October 2017 night sky guide

To help you learn about the southern night sky, Sydney Observatory provides a guide and a sky map each month. This month’s guide is presented by Dr Andrew Jacob, Sydney Observatory’s Curator of Astronomy.

This month, learn how to find the South Celestial Pole, and where to find stars and constellations including Crux (the Southern Cross) and the Pointer stars, the Large and Small Magellanic Clouds, the bright star Antares at the heart of Scorpius. Andrew also tells us how to find the planets Jupiter, Saturn and Mercury in the evening sky. And when to look out for the meteors of the Orionid meteor shower.

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 a October 2017 Night Sky Map (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 for October 2017.

This is Andrew Jacob, and I’m the Curator at Sydney Observatory, part of Sydney’s Museum of Applied Arts & Sciences. Come on a tour of the night sky with me. Learn what stars and constellations are visible, where to find the planets and what special events are happening overhead this month.

 

To make the most of this guide, you should begin by gathering a few items together. Firstly, you will need a star map. You can download a free one from Sydney Observatory’s website where you’ll find it in the Astronomy Resources section under Monthly Sky Guides. The star map will show you what stars and constellations are visible in the night sky this month and I’ll be referring to that star map in this guide.

As well as the star map, a torch with a red LED, or one covered with a few layers of red cellophane, will be very useful. The red light will allow your eyes to remain dark adapted during the evening, yet still allow you to read your star map.

Finally, a pair of binoculars or a telescope can be very handy. They’re not essential for following this guide but if you do have them they will help you see a few of the fainter objects more easily and in more detail.

Now that we have our equipment together we need to know a few directions and also how to measure angles across the sky.

You can find the cardinal directions – North, South, East and West – from a compass app on your mobile device, or just remember, of course, that the Sun rises in the east and sets in the west. And if the Sun is setting at your right shoulder, then you must be facing south. Another useful direction to remember is the zenith. This is the point directly overhead.

To find your way around the night sky, it helps to know how to measure angles across it. It makes no sense to say, for instance, that one star is “2 centimetres” to the left of another or that a shooting star left a trail “half a meter” long! Instead we should use angular measurements. The distance around the horizon, from North, through East, South, West and back to North is 360 degrees. And from the horizon vertically upwards to the zenith overhead is 90 degrees.

But how do we measure smaller angles? Well, despite the great variety of human form our fingers, hands and arms are all pretty much in the same proportions. If you hold your hand out at arm’s length and stretch out your little finger and thumb to make a hand span, they span an angle across the sky of about 20 degrees. A fist held out at arm’s length makes an angle across the sky of about 10 degrees. And a finger held up at arm’s length is about 1 degree, or twice the width of the Moon or the Sun. I’ll be using these measurements during this guide. So, a hand span at arm’s length is 20 degrees, a fist is about 10 degrees, and a single finger is about one degree across the sky.

One final point to note before we get started. The Earth rotates and so the sky changes hour by hour. My descriptions of the constellations and stars in this guide fit the time of one to two hours after sunset.

 

Let us begin our tour of the night sky by looking towards the south. If you’re facing south and you look to the right of south – about one and a half hand spans to the right – almost to the southwest, you should be able to see two bright stars, one above the other. If you’re in a bright, light‑polluted location, such as central Sydney, they may be the only two stars you can see in that direction. Away from the city and other bright lights, they’ll be the brightest pair of stars you see towards the southwest.

These two bright stars, one above the other, are known as “The Pointers,” the two pointer stars which help us find the Southern Cross. The Southern Cross is the best known constellation in the southern skies.

On your star map, the two pointers are clearly labeled. The Southern Cross, however, appears as Crux, which is the Latin name for cross. The brighter of the two pointers is called Alpha Centauri. The other one is called Beta Centauri. Now, Alpha Centauri is the third brightest star in the night sky, and Beta Centauri is the 10th brightest star.

Alpha Centauri is a very interesting star. Although it looks like a single star to your eye, it is in fact a group of three, too close together for your eye to distinguish. Through a medium-sized telescope, two of the stars are visible. Both are very similar in size and colour to our Sun. They’re orbiting about each other with a period of around 80 years. The third star is called Proxima Centauri, and it is the closest star to the Earth, after our Sun.

Proxima Centauri, the closest of these three stars of Alpha Centauri, is a red dwarf and is only visible through large telescopes. It is believed to be orbiting the first two stars. Proxima Centauri is about 4.2 light‑years away from us, or approximately 42 million million kilometres. In 2016 a small planet was discovered orbiting Proxima Centauri. With a year of just 11 Earth-days and orbiting its star closer than Mercury orbits the Sun it’s slightly heavier than Earth and lies within something called the ‘habitable zone’. Overnight this planet called Proxima b, orbiting the star Proxima Centauri, became one of the prime candidates in the search for extra-terrestrial life.

