Curiosities: Why do stars appear to twinkle in the night sky?

Stars twinkle because we view them through our atmosphere, says James Lattis, director of University of Wisconsin–Madison Space Place.  “Seen from the moon, where there is no atmosphere, stars do not twinkle at all, but here on Earth starlight passes through many miles of air on its way to our eyes.”

A ray of starlight bends slightly each time it enters warmer or cooler air, Lattis says, and this happens frequently as the ray passes through the atmosphere. “Each temperature zone is something like a bubble of warmer or cooler air, and they are usually rising and falling, which means that the path of the starlight is constantly and randomly shifting,” Lattis says. “We see that unsteady shifting as twinkling.”

So why don’t planets twinkle? “Except for the sun, stars are so far away that they look like dimensionless points to us, so their rays of light reach our eyes in a very narrow beam,” Lattis says. “As that thin beam twitches, the star twinkles.”

Planets are so much closer that we can actually see their disks, which means a much thicker bundle of light rays reaches our eyes, and each takes a slightly different path through the atmosphere, Lattis says. “The shifting of the atmosphere causes a twinkle in any given point on the planet’s disk, but these points average out, so planets generally don’t twinkle nearly as much as stars.”