2021 November 15
Observer’s Challenge – November’s Tricky Partial Lunar Eclipse
On the morning of 19 November 2021, from the UK a tricky to see partial eclipse of the Moon occurs. This event favours those living further north and west in the British Isles, the majority of the country getting just a glimpse of the main part of the Earth’s shadow passing across the Moon’s disc before moonset. For those lucky enough to live in parts of the world where the entire event is visible while above the horizon, this is an almost total lunar eclipse, the Earth’s dark umbral shadow reaching 97% of the way across the lunar disc.
If conditions are clear on the morning of 19 November, the event will play out as follows. The bright full Moon will appear to be shining in the sky from sunset on 18 November through the night and into the early hours of 19 November. As the start of astronomical twilight approaches around 06:00 UT, the Moon will be approaching the weaker penumbral part of the Earth’s shadow. Contact with the penumbral shadow occurs at 06:02 UT but this is unlikely to be noticeable.
The penumbral shadow represents the region where an observer would see the Sun partially eclipsed by the Earth. As the eclipse progresses the Moon’s disc will move ever deeper into the penumbral shadow towards the much darker and better-defined umbral shadow.
The umbral shadow represents the region where an observer would see the Sun totally eclipsed by the Earth. The darkness of the umbral shadow increases from the outer edge of the penumbral shadow towards the outer edge of the umbral shadow. The Moon reaches the umbral shadow at 07:19 UT but the deepening darkness of the inner penumbral shadow should be evident before this time, causing the Moon’s northwest limb to appear significantly shaded.
As mentioned earlier, the best views will be had from locations further north and west within the UK. From London, the umbral shadow crossing starts only 6 minutes before moonset, the eclipse only reaching a maximum magnitude of 3% before the Moon is lost below the horizon. Here, eclipse magnitude describes how much of the Moon’s diameter is covered by the umbral shadow. Stornoway on Lewis and Harris experiences just over an hour of true umbral shadow, the partial from this location reaching a peak magnitude of 74% before the moon sets. The umbral eclipse begins an hour before moonset from the west coast of Ireland, the peak magnitude reaching 68%. From the centre of the UK given a flat west-northwest horizon, the eclipse will reach a peak magnitude of 18%, the Moon setting 20 minutes after the start of the umbral phase.