Lunar Eclipse December 2011
On Saturday 10 December 2011 the Moon will enter total lunar eclipse between the hours of 1405 and 1457 GMT. The full eclipse process begins at 1131 and ends six hours later at 1731 GMT, but outwith the 52 minutes of total eclipse the moon will only be partially in the Earth’s shadow.
A lunar eclipse occurs when the Moon, in its orbit around the Earth, passes into the Earth’s shadow, as cast by the Sun. You might imagine that this would happen once every lunar orbit, or once a month. That it does not is due to the fact that the Moon’s orbit around the Earth is tilted by around 5 degrees compared with the Earth’s orbit around the Sun. So in most orbits the Moon passes above or below the Earth’s shadow.
However, once in a while (there are at least two lunar eclipses each year) the orbital planes will align so that the Moon passes through the Earth’s shadow, sometimes just grazing it, in which case we get a partial lunar eclipse, and at other times passing right through the shadow, when we get a total lunar eclipse.
The Earth’s shadow has two distinct regions, forming two concentric cones: the inner, darker, part of the shadow is called the umbra, and objects within this part of the shadow receive no direct light from the Sun. The outer, lighter, part of the shadow is called the penumbra, and objects within this part of the shadow can receive direct light from the Sun, but part of the Sun’s disk will be obscured by the Earth, and so less light than normal falls on the object.
There are several distinct phases of a lunar eclipse, as the Moon travels through the penumbra and umbra. For this lunar eclipse the total time during which the Moon is at least partially in the Earth’s shadow is 360 minutes, and 52 minutes of this is spent entirely within the umbra, i.e. in total eclipse.
These phases are given the names: P1, the time when the Moon’s disk enter the penumbra; U1, the time when the Moon’s disk enters the umbra; U2, the time when the entirety of the Moon’s disk is within the umbra; U3, the last time when the entirety of the Moon’s disk is within the umbra; U4, the last time when part of the Moon’s disk is within the umbra; and P4, the last time when part of the Moon’s disk is within the penumbra.
Unfortunately, the UK is far from ideally placed to view this total lunar eclipse. The Moon will rise some time around 1530 UT, well after U3 (the end of the total eclipse), and nearing U4 (the end of the partial eclipse). It may still be worth watching for the Moon rising in the east around 1530, but at least for UK observers a blood-red total eclipse Moon will not be seen.