Atmospheric Optical Phenomena

Stars twinkle; planets do not. This general, though not inviolable, rule can be explained in terms of refraction. Stars are so far away that their light reaches Earth's atmosphere as a single point of light. As that very narrow beam of light passes through Earth's atmosphere, it is refracted and scattered by molecules and larger particles of matter. Sometimes the light travels straight toward an observer, but sometimes its path is deflected. To the observer, the star's light appears to go on and off many times per second. That is, it twinkles.

Planets usually do not twinkle because they are closer to Earth. The light that reaches Earth from them consists of wider beams rather than narrow rays. The refraction or scattering of only one or two light rays out of the whole beam does not make the light seem to disappear. At any one moment, enough light rays reach Earth's surface from a planet to give a sense of one continuous beam of light.