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Moon and Spica

Every star in the night sky is moving around the galaxy in a hurry — hundreds of thousands of miles per hour. Yet the stars are so far away that it’s not easy to measure that motion.

Consider Spica, the bright star that hangs below the Moon as night falls.

Seen from here on Earth, the Moon is about half a degree wide. Now imagine cutting that small disk from top to bottom into more than 30 thousand equal slices. The amount that Spica moves across the sky in a year is the width of one slice. At Spica’s distance of 250 light-years, that works out to a speed of about 40,000 miles per hour relative to the Sun.

That sideways motion is known as proper motion. But it tells us only part of the story of Spica’s motion through space. The other part is its radial velocity — how fast it’s moving toward or away from us.

Proper motion is measured by watching how a star moves against the background of other stars. For radial velocity, though, astronomers have to take the spectrum of a star — they split its light into its individual wavelengths or colors. Those wavelengths are shifted by the star’s motion. They shift to redder wavelengths if the star is moving away from us, and bluer if it’s moving toward us.

Spica’s light is shifted toward the red. The size of that shift tells us that it’s moving away from us at about 2200 miles per hour — part of its overall motion through the galaxy.

Tomorrow: the midnight Sun.

Script by Damond Benningfield

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