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Northern Cross

December 25, 2016

Some beautiful decorations light up the evening sky on this Christmas night. Venus, the brilliant “evening star,” is low in the southwest as night falls. Mars stands to the upper left of Venus, shining pale orange. And the Summer Triangle perches low to the right of the two planets, with its highest point marked by Deneb, the star at the tail of Cygnus, the swan.

When the swan stands high in the sky, it really does look like a graceful bird soaring through the starry Milky Way. As it noses toward the horizon, though, the pattern looks more like its other name: the Northern Cross. It’s just above the horizon around 8 or 9 o’clock, with the long axis standing straight up, and the cross parallel to the horizon.

Cygnus was first drawn thousands of years ago. But if the people who created it could look at the stars tonight, they wouldn’t notice any difference in its pattern at all.

The stars are all moving around the center of the galaxy at a good clip. And they’re at different distances from Earth, so their motions across the sky are different, too. So over time, the familiar patterns in the night sky will become distorted — and, eventually, unrecognizable. But on the human scale, “eventually” is a long time. The stars are all so far away that their drift across the sky is imperceptible — even over the span of many lifetimes.

So the graceful swan — and the Northern Cross — will continue to decorate the night sky for millennia to come.


Script by Damond Benningfield


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