Starlight sprinkles down on Earth like raindrops in a summer shower. And like 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 sides 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. The effect is like watching raindrops fall from inside a car. If you’re sitting still, the raindrops might 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 – 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.

Eltanin is high in the north-northeast at nightfall. The moderately bright star stands to the left of brilliant Vega.

Script by Damond Benningfield