The great lunar eclipse of 2018

This month there will be a spectacular total lunar eclipse partially visible from the UK. The date for your diaries is the evening of Friday, 27 July.

lunar eclipse
Lunar eclipse in 2004. Photo: Robin Scagell

The Moon will rise fully eclipsed, so we won’t see the early stages of the event. But that will actually add to the spectacle, because the Moon will be a deep red colour against the blue sky (assuming of course it’s good and clear!) The media are calling it a ‘blood Moon’ – though that’s not a term astronomers traditionally use!

A lunar eclipse, just to remind you but you really knew anyway, happens when the Moon goes into the Earth’s shadow, so it goes from being the familiar bright moon to a very dark one. As well as being interesting to watch, lunar eclipses can be very beautiful because of the unusual colours that occur.

Lunar eclipses explained

Normally during a lunar eclipse the Moon starts out as a regular full Moon, a complete disc, but then we start to see its left-hand edge become gradually darker until it seems to have a circular bite taken out of it. This is quite different from the phases of the Moon, caused by the Sun shining on it from different angles as the Moon goes around the Earth every month – starting with a crescent, then going to a half Moon, then gibbous when it’s nearly a complete disc, then Full. A lunar eclipse can only take place at full Moon, because that’s when the Sun is exactly opposite the Moon in the sky. In fact, the true full Moon always rises exactly opposite the Sun, and at the time the Sun is setting. And it doesn’t happen at every full Moon, because more often than not the tilt of the Moon’s orbit means that it goes above or below the Earth’s shadow.

So to continue the story of the eclipse, which takes place over a matter of a few hours, eventually the Moon is completely within the Earth’s shadow, so all the Sun’s light is cut off. But it doesn’t go completely black. There’s always a bit of light refracted around the edge of the Earth, even when the Sun is completely covered by the Earth. If you could stand on the Moon during a total eclipse you’d see the Sun gradually being hidden by the Earth’s larger disc, but even when it’s totally behind the Earth there would be red light bent around by our atmosphere.

Lunar eclipse
Stages of a lunar eclipse. The Moon moves from right to left through Earth’s shadow, though it moves from left to right during the evening

The Sun’s light would have to travel through a lot of atmosphere to reach the Moon, so its becomes reddened – we see this every sunset, when the Sun’s light passes through the lowest and densest parts of the atmosphere. So being on the Moon would be like seeing all the sunsets around the Earth at once.

No human has stood on the Moon during a total eclipse, but in 1967 a spacecraft on the Moon, Surveyor 3, did photograph the event with its black and white camera. It showed that some parts of the atmosphere were brighter than others. Where there was a lot of cloud, it was dark. So if all the parts of Earth at the rim as seen by the Moon happened to be cloudy, the eclipse would be dark, and if they were clear, it would be bright. Another thing that affects the brightness is whether there have been any major volcanic eruptions in the past year or so. The eruptions in Hawaii and Guatemala might make this eclipse darker.

Lunar eclipse seen from Moon
The total lunar eclipse of 24 April 1967 seen from the lunar surface by Surveyor 3. The Earth appears with a bright ring of light around its darkened surface as seen from the Moon. Photo: NASA/Galaxy Picture Library

After totality, the Moon slowly comes out of the deep shadow, called the umbra, and eventually starts to go through the paler outer shadow, the penumbra. Eventually it leaves the shadow completely and shines at its full brightness again.

What to expect

So what will we see on Friday 27 July? The Moon will rise as usual, around 9 pm BST (the exact time depends on where you are in the UK). And it will be exactly opposite the Sun in the sky. But instead of being bright and easily visible, assuming there are no clouds of course, it will be dark and red. Just how dark we can’t say. It could be completely invisible, or it could be deep red or orange in colour. The Moon usually is yellowish or reddish when it rises anyway, because it’s so low in the sky, but on this occasion it will be a much deeper colour. Maximum eclipse is at 9.21 pm, when the Moon is only a few degrees above the south-eastern horizon as seen from the south-east of the UK, though from the north-west the Moon doesn’t rise until around 9.30 pm.

The exact rising position also depends on where you live. From central England the azimuth (measured from north through east) is 122º, while from northern Scotland it is 127º. Tip: Use Google Earth’s Path tool to draw a line at this azimuth from possible observing sites to see which works for you.

Lunar eclipse
Total lunar eclipse of 16 September 1997, when the Moon rose during the total phase of the eclipse. Photo: Robin Scagell/Galaxy

But around 30 minutes after sunset it should be easier to find, as it gets higher and the sky gets darker. It will probably be a very deep red, and even non-Moon watchers will realise that this is a very unusual appearance. Then the left-hand edge of the Moon will start to get a bit lighter until at 10.13 pm sunlight starts to return to the rim of the Moon and the total eclipse becomes a partial one. Over the next hour or so the dark shadow slowly leaves the disc, and by 11.19 the Moon is only in the outer shadow, so it just appears darker than usual at the right-hand side. By half past midnight it’s all over and the Moon is back to being a full white disc again.

You’ll be able to see the eclipse from anywhere in the UK, though you’ll need a clear south-eastern horizon to see the earlier stages when it’s rising. And from about 10 pm you’ll see the planet Mars right below it, appearing red itself because Mars really is red.

The timings

Moon enters penumbra (Earth’s outer shadow) 18:12 BST (not visible from UK)

Moon enters umbra (Earth’s dark inner shadow) 19:24 (not visible from UK)

Totality begins (Moon fully within umbra) 20:30 (not visible from UK)

Mid eclipse 21:21

Totality ends 22:13

Moon leaves umbra 23:19

Moon leaves penumbra 00:30

eclipse timings
How the eclipse will progress on 27 July. From the UK the Moon doesn’t rise until around mid eclipse

Although at least part of the eclipse will be visible from all over the UK, how much you see of it will depend very much on where you live. The farther south and east you are, the better. From Dover, the Moon rises at 20:42 BST, and by mid totality it will be 4º above the horizon, but from Manchester the Moon is only 1.4º altitude at mid eclipse. From most of England and Wales the Moon has risen by mid eclipse, but from Scotland and Ireland the Moon rises after mid eclipse.

The eclipse will be an eerie sight, with the red totally eclipsed Moon barely visible as it rises, within a few minutes of sunset. How well it will be visible will depend on how clear the sky is. The Moon at this time of year is low in the sky and appears large, so could be very photogenic with a suitable foreground.