2 min
Why do stars twinkle while planets shine steadily?
Stars never actually blink — the Earth's atmosphere does. Planets hold steady because they are close enough to look like tiny discs rather than single points.
Answer
Look up on a clear night and the stars seem to twinkle like they are winking at you. But have you noticed that one especially bright light — Venus or Jupiter — just sits there, perfectly steady? They are all points of light in the sky. Why the difference?
Stars never blink. The Earth's atmosphere does.
A star is so far away that even through a huge telescope it stays a point, with no size at all. That pinpoint of light has to travel through a hundred kilometres of air that is unevenly heated and constantly churning. Pockets of air with slightly different densities bend the light by slightly different amounts, so it arrives at your eye flickering and shifting. Astronomers score this wobble as "seeing", measured in arcseconds: above a city it is often 2 to 3 arcseconds, while at a top site such as Mauna Kea the typical value is about 1 arcsecond or better.
Planets do not twinkle because they are close enough to show up as a tiny disc instead of a point. The disc contains countless points, each flickering on its own, and they average each other out. The old rule "planets do not twinkle" was a way for sky-watchers two thousand years ago to tell planets from stars.
Stars near the horizon twinkle hardest, because their light cuts through much more air. And space telescopes like Hubble and Webb sit outside the atmosphere entirely, so their images are always sharp — which is the plainest reason they have to be up there at all.
In one line: a star's twinkle is special effects added by our own atmosphere, and a planet looks steady because it is close enough to average those effects away.
Michael's Thought
I always thought the stars were winking. It is our own air wobbling. So what I am really looking through is the atmosphere, not just the star.
Sources
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