Thursday, December 8, 2011

Constellation of the Month - Cassiopeia, the "W"

Cassiopeia is a "W"- shaped constellation almost directly overhead and slightly north in December soon after sunset. Cassiopeia is an interesting constellation for several reasons.
1. It's Circumpolar: It's one of a few constellations that never rise and never set. They just (appear to) go in circles around the North Star -- every day a circle, and every year a much slower circle. At 6AM tomorrow it will be low in the sky to the north, and 6 months from now in the evening, it will be about the same place.
If you lived at the North Pole, all the stars you could see would be circumpolar. But they'd only be half the stars in the sky, because the other half -- which you would never see -- would be circumpolar for those penguins looking up from the South Pole.
2. It's always opposite the Big Dipper, with the North Star about midway between them. So, if you look north from Cassiopeia, and keep going north and lower in the sky, you'll find the Big Dipper low in the north. 3 months ago and 3 months from now, they were opposite each other east to west.
3. The band of the Milky Way goes through it. We're in the Milky Way galaxy, which is shaped like a disk. When we see it as a fuzzy band of many, many stars, we're looking along the plane of the disk, so we see what looks like a band of stars. When we look anywhere except along this plane we just see a few stars that are close, and that's what the rest of the sky (outside the band) looks like.

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