Stars twinkle because Earth's turbulent atmosphere bends their light before it lands.

The longer version

Turbulent air constantly shifts temperature and density, and each pocket bends starlight by a slightly different amount. Starlight reaching the eye is effectively a single narrow beam, so those small deflections make the point seem to flicker in brightness and wander slightly. The effect is strongest near the horizon, where the light passes through more air.

Planets twinkle far less because they appear as tiny disks rather than points, and the light from different parts averages out the distortion. From above the atmosphere, stars shine steadily. This blurring is exactly what adaptive optics and orbiting observatories are built to overcome.

Where this entry sits

The night sky looks calm, yet almost everything up there is moving, burning, or slowly changing. These facts cover planets, stars, the Moon, gravity, black holes, comets, and the story of how people began exploring beyond Earth. The full list sits on the Space and Sky page, where every entry is listed in order.