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What's Up for September 2017

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 17

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 17 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 it isn't shown on our map, Saturn is in the evening sky at the moment, but it's only visible earlier in the evening than our map is set for. Look for it about 9 pm, when there's still a bit of light in the sky, quite low down on the south-western horizon. 

Saturn is always worth a look through a telescope, even though it is low down. 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 sinks out of view at the end of the month.

Jupiter is also in the western evening sky, some way to the west of Saturn, but it sets soon after the Sun.

Venus, Mercury and Mars are in the early morning sky at the moment, with Venus easily outshining the others. They are all rising in the eastern sky before sunrise, so you'll need to be up about 5.30 am to see them. They are all strung out in a line, with Venus highest, then Mercury and Mars lower down, best seen after the second week in the month. Mars is much fainter than Mercury, so will not be easy to see. The bright star Regulus is in there as well. As the days (or rather nights) go by, they change positions, Mercury being the fastest mover. 

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

Harvest Moon. Photo: Robin Scagel

What about the Moon, then?

It begins the month just after first quarter – that’s a half Moon – which was on 29 August, and then full Moon this month is on the 6th. Last quarter is on the 13th, then comes New Moon on the 20th. You might just see the crescent Moon in the western twilight on the 21st about half an hour after sunset, just to the right of Jupiter. Finally there's first quarter on the 28th.

The full Moon on the 6th 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.

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