Why Stars Twinkle but Planets Don't: The Simple Science

Look at the night sky, and you may notice that stars seem to flicker or twinkle. Planets such as Venus and Jupiter usually look much steadier.

The reason is Earth's atmosphere. Starlight must travel through layers of moving air before reaching your eyes. These layers have different temperatures and densities, so they bend the light slightly in changing directions.

Stars are so far away that they appear almost like tiny points of light. Small changes caused by the atmosphere can therefore make their brightness and apparent position change rapidly. This effect is called atmospheric scintillation.

Planets are much closer and appear as tiny disks rather than perfect points. Light arrives from many parts of each disk, so atmospheric changes tend to average out. That makes planets twinkle much less.

Interestingly, planets can still twinkle when they are low near the horizon because their light passes through more atmosphere.

Practical tip: a bright object shining steadily is more likely to be a planet than a star, although this is not a perfect identification method.