Moon goes dim this Friday

A partial lunar eclipse takes place in the early morning of 19 November 2021. It could be spectacular over much of the northern and western UK – but you need to know just where and when to look.

A lunar eclipse occurs when the Earth comes between the Sun and the Moon, so its shadow crosses the full Moon. In the case of a partial eclipse, the Moon doesn’t go centrally through the shadow, but only through part of it. The Moon has a curved shadow across the top or the bottom of it. 

This is what happens on 19 November, but from the UK the Moon sets quite soon after the eclipse begins, so we only see part of the event. Those farther west, such as in most of  North America, get to see the full eclipse. But at no time is the Moon completely immersed in the Earth’s shadow.

As the Sun, Earth and Moon have to be in line for an eclipse to occur, lunar eclipses can only take place at full Moon, when the Sun and Moon are exactly opposite each other in the sky. on this occasion, from the UK not only will the Moon be low in the sky and about to set, it will occur close to the time of sunrise and the sky will be quite bright. Although this makes the event more difficult to observe, it can also be more spectacular and photogenic, as the Moon will be close to objects on the horizon. If it’s clear, there will be a good opportunity for careful choice of observing position to make the most of the event.

The eclipse could look similar to this from the northern UK at about 7:30 am. Photo taken in December 2010. Photo: Robin Scagell

However, not all of the UK will be favourably placed. In the extreme south-east, the Moon sets before the main partial eclipse even starts. To the north and west, it is higher in the sky and the eclipse has progressed more by the time the Moon sets.

When and where to look

The event starts at the same time wherever you are. The Moon enters the Earth’s outer shadow – the penumbra – at 06:00 on Friday 19 November. The upper part of the Moon will start to look increasingly dusky from this time. The sky will still be fairly dark and the Moon will be between 8º and 13º above your western horizon depending on your location.

At 07:18 the Moon starts to enter the dark part of the Earth’s shadow, the umbra. However, by now the sky is getting quite bright and the Moon is rather low. It has set in the north-west as seen from Dover, and is only 1º above the horizon seen from London. From Birmingham it is 2¼º, from Manchester 3º and from Belfast over 5º in elevation at this time. 

The time of sunrise also depends on where you are: 07:36 in London and 8:02 in Belfast. However, unless you have both a good clear sky and a good horizon in the direction of the Moon you may find it hard to locate it. The best way is to use a sky mapping app on a good smartphone, which should show you the Moon’s position in the sky as you hold it up to the western horizon. 

Please note that the timings given in the November issue of Popular Astronomy are in error.

The position of the Moon within the Earth’s shadow as seen from Carlisle at 07:30 UT on 19 November. The Moon has an elevation of 2º 15′. The darker area is the umbra and the lighter area is the penumbra. Diagram adapted from SkyMap.