Stars twinkle because their light arrives from an almost point-like source and is repeatedly refracted by moving layers of Earth’s atmosphere. Planets appear as tiny disks, so light from different parts of the disk averages out much of that fluctuation.
The atmosphere is always moving
Air is not equally dense at every height. Temperature differences create pockets and layers with slightly different densities, and light changes direction when it crosses between them. These changes are small, but starlight travels through a long column of atmosphere before reaching your eyes.
The result is a rapid change in apparent brightness and position. This atmospheric effect is called scintillation, and it is strongest when a star is low above the horizon because its light travels through more air.
A star is effectively a point
Even the nearest stars are so far away that the unaided eye sees them as points of light. If a moving air cell bends that narrow beam, the whole visible source seems to brighten, dim or jump. The star itself is not switching on and off.
Stars can also show colour flashes near the horizon because different wavelengths are refracted by slightly different amounts. The atmosphere is adding the shimmer.
Why planets look calmer
Planets are much closer. Through a telescope they show a small disk rather than a single point. Each part of that disk sends light through a slightly different atmospheric path, so the fluctuations partly cancel when combined in your eye.
“Usually” matters: planets can shimmer when the air is very turbulent or when they are close to the horizon. The steadier appearance is a statistical effect of their apparent size, not a special kind of light.
Sources and evidence
- NASA Space Place — Why do stars twinkle? ↗
Atmospheric turbulence and the difference between stars and planets.
- European Southern Observatory — Atmospheric turbulence ↗
How changing air distorts astronomical images.
Sources consulted: 9 October 2026. Written with AI assistance and checked against the linked references; not independently reviewed by a subject specialist. Sources do not endorse this article. See our editorial policy.
Revision: Explained scintillation with the point-source versus disk-source comparison.