A moment ago I mentioned “light‑years”. What is a light year? It’s a measure of distance, even if it sounds like a time. If you have a torch shine its light into the sky for a moment. The light from your torch travels incredibly fast. In just one second it goes almost 300,000 kilometers. That’s seven and a half times around the Earth or almost the distance to the Moon. In fact in just one and a quarter seconds your torch light would reach the Moon, 380,000 kilometres away. In eight and a half minutes the light would reach the Sun, 150 million kilometres from Earth. After five hours light from your torch would pass Pluto. Finally, after one year of time your torch light will have travelled one light-year of distance. Yet we still haven’t reached the next nearest star!

Proxima Centauri, our closest star after the Sun, is about 42 million million kilometres away from us. Your torch light would take about 4.2 years to reach it. So we can say that Proxima Centauri is about 4.2 light years away from Earth. This also means that we see Proxima Centauri, the star, as it was about 4.2 years ago. We are looking into the past to see everything in the universe.

So a light year is simply a distance, and one lightyear is about 10 million million kilometers long.

So let’s get back to the night sky. We’ll return to Alpha Centauri. If you draw an imaginary line downwards through The Pointers – from the brighter Pointer, Alpha Centauri, through Beta Centauri – and onward you’ll reach the Southern Cross. The Cross is lying on its right‑hand side, but otherwise it looks just as it does on the Australian flag.

The Southern Cross is very useful as it can help us find the direction of true south. Hold out your arm, and measure the length of the long arm of the Cross using two fingers, say your little finger and your index finger. Follow the direction the long arm of the Cross is pointing and measure this distance four times to the left, and upwards, from the Cross.

The point you end up at is called the South Celestial Pole. You can also see that on your star map – the south pole of the sky. It’s the point in the sky about which all the stars are rotating, but there are no bright stars in this area. It’s just an imaginary point in the sky.

Now, if you drop a vertical line from the South Celestial Pole straight down to the horizon, you’ll find the direction of south on the ground. And here’s something interesting you can do if you have a camera that allows you to leave the shutter open. If you take a photograph of the sky to the south, including the South Celestial Pole, and leave your camera shutter open for 10 minutes or more, you’ll find beautiful, circular star trails in your photograph.

Now that we can find the South Celestial Pole using the Southern Cross, take your hand span, which is approximately 20 degrees, another two times in the same direction beyond the Celestial Pole position, so about 40 degrees further. You should come across a fairly bright star in the southeast of the sky.

This bright star is called Achernar. Achernar is a hot blue‑white star about 144 light years away from us. It represents the end of the River Eridanus in the sky. Eridanus is the sixth largest of the 88 constellations in the night sky and it’s also one of the longest. The river meanders across the sky over to the eastern horizon and beyond. It’s been associated with the rivers Nile, Euphrates, Danube and others throughout history.

If you’re in a dark location, well away from city lights, you might have noticed, as you were measuring your hand spans across the sky to Achernar, two faint, fuzzy, cloud‑like objects. They’re located between the South Celestial Pole and the star Achernar. These two hazy clouds are called the Magellanic Clouds.

They were first seen by Europeans hundreds of years ago, and are named after the explorer, Magellan. One of the clouds is slightly larger than the other. This Large Magellanic Cloud, which is about 160,000 light years away, is about halfway down to the horizon below Achernar.

The Small Magellanic Cloud is about 200,000 light years from us. Both clouds are small galaxies that orbit our Milky Way galaxy. And they’re gradually being torn apart and absorbed into the Milky Way galaxy in a long slow-motion act of galactic harassment and cannibalism.

If you have binoculars, take a look at the Small Magellanic Cloud. You’re seeing light from stars that has taken 200,000 years to reach your eyes. Just above the cloud, through your binoculars, you should see a small, fuzzy ‘star’.

In fact, it’s not a star, it’s something called a globular cluster. It’s a ball‑shaped group of several hundred thousand very ancient stars. This particular one is called 47‑Tucanae. It’s about 16,000 light years away, barely a tenth of the way towards the Small Magellanic Cloud, but still far beyond our solar system.

 

Let’s turn to the west now. If you’re facing due west, hold out your arm and measure two hand spans, or 40 degrees, above the western horizon. You should reach a bright, orange‑coloured star, perhaps twinkling away due to the effects of the Earth’s atmosphere. If you’re using your star map, hold it up in front of you and rotate it so that the horizon labeled West is at the bottom edge. This will orient the map to match the western sky in front of you.

Now, I remember having great trouble seeing colour in stars when I first began looking at the sky. So if you don’t see the orange colour tonight, don’t worry.

This star is called Antares which means ‘in the likeness of Mars’ because of its reddish colour. It’s an enormous, red supergiant star, around 400 times the diameter of our Sun. If you placed it where our Sun is, it would reach out through the solar system and engulf the Earth. It’s a star coming to the end of its life.

