the vacuum of space. It’s a long way. For those of you who like mathematics, it’s about 9.5 x 10 to the power of 15 metres. For those of you that like kilometers, it’s roughly 9,500 billion kilometres. It’s not a very convenient way of thinking about it to be honest, so we tend to use just ‘light year’. You’re seeing this star is as it was about 600 years ago. You are looking back in time, how cool is that?

Antares, its name came about because every now and then, roughly every 800 days or thereabouts, the planet Mars wanders fairly close by. The Greek name for Mars is Ares. As Ares, god of war, would go past this fairly bright reddish star they looked similar. This star was called the rival of Mars, anti-Ares, Antares.

When you look into the night sky, you might be able to see around 2-3000 objects depending on your age and eyesight. It’s very hard to remember which one is which, unless you have a memory aid, something to help you. A dot-to-dot picture with a good story behind it is a fairly handy way of doing it.

To the ancients, the area of sky that we’re looking at now appeared like a small but nasty animal. Antares marks its heart. There’s a star on either side which should make up the body. If you go down just a little bit toward the Western horizon, there’s a perpendicular line of stars to the ones we’ve just mentioned, which will be the head and claws on either side.

From the head and claws, go back up through a line of three stars with Antares in the middle then curling up around will be the long and dangerous tail of the scorpion.  Scorpius is a constellation and a constellation is simply a region of the sky. Many of the figures in constellations are hard to identify until you see them on a map or someone points them out but once you’ve seen them they are easy…with a little practice. And constellations are a bit like a suburb. There are many suburbs in a city. As soon as someone mentions a particular one it gives you a rough idea of where it is and it’s much the same with the sky.

Over thousands of years the sky has been mapped and broken up into, now, 88 different sky suburbs or constellations and Scorpius is one of the more famous. It has many good stories about it but as you can imagine the stories that we have now may not be the same as they were thousands of years ago.

One that I particularly like revolves around another very famous constellation, the mighty hunter Orion. He boasted that he could kill any animal on the planet. This displeased Gaia, the Goddess of the Earth and protector of animals, so she created the giant scorpion ‘Scorpius’ to kill him.

So epic was the battle that even Zeus, king of the gods, stopped what he was doing and watched the fight. Ultimately Orion was killed and Zeus placed the scorpion in the sky for us all to see. But Artemis, goddess of the hunt who was a good friend took pity on Orion and asked Zeus to also place Orions body in the sky as well but on directly the opposite side of the sky so that the two would never fight again. This very simple stick figure of a scorpion therefore has a wonderful story behind it.

When you’ve been able to see the full constellation of Scorpius, if you have a pair of binoculars or a small telescope, scan around the tail region. There are some nice objects there including some lovely clusters of baby stars called M6, the Butterfly cluster and M7. They’re simply the sixth and seventh objects in the catalog developed by a man whose name began with M, Charles Messier. It was done quite some time ago but we still use it.

Once you’ve seen the star clusters go a little bit higher but you will need to have our printable map because the next group of stars is rather hard to see. What you are looking for is a half-man half-horse with a bow and arrow. Can you see it? I’ll give you a few seconds. [hums] Found it yet? I don’t think so. It’s nearly impossible. If you can see a half-man half-horse with a bow and arrow you’re doing very well or perhaps it’s been shown to you before because to many people it looks much more like an old-fashioned teapot.

Yes, that’s right. Those of you out there with the birthday star sign of Sagittarius – you’re now a teapot, unofficially of course. This group of stars is fairly important to look at because it points to the centre of our Milky Way galaxy.

If you can get away from the bright city lights and there’s no Moon and you look up at this time of year you should be able to see a glorious view of Via Lactea, ‘By Milk’, our galaxy, the Milky Way. It’s simply breathtaking from a dark clear site.

