Popular Astronomy

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What's Up for September 2016

It's September and term has started again.  But look on the bright side – or rather the dark side. It gets dark at a reasonable hour in the evening and at least you won't get frostbite. But how do you make sense of all those stars?

That's where we come in. Use our easy-to-follow sky guide and you'll be out there stargazing in no time. You will need a star map, and you will need to know how it works. This may seem obvious, but there is a knack to understanding these things. Here's our map for this month:

Sky for September 16

Wrong way round?

Now don't write in and tell us that we have the points of the compass all wrong. This is a map of the sky, so you have to hold it over your head. When you do this, east and west will be the right way round.

It shows the whole sky, so the scale is quite small. Normally you turn to see different parts of it, so to see the view looking north, for example, hold the map upside down with north at the bottom.

The map shows the sky in the middle of the month at about 10 pm, at the start of the month at 11 pm, or by the end of the month at 9 pm. All times are BST.

TIP If you aren't sure of the direction of north from your location, click here for a page on Getting Your Bearings.

Making sense of the stars

This still being summer, almost, the best way to find your way around the sky is to use the Summer Triangle. Actually this is not as obvious as it seems, because this triangle still remains visible well into the autumn and even the winter, but the fact is that it's a key feature of the summer skies so we might as well stick with the name.

Find it by looking fairly high up to find a really bright white star, Vega. It's the only bright star anything like overhead, so ignore all other stars and look really high up. The next star of the triangle, Altair, is halfway between Vega and the horizon. Altair has a fainter star on either side of it – look at the map to get the idea. The other star is Deneb, which is now more or less overhead. If you can't spot these three stars straight away, remember that the map above is on quite a small scale, so think big and you should spot it.

Many people recognise The Plough (or Big Dipper), which if you want to get your bearings is low down in the northwest. We've picked it out on the map. If you know how to use the Plough to find the Pole Star, keep going and you'll find the W-shape of Cassiopeia, one of the most easily recognised constellations in the sky.

Now use the map below to find more patterns nearby, but don't expect to see those convenient lines helping you to see the patterns. If you do see them, you've been watching too much TV and ought to get out more.

Map for September 16 with constellation names
All maps produced using Stellarium software.

Other constellations to look for

Once you have found the Summer Triangle, you can now start to look for some constellations. There's Cygnus, the Swan, also known as the Northern Cross. It is a large cross-shape with Deneb at the top, marking the tail of the swan which flies down the Milky Way with outstretched wings. "What Milky Way?" you ask. Fair point. You live in Neasden or Newcastle and can't see this Milky Way which is shown on the map. You'll just have to take our word for it that it's there.

To find out more about Cygnus, including why it is unsuitable for children, click here.  One of the best-known stars in Cygnus is Albireo, a fairly faint star marking the head of the Swan, or the foot of the Cross. Midway between Altair and Albireo is a rather cute constellation called Sagitta, which means Arrow, along with two neighbours, Vulpecula and Delphinus.  They may be small, but they have a lot going for them, so take a look.

Going back to the Summer Triangle, find Altair and follow the line of the three stars down towards the south until you came to a brightish star as shown on the map, Alpha Capricorni. You should be able to pick out that this consists of two stars – a double star.  Actually, the fainter star is about six times the distance of the brighter one.

The map shows the Milky Way as a pale band crossing the sky. This is a good time of year to look for it, though you won't see it from light-polluted areas. The best chance of seeing it is high up in Cygnus, but if you go on holiday to a dark-sky area it can appear so bright you will wonder why you can't see it at home. The map shows the Great Rift, a dark zone down the middle of the Milky Way caused by dust clouds.

If you want a map with all the constellation names on it, click here.

Where are the planets?

Though they aren't shown on our map, Saturn and Mars are both in the evening sky at the moment, but they're only visible earlier in the evening than our map is set for. Look for them about 9 pm, when there's still a bit of light in the sky. They are quite low down on the south-western horizon, with Mars being the brighter one, on the left. You'll probably notice its reddish colour as well. 

Actually, Mars will be with us in about the same position in the sky for several months to come. If you know your planets, you'll be aware that it's the next planet out from Earth. This means that it goes a bit slower in its orbit than Earth does, but is almost keeping pace with us. Through a telescope, though, it's a bit of a washout right now. We are leaving it behind, and it's now getting quite distant, which means that it's rather small as well. You should just be able to make out its disc, but from the UK it is so low that you're looking through a lot of our own atmosphere, which really messes up the view. It can look quite colourful, with red at one side and blue at the other, but this is just our atmosphere acting a bit like a prism and those colours aren't really on the planet.

Saturn is a better bet through a telescope, as it is a lot larger. You'll still be able to see the rings, even with a small telescope, so take a look at it if you can before it gets too low.

All the other bright planets are a bit too close to the Sun to be seen easily at the moment.

For a detailed list of things happening in the sky, click here.

What about the Moon, then?

Penumbral eclipse of the Moon
Photo: Robin Scagell

It begins the month at New Moon on 1 September, when it’s in the same part of sky as the Sun and isn’t visible. Having New Moon on the first of the month means that the day of the month will tell you the age of the Moon as well. You might be able to see a thin crescent after sunset over in the west on the 4th. First quarter – that’s a half Moon – is on the 9th and then full Moon this month is on the 16th. Finally, the Moon ends the month with last quarter on the 23rd. This is known as the Harvest Moon, because in olden times before tractors had several kilowatts of lighting on them, farmers relied on the Moon to help them get in the harvest. It so happens that in September, the full, or nearly full, Moon rises more or less at the same time each evening, thus prolonging the twilight.

Actually the harvest is pretty well in by September, and always was, so why the Harvest Moon is the September one is a bit unclear, but maybe it was invented by some Victorian writer who thought it sounded romantic.

This year there is a lunar eclipse of the Harvest Moon, on Friday 16 September, but don't get too excited – it is what we call a penumbral eclipse, where the Moon just goes through the outer part of the Earth's shadow. It won't go really dark, as in a total eclipse of the Moon, but you should see some darkening towards the upper side of the Moon. It won't make a lot of difference to those farmers, not that they need the Harvest Moon anyway these days.

The Moon rises with the eclipse in progress, and mid eclipse is at 19.54 BST, so it all happens at a civilised time. The show is over by 21.55. Well worth a look, and you don't have to lose any sleep over it. There's another penumbral eclipse next February, but we don't get a total lunar eclipse until 27 July 2018.

We have a whole section of the website devoted to the Moon and its features.

Get more helpful info

OK, you've read all this for nothing, now comes the plug. This page is brought to you by the Society for Popular Astronomy, which is a really great society to join. It's based in the UK but there are members in other countries as well. It doesn't cost much to join, and there is a special rate for Young Stargazers. At least take a look at what we have to offer. 
Text by Robin Scagell