Why Do Stars Twinkle? The Science in Plain English

Why do stars twinkle? Look up on a clear night and you will see them shimmer, flicker, and shift in brightness, as if the sky itself were breathing. It is one of the oldest questions in skywatching, and the answer has almost nothing to do with the stars and everything to do with the air above your head.

Why do stars twinkle? It happens in our atmosphere

A star is so far away that it arrives as a single, razor-thin point of light. To reach your eyes, that light must travel through dozens of miles of Earth's atmosphere, which is full of pockets of warm and cool air constantly moving and mixing. Each pocket bends the light slightly, like ripples on a pond bending the view of a coin below.

Because those air pockets shift moment to moment, the star's light bends in tiny, ever-changing directions. The result is the rapid dance of brightness and color we call twinkling, or, more formally, stellar scintillation. The star is perfectly steady, your atmosphere is not.

Why planets do not twinkle (much)

Here is a wonderful trick for telling stars and planets apart. Stars twinkle, but planets usually shine with a calm, steady glow. The reason is distance. A planet is close enough that it appears as a tiny disc rather than a single point, so the shimmer from many points across that disc averages out into a smooth, untroubled light.

Stars near the horizon twinkle the most, because their light slices through the thickest, most turbulent slab of air. Stars high overhead pass through less atmosphere and look steadier.

• Compare two bright objects: the one flickering is almost certainly a star, the steady one is likely a planet.
• Watch low on the horizon for dramatic twinkling and flashes of color.
• On windy or unsettled nights, expect more shimmer across the whole sky.
• For the steadiest views, look on calm, clear nights well after sunset.

If you are just getting started, our guide to stargazing for beginners and your first night under the stars walks you through the basics. And to put the twinkle test into practice, try spotting the planets with your naked eye and watch how steady they look next to the shimmering stars.

Ready to find a twinkling star for yourself? Open the Starly sky map, let it tune to your location, and step outside to watch the atmosphere put on its quiet, endless show.