Moon, Regulus, and Saturn

StarDate: June 25, 2009

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The Moon teams up with Regulus and Saturn over the next few evenings. Tonight, Regulus is to the upper left of the Moon, with Saturn farther along the same line.

Regulus is the brightest star of Leo, the lion, while Saturn is one of the biggest planets of the solar system.

But with your eyes alone, it's hard to tell the difference. Both of them look like stars. Saturn is a little brighter than Regulus, but that doesn't help much -- a good many true stars outshine Saturn. And Saturn shows a golden color, but that doesn't help much, either -- quite a few stars also look yellow or orange.

One difference is that Regulus twinkles more than Saturn does.

Stars twinkle because their light is bent a little as it travels through layers of air of different density. Since those layers are in constant motion, the stars look like they're dancing and flickering through the sky.

The light of a planet passes through those same layers of air. But the planet is much closer, so it forms a tiny disk in the sky instead of a pinpoint, as a star does. Each point in the disk twinkles, too, but since the disk is so big, the points all smear together, and the planet shines steadily.

Of course, if you want to clear up any doubt, just look through a telescope. Regulus still looks like a star, but Saturn looks like a pale striped beachball encircled by beautiful rings.

More about the Moon and its companions tomorrow.

Script by Damond Benningfield, Copyright 2009

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

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