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June 4, 2016

Starlight sprinkles down on Earth like raindrops in a spring shower. And like the raindrops, the sprinkles of starlight can appear to change direction.

This effect is known as aberration of starlight. It was discovered three centuries ago by an astronomer who was trying to measure the distance to Eltanin, the brightest star of Draco, the dragon.

James Bradley was looking for the star’s parallax. The idea is to observe a star when Earth is at opposite ends of its orbit. That causes stars that are close to us to shift position relative to stars that are farther away. Measuring the size of that shift reveals the star’s distance.

Bradley did see a shift in Eltanin’s position, but it wasn’t related to the star’s distance. Instead, it was caused by Earth’s orbital speed. It’s like watching raindrops fall from inside a car. If you’re sitting at a red light, the raindrops appear to fall straight down. But if you’re cruising down the highway, they appear to come toward you at an angle. The raindrops aren’t moving any differently, but you are.

The same thing happens with starlight. Thanks to Earth’s motion around the Sun, the “drops” of light appear to come in from different angles at different times of year. That causes a star to appear to shift position — the result of the aberration of starlight.

Look for Eltanin about halfway up the northeastern sky at nightfall. It’s a moderately bright orange star, to the upper left of brilliant Vega.

Script by Damond Benningfield

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