Popular Astronomy

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What's Up for September 2015

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 15

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 15 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. 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?

This is not a good time of year to be looking for bright planets in the evening sky. Saturn is way down in the early evening twilight at the moment, setting not long after the Sun, in the south-west, and not easy to view. It misses being on the map because it has set by that time.

Venus and Mars are in the early morning sky at the moment, rising over in the north-east after about 4 am. If you are in the habit of getting up before dawn you might see Venus as a bright star low down in the east. Jupiter rises even later, just before it gets light.

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

Harvest Moon
Photo: Robin Scagell

What about the Moon, then?

It begins the month just after full Moon, which was on 29 August,  so not much observing of faint objects will be possible then. Last quarter is on the 5th and then we have new Moon on the 13th. First quarter – that's a half Moon – is on the 22nd and then full Moon this month is on the early morning of the 28th.

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, it's extra special as there is a total lunar eclipse of the Harvest Moon, the first time this has happened at a Harvest Moon since 1996. Trouble is, in the UK it takes place in the early morning when not only Young Stargazers but most other people are asleep. See below for more about this.

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

The great Harvest Moon eclipse

Not everyone knows...

The full Moon always rises opposite the Sun in the sky and at more or less the same time!

This year, on Sunday 27 September, the Harvest Moon will rise as normal, slightly before sunset. It will look like a full Moon, and only with a telescope would you see that there's just a bit of shadow left at the lower left edge. By the time you normally go to bed it's still up there, shining happily. But at 1:10 am on (BST – times are different elsewhere) the total eclipse starts. You won't notice it at first, but after a minute or two you'll notice that the upper left edge seems to be a little darker than usual. The Moon is entering the Earth's shadow.

As you watch – assuming you're allowed to stay up – the dark area gets more noticeable and by about 2.10 there is a definite shadow creeping across the Moon, so a bit of it looks quite black. Half an hour later and the Moon is still quite high up but has a curved shadow across it. Then at 3.11 the Moon is completely in the Earth's shadow, and appears very dark. Stars are visible around it, whereas normally the full Moon is so bright you can't see anything near it.

An instead of being the usual white colour, the dim Moon is likely to be noticeably reddish, with maybe a few other colours thrown in as well toward the bottom edge. The red colour is because the only light reaching the Moon during totality comes from the Earth's atmosphere. If you could stand on the Moon and look back at Earth, you'd see the Earth hiding the Sun, but with its atmosphere shining orange like a ring of fire as sunlight is refracted around it. You know how the Sun appears red when it's low in the sky, at sunrise or sunset, because of the thickness of atmosphere in the way. During an eclipse the light from all those sunsets and sunrises is shining on the Moon. Because the Moon is not central within Earth's shadow on this occasion, the southern edge may appear brighter than the northern edge.

The exact colour and brightness of the Moon during a total eclipse depends on the conditions in our atmosphere at the time of the eclipse. Some are very dark, and some are quite bright. Either way, it's an eerie sight. Imagine how it would have affected our distant ancestors, who relied on the Moon to see at night, when it suddenly turned red without warning.

Mid eclipse is at 3.47 BST, then the light starts to return to the Moon at 4.23 and the whole eclipse is over by 6.23, by which time it's getting light anyway. And it's a Monday morning so you'll probably just have to see the pictures on TV instead of staying up.

Never mind, you say, I'll wait for the next total lunar eclipse. That's on 27 July 2018. In that case, the Moon rises when fully eclipsed after 9 pm in the evening, so it will be at a good time to see it, but the total phase ends at 22.13, before it gets properly dark, so you won't see the Moon in a dark sky at all. The next total lunar eclipse visible in the evening so you get to see the eclipse in a dark sky won't be until 31 December 2028! Who knows where you'll be then!

Here's a link to an animation of the eclipse.

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