Why do stars twinkle in the night sky?

Starlight passes through a thick atmosphere before reaching Earth. Air temperature and density vary slightly with height and location, so the light bends a little each time it passes between different regions.

These pockets of air are always moving, making starlight enter your eyes and then miss them slightly, over and over. The star therefore seems to brighten and dim. It is similar to a distant car shimmering above hot asphalt in midsummer. Astronomers call this twinkling scintillation.

Stars are so far away that even through a telescope they look like single points. A small shift in light from a point is easy to notice.

Planets such as Venus and Jupiter are much closer and appear as very small disks. Light from different parts of a disk shifts differently, so the effects mix and cancel out, producing a steadier glow. But both stars and planets low near the horizon send their light through more air and therefore twinkle more.

Look at Sirius low in the sky on a winter night and it may flash red, green, and blue, so much that it is sometimes reported as a UFO. The air acts like a prism, bending each color of light slightly differently, and the shimmering happens separately for each color. This happens with other stars too, but Sirius is the brightest star in the night sky, so the changes stand out.

It is easy to think a night of strong twinkling is a good night for stargazing. For astronomers, though, it signals that the air is that turbulent. Through a telescope, the image wavers and blurs. That is why large observatories are built on high mountaintops where the air is thin and calm, and space telescopes such as Hubble go above the atmosphere altogether to capture starlight without the shimmer. If you plan to take a telescope out at home, it is better to choose a night when stars twinkle less. When looking for planets with the naked eye, find a bright point that flickers less and compare it with a nearby star.