Thursday, September 25, 2008

The Ellensburg sky for the week of 9/27/08

Today: Did you time the exact length of the day and night last Monday on the first day of autumn? They were not equal in duration. Many people think that the day and night are the same duration on the autumnal equinox. The day is a little longer than the night for two reasons. First, the Sun is an extended object so even when the middle part has set, the upper half is still above the horizon lighting the sky. The second, and more influential reason, is that the atmosphere acts like a lens, bending light from the Sun above the horizon when the Sun is really still below the horizon. Day and night are closest in duration today.

Sunday: Tonight’s Moon is new. Don’t bother looking for it. The new moon is the phase where the Moon is directly between the Earth and the Sun. Hence the side of the Moon facing Earth is not receiving any sunlight and cannot be seen.

Monday: Jupiter is two fists held upright and at arm’s length above due south at 7:30 p.m.

Tuesday: “You know Aries and Cancer and Draco and Libra. Leo and Pisces and Virgo and Hydra. But, do you recall, the pointiest asterism of all? Triangulum, the three sided asterism, had a very pointy edge….” Sorry. Some stores have started putting up their Christmas decorations and that put me in the mood. Anyway, Triangulum is a small constellation between the more prominent Andromeda and Aries. Its main feature is a skinny triangle oriented parallel to and nearly four fists above the east horizon at 10 p.m.

Wednesday: Venus is a half a fist above the southwest horizon at 7 p.m. Even though it is so close to sunset, Venus is bright enough to poke out through the twilight.

Thursday: Saturn is one fist above due east at 6 a.m.

Friday: Since Halloween is coming up, the stores are filled with bags of candy clusters. Instead, take time to look at a star cluster. The Hyades cluster is an open star cluster that represents the V-shaped face of Taurus the bull. It is one of the biggest and nearest star clusters with about 200 stars 150 light years away. The Hyades cluster was the first cluster to be the subject of detailed motion studies. These studies allowed astronomers to pinpoint the distance to the Hyades and provide important information about the scale of the universe. Aldebaran, one fist above the east horizon at 11 p.m., is a foreground star and not a part of the Hyades cluster.

The positional information in this column about stars and any planet except Mercury is accurate for the entire week.

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