Light emitted from stars does not actually twinkle but only appears to twinkle when viewed from Earth. As starlight passes through the different layers of Earth’s atmosphere, turbulence causes the starlight to bend. This distortion of the starlight makes the star appear as if twinkling. Technically referred to as “scintillation,” this visual special effect is just variations in temperature and density of high altitude air flows that create a light show for observers on the ground. The next time someone sings “Twinkle, Twinkle, Little Star,” remember that it is only an illusion.

Another reason that stars twinkle is their vast distances from Earth. They appear as tiny points of light and those specks are more likely to be buffeted about in the atmosphere. The planets in Earth’s solar system, however, generally do not twinkle because they are closer to Earth and thus appear as small disks of light. Even if they still look like tiny dots, they appear bigger than faraway stars. This larger light from planets cancels out the distortions created as their light passes though Earth’s atmosphere. If you see a point of light in the sky and it does not twinkle, it is probably a planet.

The direction of a star relative to an observer can also impact the degree of apparent twinkle. The light from a star directly overhead passes through less atmosphere than a star on the horizon, which passes through more air. So a star closer to the horizon will generally appear with more scintillation. Even a planet on the horizon could appear with a tiny twinkle to your eye.