Dog Days

StarDate: August 7, 2009

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If you look quite low in the east-southeast during twilight the next few mornings, you may just make out the brightest star in the night sky: Sirius.

Its first appearance in the dawn sky is known as its heliacal rising -- a word that means "with the Sun." In other words, it's rising around the same time as the Sun does.

In ancient times, this first appearance held great significance. In ancient Egypt, where the star represented the goddess Isis, it marked the beginning of a new year. The Nile began its annual floods around that time, bringing water and fertile soil for Egyptian fields.

And in ancient Greece, Sirius ushered in the "Dog Days" -- the hottest, stickiest time of the year. The name comes from Sirius's constellation -- Canis Major, the big dog.

At the time, Sirius made its first dawn appearance a little earlier in the year than it does now. It's moved later because of an effect known as precession.

Earth wobbles on its axis a bit like a spinning top. As it wobbles, the north pole draws a big circle on the sky. So over time, the pole aims at different points. At the same time, over the millennia the Sun appears against different stars at any given date. And that, in turn, means that stars rise at different times in relation to the Sun.

But even though Sirius climbs into view later in the year than it did long ago, the hot, sticky days of summer are still named in its honor -- the Dog Days.

Script by Damond Benningfield, Copyright 2009

For more skywatching tips, astronomy news, and much more, read StarDate magazine.

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