Why do stars twinkle but planets do not?

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Dear Tom,
Why do stars twinkle but planets do not?

Cleo Lerdstrom, Moline

Dear Cleo,
Light we view from stars does, indeed, appear to oscillate more than
that from planets. Stars are so distant that they appear as small,
single points of light in the sky. Light from all celestial bodies tends
to “swim” when seen from Earth. Air turbulence bends incoming light and
this leads to the effect we call “twinkling.” Light from stars comes to
us as a tiny single shaft of light. On the other hand, multiple
reflected rays of light make up the images we see from planets. These
twinkle as badly as those from stars, but because we simultaneously see
a collection of twinkling shafts of light from planets, the effect is
averaged out. This makes the combined rays of light from planets appear
steadier than the light from stars.

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