Fall begins this week on Sept. 22 at 9:04 p.m. EDT with the autumnal equinox, a moment at which the sun crosses the celestial equator from north to south. The celestial equator is an invisible line, of course. So what does it mean to the lay person? For starters, the days and nights are equal (sort of--because of the way we calculate sunrise and sunset, it’s not quite true). The sun rises and sets due east and due west, something that happens only on the equinoxes. For those of us in the northern hemisphere, the sun’s path across the sky is exactly halfway between its extreme height in June and its extreme low in December. Each acre of earth in Adams county absorbs energy from the sun that’s in between the extremes of June and December. Finally, days are getting shorter at their fastest rate of the year; each day has 2 minutes, 35 seconds less daylight than the one before.

This month brings us the same parade of three planets as the last. Saturn and Jupiter can be seen in the east to southeast as it gets dark, with Mars rising about 11 p.m. Look for the moon near Saturn on Oct. 5, Jupiter on the 8th, and Mars on the 14th. The moon will be full on Oct. 9. Speaking of the moon, this will be a good month to look for a very “old” and very “young” crescent moon. These terms refer to the very thin crescent moons we see just a day or two before or after the new moon. By nature these sightings are always close to the sun, either just before dawn or just after sunset. This month the moon is new on Sept. 25. You may be able to catch an extremely thin crescent in the eastern sky on Sept. 24 or the western sky on Sept. 26. Look about a half hour before sunrise or after sunset.

Ian Clarke is the director of the Hatter Planetarium at Gettysburg College. More information and schedule of public programs is available online at http://www.gettysburg.edu/hatterplanetarium

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