Summer's heat may still be with us, but the night sky is foreshadowing the changing of the seasons. In particular, one of the key stars of summer is giving way to one of the key stars of fall.
This changing of the stellar guard is best seen around two or three hours after sunset. Look low in the southwest for Antares, the bright orange star that marks the "heart" of the scorpion -- one of the beacons of summer skies. Then look at about the same height in the southeast for Fomalhaut, one of the beacons of autumn skies. It's about the same brightness as Antares, but it shines pure white.
As these two stars show, the night sky is a giant calendar. The same stars climb into view during the same season year after year, decade after decade.
They do so because Earth actually has two "days."
The one we're all familiar with is the solar day -- the one that's 24 hours long. The other is the sidereal day -- the time it takes for the stars other than the Sun to return to the same position in the sky.
The sidereal day is four minutes shorter than the solar day, so the stars rise four minutes earlier each day in relation to the Sun. Add it up, and they take exactly one year to return to the same position in the sky at the same time of night.
So each star has its own "primetime" viewing season. In the case of Antares, it's summer -- a season that's on the way out. And in the case of Fomalhaut, it's autumn -- a season that's on the way in.
Script by Damond Benningfield, Copyright 2009
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