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Tue, 22 Sep 2015

The lunar eclipse: What you need to know

A total lunar eclipse. Copyright Robin Scagell

On the early morning of Monday, 28 September we’ll see a total eclipse of the Moon, a Harvest Moon, and a ‘supermoon’ all at the same time. What more could you want? Well, actually, there is one thing that would improve matters – they could occur at a sociable time.

As it is, for UK observers this all happens at just about the worst time for most people – around 4 am BST! For those in the US, however, it occurs in the evening. And, as often happens, some people may get it completely wrong. So here’s when to look and what will happen. Weather permitting, of course!

When to look
This event takes place in the early hours of Monday 28 September, so if you want to see it from start to finish that means staying up on the night of Sunday 27 September. Confusion could be caused because in America it’s still Sunday evening when the eclipse takes place. For the exact timings read on, but the crucial bits are that the eclipse starts at 1.10 BST, mid-eclipse occurs at 3.47 BST, and it’s all over by 6.23 BST.

And because this is a lunar eclipse, not a solar one, these times apply wherever you happen to be. The eclipsed Moon will look exactly the same from Dundee as from Dover at any particular time. The only thing that changes is the time zone from country to country.

What you’ll see
What’s happening is that the Moon passes through the Earth’s shadow. The full Moon will be in the sky from sunset on Sunday evening, but nothing special happens until well after midnight. Soon after the eclipse begins at 1.10 am on Monday morning you will see that the Moon‘s top left edge is looking a bit dusky. Slowly the shadow spreads across the bright Moon. The shape of the shadow is quite different from the usual phases of the Moon, which change only slowly from night to night. Earth’s shadow progresses minute by minute until by 3.11 am the Moon is totally within the shadow.

So what should be a bright full Moon is actually a dark full Moon, with mid eclipse at 3.47 am. But instead of being totally black, there will still be some sunlight light shining on the Moon, refracted around the edge of the Earth’s atmosphere, and reddened just in the same way that the setting Sun is reddened.

If you were on the Moon you’d see the Sun totally eclipsed, with the dark body of the Earth going across it. Our atmosphere would appear as an orange or red rim, with tinges of blue around the edge. From Earth, the Moon goes a dull orange or red. Just how dark it is depends on the weather systems around the rim of the Earth at the time. Although the Moon does often appear red when it’s rising, being low in the sky, when it is high up it is usually much brighter.

From 4.23 am onwards the Moon starts to leave the full shadow, and by 6.23 am, as the new day is dawning, the last traces of the shadow have left its surface. Early risers may not notice it at all, as by this time it is very low in the western sky.

Here are the full timings (BST):

Penumbral eclipse starts 01:11:46 (Moon starts to enter outer edge of shadow)

Umbral eclipse starts 02:07:12 (Moon starts to enter darkest part of shadow)

Total eclipse starts 03:11:11 (Moon fully within dark shadow)

Mid eclipse 03:47:09

Total eclipse ends 04:23:07 (Moon starts to leave darkest part of shadow)

Umbral eclipse ends 05:27:06 (Moon fully leaves darkest part of shadow)

Penumbral eclipse ends 06:22:33 (Moon leaves shadow altogether)

How the Moon moves through Earth's shadow.
But the Moon also moves through the sky from east to west through the night

How to view the eclipse
You don’t need any special equipment to view the eclipse – you can follow it all with the ‘Mk 1 eyeball’ perfectly well. Binoculars will give a more dramatic view of the lunar surface, and may enhance the colours. A telescope is OK as well, but you a low magnification is best so you can see the whole Moon at once.

When the eclipse starts the Moon is due south, in mid sky due south. During the event it gets carried westwards along with the rest of the stars and planets, and towards the later stages it is quite low above the western horizon.

Is it safe to watch?
Yes, unless you trip over something in the dark or catch cold. Better carry out a full risk assessment first....

Where's the Moon?
This seems a silly question, but many people will look out and not see the Moon at all! This is partly because it will be much darker than usual, so you won't see that familiar glow to tell you where it is, and partly because during the total stages it will be over in the western part of the sky, and may even be hidden behind a house or tree from your viewing point.

But here's a simple tip for knowing where to look. At this time of year the eclipsed Moon's position mirrors that of the Sun at the same time in pm or am. So check where the Sun is at, say, 3 pm, and that is where the Moon will be at 3 am the next morning. This won't apply at other times of year or other phases of the Moon, though.

A Harvest Moon.
Photo: Robin Scagell

The Harvest Moon and supermoon
This eclipse coincides with a Harvest Moon and supermoon. A Harvest Moon is always the full Moon closest to the equinox, which this year is on 23 September, when day and night are equal all around the world. Traditionally, its light helped farmers to gather in their harvest, and it so happens that at this time of year the Moon rises at roughly the same time for several nights in a row, helping to extend the working day in the absence of artificial light.

In September, the Moon rises as little as 31 minutes later each night, but in March it’s over an hour later, so it’s not always there just after sunset.

The ‘supermoon’ idea is anything but traditional. The term was apparently invented by an astrologer, not an astronomer, in 1979, to refer to a new or full Moon at the closest point in its orbit to Earth. The Moon’s distance from Earth varies slightly, and when at its closest the Moon appears some 14 per cent larger than when at it’s farthest. The difference isn’t obvious, nor does it have any strong effects other than that the tides are slightly higher than usual, by a matter of inches only.

Why does the Moon look big?
Everyone notices that when the Moon is rising it looks much bigger than when it’s high in the sky, and when the Moon rises on Sunday evening will be very obviously large. Some people think that our atmosphere is magnifying it, but this isn’t the case. If anything, the atmosphere actually squashes the Moon down a bit.

The real reason lies in the way our brain perceives the Moon compared with the horizon, which we know is comparatively close. If you measure it, you’ll find that it is just the same size when high in the sky as when it’s rising. But it’s hard to shake off the belief, and even astronomers see it as large, despite knowing better!

When's the next one?
If this eclipse is a bit too unsocial for you, you might decide to 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!

There are penumbral eclipses of the Moon in September 2016, February 2017 and August 2017, where the Moon goes through the outer edge of the Earth's shadow and just gets a little dusky along one edge; then there's the total eclipse in 2018 mnetioned above, and a similar early morning one in January 2019.

The last total lunar eclipse seen fully from the UK was back in February 2008, which was another early morning event.


Added by: Robin Scagell