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July 30, 2012

When the weather turns colder, many birds head south. But the swan heads west.

Cygnus, the celestial swan, is high in the east as night falls during the warm nights of mid-summer. Its body stretches roughly parallel to the horizon, with its tail at the left and its head at the right. Its graceful wings attach about a quarter of the way from the tail to the head, and extend far above and below the body.

The tail is marked by the star Deneb, which also forms one of the points of the widespread Summer Triangle. Yet summer isn’t the only time to see Deneb in the evening sky.

As Earth turns on its axis, the distant stars return to the same position in the sky every 23 hours and 56 minutes. If you’re quick with numbers, you’ll notice that’s four minutes shorter than the length of a day. That’s because as Earth moves in its orbit around the Sun, the viewing angle to the Sun changes just slightly. So it takes four minutes longer for the Sun to return to the same spot in the sky compared to the other stars.

And that means the other stars rise and set four minutes earlier each day. So Cygnus will be a little higher in the sky at the same hour tomorrow night than tonight. By autumn, it’ll be high in the south at nightfall. And by winter, it’ll be in the west, with its beak pointing down toward the horizon. At that angle, the swan’s outline looks like another shape, giving Cygnus a second name: the Northern Cross.

More about Cygnus tomorrow.


Script by Damond Benningfield, Copyright 2012


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