Total Lunar Eclipse
BEST TIME TO SEE: 21 January from 03:34 UT until 07:00 UT
The Moon enters the Earth’s umbral shadow in the early hours of 21 January producing a total lunar eclipse. Given clear skies, the entire event will be visible from across the UK.
The Earth’s shadow at the distance of the Moon has two parts: a dark inner umbra surrounded by a lighter penumbra, arranged like the inner and outer bullseye at the centre of a dart board. The Moon’s edge first touches the penumbral shadow (P1) at 02:37 UT, when it will be at an altitude of around 20° in the southwest, between Cancer and Gemini. However, you probably won’t be able to see any signs of this weak shadow at this point.
Looking at the Moon’s disc at P1 shows it to be completely full, an event that is rarer than you might realise. A ‘normal’ non-eclipse full Moon passes just north or south of the Earth’s shadow. Consequently, a telescopic view of a regular full Moon still shows a tiny sliver of terminator shadows at the extreme north or south of the lunar disc.
As the Moon creeps deeper into the penumbra, the shadow’s intensity increases and you should eventually be able to detect its presence. On page 61 is this month’s Challenge: to see how early you can spot evidence of the penumbral shadow.
The edge of the Moon first touches the edge of the darker, central umbral shadow (U1) at 03:34 UT, marking the start of the first partial phase of the eclipse.
The partial eclipse grows in depth until the following edge of the Moon reaches the edge of the umbral shadow (U2) at 04:41 UT. This marks the start of the total eclipse. The Moon continues to move into the umbra, reaching the point of greatest eclipse at 05:12 UT. For this particular eclipse, the centre of the Moon lies half an umbral shadow radius north of the shadow’s centre at greatest eclipse, so expect the northern edge of the Moon to appear brighter than the southern edge. Although the umbral shadow represents the Sun’s light being completely blocked by the Earth, our atmosphere refracts sunlight into the shadow. As our atmosphere is good at scattering blue light, the infill light is mostly orange and red. Consequently the Moon’s disc will typically appear orange during a total lunar eclipse. The clarity of Earth’s atmosphere also has an effect and each eclipse is different, appearing anything from a light coppery yellow to a really dark brown that virtually disappears against the background sky. Totality ends at 05:43 UT (U3) when the Moon reaches the far side of the umbral shadow. A second, decreasing partial eclipse follows with the Moon leaving the umbral shadow completely at 06:51 UT (U4) as the dawn sky starts to brighten. The departure from the penumbra will be complete at 07:48 UT (P4), with the Moon hanging low above the west-northwest horizon in a twilight sky.