Why do stars twinkle?

Learn why moving air makes distant stars shimmer and why planets often look steadier.

The short answer

Starlight travels through Earth's atmosphere before reaching our eyes. Moving pockets of air with slightly different temperatures and densities bend that light by changing amounts. The result is the apparent shimmer we call twinkling.

Why don't planets twinkle as much?

Stars are so distant that they look like points of light. Planets generally appear as tiny disks, so the fluctuations from different parts of the disk tend to average out. Planets can still twinkle, especially low on the horizon.

The surprising connection: looking through moving glass

Twinkling is related to refraction—the bending of light when it passes through materials with different optical properties. Similar effects create the shimmering look of air above a hot road.

Try it

Compare a bright planet and a bright star on a clear night, especially when each is high above the horizon.