Total Lunar Eclipse on the Winter Solstice

This Tuesday 21 December 2010 is the Winter Solstice, and in addition there will be a total lunar eclipse, occurring at sunrise in the UK.

Total Lunar Eclipse, image by Nick James

Total Lunar Eclipses occur when the Moon passes into the shadow of the Earth. That this does not happen every 29 days (the time it takes for the Moon to orbit the Earth) is due to the fact that the Moon’s plane of orbit is not the same as the Earth’s orbital plane around the Sun, and so the Moon passes above or below the cone of the Earth’s shadow most of the time. Every so often, however, these two planes align to create the conditions for a Lunar Eclipse.

When this happens, the Moon will begin to darken in the sky, eventually turning a dark red colour. Unlike a Solar Eclipse, where the Sun’s light is totally blocked out by the Moon, in a Lunar Eclipse the Moon is still visible.

Tuesday’s Lunar Eclipse will begin at 0528 GMT when the Moon enters the Penumbra, the outer part of the Earth’s shadow. This will begin a slight darkening of the Moon, the darkness extending across the Moon’s surface slowly, taking around an hour. At 0632 GMT the Moon will enter the central, darkest part of the Earth’s shadow, form which point it will darken appreciably until, at 0740 it will be in total eclipse, with the full face of the Moon darkened red. This will last until 0854 GMT, at which point the Moon will slowly begin to darken again.

At this point, however, the Moon will have set for some UK observers, or be very low in the sky, on the western horizon, as it is about to set. The time at which it finally sets depends on where you are. In London it sets at 0812 GMT, while in Glasgow (my home town) it sets at 0857 GMT, just minutes after total eclipse ends.

This means that observers in Scotland will have the best view, and the further north you are the more you’ll see.

This lunar eclipse also has the rare distinction of being one where you can see the eclipsed Moon and the Sun in the sky at the same time, as Sun rises around 11 or 12 minutes before the Moon sets, wherever you are in the UK.