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July 11, 2012

The stars that dust the night sky are all moving around the center of the galaxy at hundreds of thousands of miles per hour. Yet they’re so far away that their motion is imperceptible across not just a human lifetime, but hundreds of lifetimes. In fact, even one of the fastest stars as seen from Earth will move just one degree over the next 5,000 years — less than the width of a finger held at arm’s length.

Altair is the brightest star of Aquila, the eagle. In fact, the name “Altair” means “the flying eagle.” The star is in the east at nightfall, at the lower right corner of the bright, widespread Summer Triangle.

Altair is only about 17 light-years away — closer than all but a handful of the stars that are visible to the unaided eye. That’s the main reason it’s moving in such a hurry. It’s like watching race cars on opposite sides of a track. Although the cars are all moving at about the same speed, those on the side closest to you cover a larger angle in the same amount of time than those on the far side.

Yet that motion is too tiny to plot with the eye alone. Instead, astronomers make extremely precise measurements of its position against the background of more-distant stars. Comparing Altair’s position over a period of years reveals its apparent speed across the sky.

Again, look for Altair almost due east at nightfall, climbing high across the south during the night, and almost due west at first light. More about Altair tomorrow.


Script by Damond Benningfield, Copyright 2012


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