Friday, 2 March 2007

Don't forget the Total Lunar Eclipse on Saturday night!

A Total Eclipse of the Moon occurs when the Moon passes into the shadow of the Earth. It can only happen at Full Moon, but it doesn't happen every Full Moon because the Moon's orbit is tilted to ours by about 5 degrees, and so usually it passes above or below Earth's shadow, rather than through it. And sometimes it only passes through the edge of the shadow, giving a partial eclipse. The next total one visible from Ireland will be on Feb 21, 2008.

But this is a good one for Ireland, with all stages of the eclipse visible, and the main part occurs before midnight! If it's clear we'll see the Moon turn a glorious reddish colour - anything from a sort of ochre, to deep red. The exact colour can't be predicted, which is one of the fascinating things about such eclipses. The colour is due to the Sun's light passing through the Earth's atmosphere, which acts like a giant lens, focussing the light onto the Moon so that it never goes totally dark.

The Moon starts to enter the Penumbra, Earth’s faint partial outer shadow at 20.18, and then begins to enter the darker main shadow or umbra at 21.30. It will be completely inside the umbra, i.e. the eclipse will be total, from 22.44 to 23.58, with Mid Eclipse at 23h 21m.

It will have left the umbra at 01.00, and will finally exit the penumbra at 02.24, marking the end of the eclipse.

The penumbral stages are barely noticeable, and it will probably be a few minutes after the start of the umbral phase before you'll notice the lower left side of the Moon start to darken slightly: say about 21.35.

This darkness will gradually spread across the Moon until by about 22.45 it will all be immersed in the shadow of the Earth. However, it won't all appear equally dark, as the Moon does not pass through the centre of the shadow, and even at mid-eclipse at 23h 21m you'll notice that the top left of the Moon won't be as dark as the bottom right, which will be closest to the centre of the shadow.

The Moon will be quite high up in Southern Leo, near Chi Leonis, with Saturn about 24 deg away to the upper right.

The Moon will occult 5th magnitude 59 Leonis after the total phase ends, at about 00.43 for observers in Belfast, and about 00.30 for observers in Cork. That event would be visible in a small telescope.

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