Antares is about 604 light years away from the Earth. As a supergiant, it will eventually die as a supernova, a powerful stellar explosion. However, it’s so far away that this will have no effect at all on the Earth, although it will be a spectacularly bright sight. Unfortunately, that’s going to happen long after our short lives have come to an end.

Antares is the heart of Scorpius, the Scorpion in the sky, one of the few constellations that really looks like its name. If you have your star map with you, hold it up towards the west with the west horizon at the bottom, as I described earlier, and locate the star Antares on your star map. Just below Antares, you’ll see a short arc of three stars (or maybe four if you are at a truly dark location) which represent the head and shoulders of the scorpion.

Look back up through Antares and above it you should see a large back-to-front ‘question mark’ of stars stretching up into the sky above Antares and reaching almost to the zenith. That’s the tail of the scorpion. At the very end of his tail, you can see his sting quite clearly. Scorpius really does look like a scorpion!

Let’s move on. To the right and above the sting of Scorpius is the constellation Sagittarius, which is supposed to represent an archer, but I’ve never been able to see an archer when I look at this set of stars. All I can see is a rather triangular teapot. On your star map, Sagittarius is highlighted as the Teapot. Can you see the Teapot pouring tea all over the tail of Scorpius?

Just off the tip of the Teapot is an interesting point in the sky. If you’re away from bright city lights and you’ve been outdoors for more than 15 minutes or so, you might have noticed the Milky Way stretching overhead. It’s a band of faint, milky light stretching from the Southern Cross over on your left up past the Two Pointer stars and then continuing to stretch over through the tail of Scorpius and, finally, off to the northern horizon on your right‑hand side.

The center of our Milky Way galaxy lies just below the tip of the Teapot not far from Scorpio’s sting. It’s a good thing we’re out here near the edge of the Milky Way galaxy and a long way from its center. At the center of our galaxy, 27,000 light years away, lies a very large black hole over four million times the mass of our Sun. We’re quite safe from it here, out on the edge of the galaxy.

 

Let’s turn to the right again and look to the north. Again, if you’re using the star map, turn it so that north is at the bottom and so the map is upside down. There are a handful of bright stars in the northern sky and some fairly faint constellations. Let’s begin by looking directly north. Just 10 degrees, or one fist width at arm’s length, above the northern horizon, you should see a prominent star, probably twinkling wildly because it’s so close to the horizon.

This is the star Deneb in the constellation of Cygnus the Swan or, as it’s sometimes called, the Northern Cross. We’ll come back to the constellation Cygnus in a moment.

Just over a hand span, or slightly more than 20 degrees, to the left of Deneb is another reasonably bright star, again, about 10 degrees above the horizon. This second star is Vega, the fifth brightest star in the night sky. It’s in the constellation of Lyra the Harp and lies just 25 light years away, quite close compared to most other stars you can see tonight.

These two stars form the base of a triangle. High above them, just over two hand spans, or 45 degrees off the horizon, you’ll find the star Altair, which is in the constellation of Aquila the Eagle. Altair may look slightly yellowish in colour. It’s 17 light years away and is about 10 times brighter than our Sun. It spins on its axis very quickly, about once every 10 hours. By comparison, our Sun only spins once in 25 days.

These three stars don’t look particularly special to us in the southern hemisphere. In the northern hemisphere, however, they’re known as the Summer Triangle. They’re very high overhead up there and form a noticeable triangle. If you ever travel to the north in summer, have a look for the Summer Triangle made up of the stars Deneb, Vega, and Altair.

So let’s return to Deneb, down by the northern horizon and due north. This star is a blue-white supergiant star over 100 times the diameter of our Sun – it really is a giant of a star. Deneb is about 3,000 light years away from us, but even at that extreme distance, much further away than Vega or Altair, it still appears as a bright star in our night sky. Most of the bright stars in the sky are not even half that distance from us, so this must be one very bright star! If it was the same distance from us as Vega (at just 25 light years) it would cast shadows at night and be visible during the day. And if we placed it where the Sun is we would be incinerated by its intense ultraviolet light.

As I mentioned earlier, Deneb is in the constellation Cygnus the Swan. If you have your star map aligned with the stars to the north (that is, it’s upside down with North at the bottom on the map) it will help you identify the swan. Its wings stretch down to the left and up to the right and its long neck stretches out and up to the left.

Altair, another one of the Summer Triangle stars, sits just above Cygnus in the constellation of Aquila the Eagle. The eagle also has its wings stretching out to the right and left but its head, Altair, points down and to the right. If you’re at a dark site, you’ll be able to see the Milky Way in this region and you’ll see these two birds flying past each other along the Milky Way, the eagle heading north and the swan migrating south to escape the approaching northern winter.

