Wednesday, December 22, 2010

Winter Solstice Lunar Eclipse

A couple of nights ago we got to see a spectacular event: a full lunar eclipse at midnight on the Winter solstice.

[Sun and Moon, Midnight and Noon, Winter Solstice]

This little chart shows a slice of the sky from North to South in order to demonstrate how to calculate positions on the solstice.

1. Find your latitude. You can do this by using your sextant to measure the altitude of Polaris. (or look it up on Google Maps.)
2. Subtract your latitude from 90 degrees to get the altitude of the equatorial line. (In this case Polaris is 43 degrees up from the horizon so the equatorial line is 43 degrees down from the zenith.)
3. The Earth is tilted 23 degrees on its axis. This is the reason for the seasons.
4. The altitude of the sun at noon on the winter solstice is 23 degrees down from the equatorial line, the lowest it will ever be.
5. The altitude of the moon at midnight on the winter solstice is 23 degrees up from the equatorial line, the highest it will ever be.

From this day on, the sun will rise and the moon will sink until the summer solstice when they will have completely changed places: the sun will be high in the sky and the moon will just skim the hills.

This eclipse was so rare because it happened, not just on the right day, but at the precise moment when the moon was at its highest point for the whole year. This was, of course, true only for people near our longitude, the Pacific time zone. For others, the eclipse just happened on the solstice.

I'm extremely glad that the snow stopped and the sky cleared just in time to see the eclipse at its apex. Wow! I don't ever remember seeing the moon so high in the sky. (I'm usually asleep by 9:00 in the Winter and the sky's usually cloudy anyhow, so it's likely that I never *have* seen it!) It was just a dark red disk almost straight up. An hour later it was heading west and still just a sliver.

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