Why do stars twinkle?

Stars do not really twinkle. Out in space, they shine steadily. The twinkling happens in the last few hundred kilometres of the light’s journey — inside the Earth’s air.

The air is never still. Warm and cool layers move and mix, and each layer bends light a little differently. The star’s light passes through many of these moving layers before it reaches your eye.

Each tiny bend shifts the light slightly. To your eye, the star seems to jump, dim and brighten many times a second. That is twinkling.

Planets usually do not twinkle. They are much closer, so they look like tiny discs rather than points. The small shifts cancel out across the disc, and the planet shines steadily. That is one way to tell a planet from a star.