Now we’ve looked at constellations and stars to the south, to the west, and to the north. We could turn to the east, but in October, there aren’t many bright stars, nor any truly distinctive constellations, in that direction. So let’s leave the eastern sky until next month.

 

What are the special events and highlights for October 2017?

Let me note that all the times I am about to mention are in ‘clock’ time – the time your clock would show, either Eastern Australian Standard Time or Eastern Australian Daylight-saving Time as appropriate. Remember that daylight saving begins this month on Sunday 1st October at 2am – put your clocks and watches forward one hour before bed on that Saturday night. And please make the appropriate adjustments for your time zone and state where necessary.

 

Let’s begin with the Moon and its phases. The Moon is the brightest object in the night sky when it is up, no matter what phase it is in. It is well worth observing its changing phases or looking closely at the craters, plains and other features with binoculars or a telescope. However, to get the best views of the Milky Way, stars and constellations it is best to avoid moon-lit hours. If the Moon is between New and Full (i.e. waxing) wait for it to set before observing. If the Moon is between Full and New (i.e. waning) observe before it rises. You don’t need a daily list of rise and set times – just watch the Moon for a few days and you will soon learn to predict its behaviour.

We start the month with a Full Moon on Friday October 6 at 05:40am. Last Quarter is on Thursday October 12 at 11:25pm. New Moon is on Friday October 20 at 06:12am. And finally First Quarter falls on Saturday October 28 at 09:22am. So for your best views of the Milky Way, stars and constellations this month avoid the first week, for the next fortnight observe after sunset but before the Moon rises, and after the 21st wait for the Moon to set before observing.

Saturday the 28th is “International Observe the Moon Night”. Why not take the time to join the world and catch the First Quarter Moon with your binoculars?

What planets are visible in October 2017?

In the evening sky Jupiter, Saturn and Mercury are up.

This is your last chance to catch Jupiter before it descends into the evening twilight in the west by the middle of the month.

Saturn remains high in the west in the constellation of Ophiuchus, to the right of Scorpius and below the Milky Way, alone once again now that the Cassini spacecraft has plunged into its atmosphere. On the 24th the crescent Moon guides you to Saturn by appearing just below and to the right of the planet.

Mercury reappears low in the west in Libra during the last few days of the month.

In the morning Venus and Mars are up, both rising during the morning twilight.

Venus is low in the east moving from Leo to Virgo during the month. Mars is also low in the east and on the 6th it passes close to Venus. You should easily see bright Venus on this morning but Mars may require binoculars or a small telescope to detect. Later in the month the Moon will guide you to these two planets. On the 17th Mars is to the right of a thin crescent moon. The next day, on the 18th, Venus is to the right of an even thinner crescent Moon.

The Orionid meteor shower occurs this month. Meteors should be visible throughout October and into November, after midnight, but it’s best seen for several nights around the 21st. This shower is associated with Halley’s comet and the meteors are fast and often bright. You can expect to see a meteor every couple of minutes on the best nights.

There are a couple of other interesting events this month, but they are difficult if not impossible to observe. On the 3rd at 10pm the Moon passes very close to Neptune from most of Australia. And for anyone in Tasmania, on the eastern coast of Victoria or in New Zealand, the Moon passes in front of, or “occults”, Neptune. And on October 12th the minor planet 2012 TC4, a 16-metre diameter rock, passes Earth at just one tenth the distance to the Moon. There is no danger of it hitting us, and it won’t be visible, but this is one of the closest minor planet passes this year.

Finally, it’s UN Space Week again from October 4-10. This year’s theme is astrobiology.

And that wraps up the special events for October 2017.

 

These Monthly Sky Guides are available from the Sydney Observatory website or from iTunes. An excellent companion is the annual “Australasian Sky Guide” book by Dr Nick Lomb. It not only contains detailed monthly sky maps but is jam-packed with astronomical information, including rise and set times for the Sun, Moon and planets, plus tide times and a detailed look at our solar system and upcoming astronomical events. It’s only $16.95 from Sydney Observatory or the MAAS store, or you can purchase it online, for which additional costs apply.

For more astronomical information, why not check the Sydney Observatory website and blogs, and keep in touch via our Facebook and Twitter accounts.

And if you’re in Sydney visit the Observatory in the Rocks area. Book in for a night tour to view the skies through our telescopes. Or tour our exhibition for free and discover the history of Australian astronomy.

And that brings to an end this Night Sky Guide from Sydney Observatory and from me, Andrew Jacob. Thank you for listening and I wish you clear skies until next time.

 

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2 responses to “October 2017 night sky guide

  • Roberto, Thanks for that. The Moon, of course, occults Neptune as you say. I have corrected the transcript above. However, the audio retains the error.

  • I know it’s already occurred by now, but your reference to the Moon occulting itself on 3 October, rather than occulting Neptune, reads very strangely.

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