Why do stars twinkle?
Stars twinkle because their light passes through Earth's moving, uneven atmosphere, which bends it slightly in different directions. This makes the star's brightness and position seem to flicker.
Stars are so far away that they look like tiny points of light. As their light travels through layers of air with different temperatures and densities, it is refracted, or bent, many times. Because the air is constantly moving, the amount of bending keeps changing, so the light reaching our eyes flickers. Astronomers call this effect scintillation.
Stars near the horizon twinkle more than those overhead, because their light passes through more atmosphere. Bright stars low in the sky can even seem to flash different colors, as different colors of light are bent by slightly different amounts. Twinkling is usually stronger on windy nights.
Why planets twinkle less
Planets are much closer than stars, so they appear as tiny disks rather than points. Light from different parts of the disk averages out, so planets usually shine with a steadier light. This is a handy way to tell planets from stars with the naked eye.
Did You Know?
Telescopes in space, such as Hubble, avoid twinkling completely, which is one reason they produce such sharp images.