Try and find the teapot because if you do find it and in particular the spout, in that direction roughly 26,000 light years away is the heart of our galaxy and an object called Sagittarius A Star. (Written as Sagittarius A *)

It is a massive black hole roughly four million times the mass of the Sun. NASA’s Swift space telescope has been observing it and in 2014 it gave astronomers enough data to work out that it’s roughly 44 million kilometres in diameter. Black holes are rather bizarre objects. At the centre of the black hole there is something called a singularity, a point, a mathematical point with infinite density and no size. Oh dear, that makes my head hurt.

Around that singularity there is a sphere, a sphere of influence called the event horizon and that event horizon is the part with a diameter of 44 million kilometres. Travel over the event horizon and…uh-oh, it’s not good times ahead. In fact, we call it ‘spaghettification’, being ripped apart atom-by-atom. But we don’t know for sure what happens once you go over that because you leave the observable Universe.

Let’s move along from Sagittarius and start to head down toward the East. You might have to  turn around a little bit so you’re now facing the East. The next constellation down is the first of the water signs.

It’s a fairly faint group of stars. What I want you to do is look for is a group that looks a bit like a triangle that’s been bent. Technically I suppose that makes it a quadrilateral and what you’re looking at, is Capricornus, the half-goat half-fish.

This is such a bizarre creature and the story behind it gave us the word ‘panic’ but I don’t have time to go into that here now. There’s not a lot else to see there, so let’s leave Capricornus and head down a little bit more. This constellation is    also very hard to see but once it’s been shown to you I think you’ll find it’s not that hard. All you’re looking for, are the shoulders, the jug and the line of water flowing from the zodiac constellation of Aquarius, he was the most handsome youth ever and his name was Ganymede. He was carried from the Earth up to Mount Olympus by Aquila the eagle where he became the water carrier.

I take it back, it’s actually pretty hard to see but from Aquarius there is a line of stars that seems to meander across the sky to a bright star that you can see about 30 degrees, so that’s one hand span for 20 degrees and one clenched fist for about 10 degrees above the eastern horizon. It’s a relatively bright star called Fomalhaut, the mouth of the southern fish or Piscis Austrinus. Long, long ago Fomalhaut used to mark the position of the winter solstice as seen from the northern hemisphere but no longer. That point has moved on because of the Earths precession of the equinoxes.

Fomalhaut is a fairly young star. It’s only about 400 million years old and about 25 light years away. At twice the size of the Sun it’s pretty big.

After you’ve been able to see Fomalhaut…. oh by the way, the rest of the constellation to me looks a little bit like a paisley swirl with the brightest star at the bottom of it as we’re looking at it right now. What I want you to do is turn to your left, that means we’re going toward the North.

It’s pretty hard to see but we’re going to go past the constellation of Pegasus which will look a bit like a big square, though it’s very low at this time. As we go a little bit further toward the North past another group of stars that looks like a large faint cross. You’ll need a perfectly clear view toward the North to see this one. It is the constellation of Cygnus the swan and the home of the first suspected black hole ever found and it’s called Cygnus X1.

Go a little bit past that and what you’re looking for is the fifth brightest star in the night sky. It’s only about 18 to 20 degrees above the horizon. Remember, as we’ve said before that’s roughly one outstretched hand, pinky tip to thumb tip, above the horizon. What you’re looking for is the star Vega. Again, like Fomalhaut, it’s only about 25 light years away and about twice the mass of the Sun. It’s less than 500 million years old so it’s a young star but the cool part is 12,000 years ago it was the North Polar Star. It seems that many people think that the stars never move but they do and the patterns change over very, very long periods of time. The Earth also wobbles over a 26,000 year period know as the precession of the equinoxes. So 12,000 years ago Vega was the North Polar Star and roughly in another 14,000 years it will be the Polar Star again but don’t worry about waiting for that one.

I love looking at this star the fifth brightest star called Vega. By the way, for the science fiction buffs, it was the destination star in the fabulous 1956 movie ‘Forbidden Planet’.

If you go from Vega at roughly 18 degrees above the northern horizon up to about 35 degrees, so that’s one hand span with the fingers wide open, one clenched fist and then half a fist. Got that? Good.

