Astronomy news for the week starting Friday, October 19, 2001.

The Moon passes through its first quarter this week the night of Tuesday, October 23. On its way there it makes a close pass to Mars in the middle of the American afternoon of Tuesday the 23rd, and by evening will make a fine configuration just to the east of the red planet, both bodies to the east of the Little Milk Dipper in Sagittarius.

Though the sky changes only slowly from week to week, it changes surely. At the same time each night, from one week to the next the stars slip another seven degrees to the west, and to see the same sight you have to look another (roughly) half an hour earlier. We therefore lose the western stars to the Sun, the loss compensated by the ever-earlier risings of the eastern stars. Two bright constellations, Taurus and Gemini, representing late autumn and true winter, and tagged with the giant planets Saturn and Jupiter, now rise around 8:30 and 10:30 Daylight Time.

The planets of course have their own motions within these constellations. Saturn, to the east of Jupiter, is now in retrograde (as a result of the Earth beginning to pass between it and the Sun) and is moving westward against the background stars. Jupiter, on the other hand, is still in direct (easterly) motion. It will not enter retrograde ("retro" in the trade) until November 2. As a result, the two planets are (as seen in the sky) moving farther apart. After Jupiter enters retrograde, they will slightly approach each other. But that is temporary. Jupiter will quickly thereafter pull away from the ringed planet, and the two will not be back together again for nearly 20 years, as Jupiter takes 12 years to orbit the Sun, Saturn nearly 30. As a result, Jupiter on the average spends about a year in a given constellation of the Zodiac, while Saturn visits each for just over two years.

Watch Cassiopeia now climb the northeastern sky opposite the Big Dipper, her "W" beginning to go over the pole like a splayed "M." Unlike the Dipper, all of whose are named, Cassiopeia, bright as it is, has few that are. Following behind is bright Perseus, whose central concentration of stars is actually a wide cluster. Between the two, those under a dark sky can make out the "Double Cluster in Perseus," the only example of two clusters, undoubtedly born at the same time from the same interstellar cloud, moving through space together. Eventually, as a result of gravitational forces from the Galaxy, they will separate. Through such forces, and as a result of simple "evaporation" (stars just leaving), each will individually mostly dissolve, as will the bright stars of the Pleiades in Taurus.
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