People in the UK will have the chance to see a total eclipse of the Moon at sunrise on the morning of 21 December, the date of the winter solstice. Unusually, for British observers this eclipse includes a brief period when both the Sun and eclipsed Moon are above the horizon and precisely opposite each other in the sky.
In a total lunar eclipse, the Earth, Sun and Moon are almost exactly in line and the Moon is on the opposite side of the Earth from the Sun. The Moon is full but moves into the shadow of the Earth and dims dramatically with the shadowed lunar surface lit by sunlight that passes through the Earth's atmosphere. Stronger atmospheric scattering of blue light means that the light that reaches the lunar surface tends to have a reddish hue, so observers on Earth will see a Moon that is much darker than usual, with hints of colour that depend on terrestrial conditions.
The Moon travels to a similar position every month, but the tilt of the lunar orbit means that it normally passes above or below the terrestrial shadow. A Full Moon is seen but no eclipse takes place.
Lunar eclipses are visible wherever the Moon is above the horizon. This time the entire eclipse will be visible from the whole of North America, the eastern Pacific and the northwest of South America. From western Europe, including the UK and Ireland, the Moon will set during the eclipse as the Sun rises.
It begins at 0528 GMT (all times that follow are GMT) when the Moon enters the lightest part of the Earth's shadow, the penumbra. Soon after the Moon should have a slight yellowish hue. The Moon enters the darker part of the terrestrial shadow, the umbra, at 0632.
The total phase (totality) begins when it is completely immersed in the umbra at 0740. From London, the Moon will then only be 3 degrees above the northwestern horizon, so observers will need a good unobstructed view to see it. In the northwest of the UK, the prospects are better. In Glasgow at the same time, the Moon will be 7.5 degrees high and in Stornoway in the Western Isles the Moon will be about 10 degrees above the horizon.
By this time the sky will be brightening as sunrise draws near, which in London happens at 0804, in Glasgow at 0846 and in Stornoway at 0912. In the three locations, the Moon sets at 0812, 0857 and 0925 respectively. Totality ends at 0854, meaning that Londoners will miss the later part of totality, Glaswegians will potentially just about see the whole total phase and residents of the Western Isles could enjoy an even better view.
In the later phases of the eclipse not visible from the UK, the Moon leaves the umbra at 1002 and the eclipse finishes when it exits the penumbra at 1106.
During the eclipse the Moon lies in front of the stars of the constellation of Taurus, although these will be increasingly hard to see in the twilight sky and will be invisible after sunrise. However, the low altitude of the Moon offers some photogenic opportunities for photographers to image the Earth's natural satellite in eclipse next to more familiar terrestrial landmarks. And unlike an eclipse of the Sun, the whole event is quite safe to watch and needs no special equipment.