If you can do that you will see another fairly bright star. This is Altair ‘eye of the eagle’. That was the eagle that carried the youth Ganymede up to Mt Olympus to become Aquarius the water bearer.

Vega and Altair, the two bright stars separated by the Milky Way, the river in the sky. Throughout Asia on the 7th of July legend says that birds come together and build a bridge over the Milky Way so the two can be together for just one day.

Oddly, nothing actually happens in the sky but I love this story and in Japan the girl, Vega is known Orihime, meaning the princess and the boy is Hikoboshi. In China it’s Zhi nu and Niulang. My pronunciation however may be off somewhat so please forgive me. This story is also quite famous in Vietnam and Korea. The sky really is a multicultural delight.

Continue now to turn to your left past where we started. To the southwest we’re looking for a second centaur half-man half-horse and this ones’ name is Chiron. He was a wonderful teacher. According to mythology from long, long ago he was the tutor to Achilles, Hercules and Jason.

What we’re looking for is a fairly bright star that makes up one of his front feet. Its altitude is about 40 degrees above the horizon. How do we do that? Aha of course, two hand spans, pinky to thumb tip, that’s about 40 degrees. If you can see one bright star there you’ve nailed it. That’s Alpha Centauri. It’s the closest star to us after the Sun and the third brightest star in the night sky.

The interesting thing about this star Alpha Centauri is that it’s a visual binary which means that if you have a small telescope this one star looks like two snuggled up against each other.

In reality they’re not. The distance between them varies enormously. They’re in a dance. A dance that takes roughly 80 years for them to go around each other once. At their closest they’re roughly the distance from the Sun to the planet Uranus. At their most distant nearly double that from the Sun to Neptune. One of them is a little bit bigger than our Sun, one of them a little bit smaller, but it’s a little more complicated than just being a binary star because there is a third star in the system. The third star is a small red dwarf going around the other two going around one another. The third star comes closer to us than other star apart from the Sun and is called Proxima Centauri at 4.24 light years. Recently in the search for exoplanets, a small planet just a little bigger than the Earth has been detected in orbit around Proxima Centauri. It takes about 11 days to go around once so it’s very close to the parent star but because it’s not as big or as hot as the Sun that actually puts this planet called ‘Proxima B’ into what’s called the ‘Goldilocks’ zone.

Theoretically it means that liquid water may exist on this planet. Stay tuned because I am sure over the next few years, or perhaps decades there will be more studies and information released about this intriguing new and close exoplanet.

Just below Alpha Centauri is the second brightest star in the Centaur so it is called Beta Centauri. It represents another of the front feet of the half-man half-horse. From there there’s a line of stars that, again, you’ll need the star map to see, that wraps around indeed the smallest of all 88 constellations and that is the Southern Cross, Crux. It’s getting a bit low in the southwest at this stage and too hard to see right now.

Keep going around to your left, past due South and into the southeast. You’ll see the 9th brightest star in the night sky. That is Achernar the brightest star in the constellation of Eridanus, the River.

It’s an amazing constellation because it winds its way across so much of the night sky. It’s an interesting star too at about 140 light years away, it’s seven times the mass of the Sun but 3,000 times brighter. It’s very, very bright, but because of the distance it fades to being the 9th brightest as we see it.

It’s also intriguing because it spins so quickly at about 250km/s, its equatorial diameter is about 56 percent greater than its polar diameter. That makes it one of the flattest stars we’ve ever seen.

When we look up and see stars we see mostly single points of light scattered here and there but that is not the reality. The majority of stars come in groups of two or more and they are as close to immortal as we can imagine. Our galaxy, and indeed the universe in which we live, is a very unusual and beautiful place, and all we have to do to explore…is look up.

Special events for September 2018 and let’s start with the Moon.

Last quarter Moon is on Monday the 3rd at 12:37pm.

New Moon is on Monday the 10th at 4:01am.

First quarter Moon will be on Monday the 17th at 9:15am

Full Moon is on Tuesday the 25th at 12:52pm Australian Eastern Standard Time or AEST.

