The air is doing it
Why Do Stars Twinkle but Planets Do Not?
Stars twinkle because our restless air bends their single point of light; planets are too wide to flicker.
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Step 1 of 5
The twinkling does not happen at the star.
Starlight leaves perfectly steadily and crosses light years without wavering. Everything that makes it flicker happens in the last few miles.
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Step 2 of 5
Our atmosphere is full of moving pockets.
Warm and cool air mix constantly, and each pocket bends light by a slightly different amount as it drifts across your line of sight.
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Step 3 of 5
A star is effectively a single point.
Even the closest stars are so distant that all their light arrives along one thread, so bending that thread moves the whole image at once.
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Step 4 of 5
A planet is a small disc, not a point.
Planets are near enough to be tiny discs. Each part of the disc is bent differently, and those wobbles average out into a steady glow.
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Step 5 of 5
Twinkling gets worse low in the sky.
Near the horizon you are looking through far more air than overhead, so a rising star flickers and flashes colours far more dramatically.
Telescopes above the atmosphere see no twinkling at all, which is the main reason Hubble was worth launching.
The short version
Stars twinkle because our moving air bends their single point of light, while planets are wide enough that the wobbles average out.
Try it yourself
On a clear night find a steady bright dot and a flickering one. The steady one is almost always a planet.
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