The spring equinox will occur on Sunday the 23rd at 11:54 am. This is simply when we see the Sun move from the Northern Hemisphere and cross the celestial equator into the Southern Hemisphere, which for us signals the start of spring. In the Northern Hemisphere however it is the Autumnal equinox. On the equinox the Sun will rise due East and set due West.

At the start of the month sunset is about 5:37pm and by the end of the month its drifted 20 minutes to 5:57pm. At the start of the month sunrise is at the start of the month is 6:14am and by the end of the month at 5:34am, a difference of 40 minutes. So an extra hour of daylight by the months end.

We continue to enjoy four planets in the sky at the moment, Venus, Jupiter Saturn and Mars.

Venus, Goddess of love and beauty in the constellation of Virgo passes close by its brightest star Spica on the first and second of September. On the 13th the 16% waxing crescent Moon will be to the north or to the right and above Venus.

Jupiter spends the month in Libra. On the 14th of September the waxing crescent Moon will be to the north, or to the right.

Perhaps the most beautiful planet in the Solar System, Saturn is high in the constellation of Sagittarius.  On the 17th the first quarter Moon is the left or west and slightly below.

If you’d like to see these planets in spectacular detail, although in reality still quite small, please come along to Sydney Observatory. We have many telescopes there and expert staff to help you enjoy the view.

You can find our monthly sky guide podcasts on iTunes. If you want more detailed sky maps, sunrise and sunset times, Moon and tidal times, and a whole lot more, we recommend that you purchase the book ‘The Australasian Sky Guide’, by Dr Nick Lomb.

It’s available from Sydney Observatory and the Powerhouse Museum shops and online of course at maas.museum. If you come in to our venues it’s only $16.95 but a little bit more if you order online with postage and handling.

For the very most up-to-date information, why not engage with us via Facebook? Go to facebook.com/sydneyobservatory, or you can follow us on Twitter, @sydneyobs. Please share any photos that you take of the of the night on our Facebook page and of course feel free to contact me or any of the team via the links on our blog.

My name’s Geoffrey Wyatt, I’m one of the team here at Sydney Observatory and the Museum of Applied Arts and Sciences. I hope you enjoyed this tour of what’s visible in the southern sky for the month of September 2018.

4 responses to “September 2018 night sky guide

  • Hi Ari,
    I am so sorry for the delay but I just saw this. Yes Sirius is the brightest star in the night sky but it didn’t rise until about 1am on the 3rd. Can you tell me what time your son was born and I can identify the brightest star in the sky at the time.
    Regards
    Geoff

  • Hey Geoffrey , this is an amazing read. I have always interested in the sky and our small visible universe from our planet.

    Thanks for taking the time for this blog and the podcast. I want to visit the park next to the observatory with my camera to do some night sky photography, will be listening to the podcast while setting my shots. 🙂

  • Hello Ari,
    Thank you for your feedback. It is always great to know the podcasts are being read and listened to. You are correct that the brightest star at night is Sirius but it depends on the ‘time of night’ that you want.
    Do you have a particular time in mind? The two brightest stars Sirius and Canopus are not visible at the start of the evening until around 10pm and later. I can happily make a map at a particular time for you if you would like to send an email to geoff.wyatt@maas.museum. Kind regards Geoff

  • Hi Geoffrey,

    I just read your tour of the Sept sky and thought it was great. I love that you include some of the mythology behind the names of the stars.

    I have a weird, maybe silly sounding request but please bear with me. My son’s birthday is 3rd Sept and I thought I’d commemorate his 1st birthday by getting a tattoo. His Chinese name means “brightest star” so I thought I’d get a tattoo of the brightest constellation in the Sydney night sky. Unfortunately, I’m having a little trouble finding out which is the actual brightest star on the night of the 3rd. I’ve read somewhere that Sirius is the brightest star, but I also read somewhere else that Cygnus is quite bright in Sept.

    Any help you can give would be greatly appreciated !

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