Stars twinkle because their light must pass through pockets of Earth's atmosphere that vary in temperature and density, and it's all very turbulent. On rough nights, a star appears to shift position constantly as its light is refracted this way and that.
It's much like watching a coin appear to dance at the bottom of a pool.
Astronomers try to overcome the twinkling by using adaptive optics, in which many small mirrors on the scope adjust constantly to allow for the atmospheric disturbances.
They can also use space-based telescopes to make observations. Telescopes orbiting Earth above the atmosphere avoid the problems caused by turbulence.
Planets, on the other hand, generally do not appear to twinkle. The planets of our solar system are much closer to Earth than the stars we see , so planets loom slightly larger within our field of vision (in reality, stars are actually much larger ). Because planets appear larger, the light reflecting from them is less shifted around by the pockets of air in the turbulent upper atmosphere, according to astronomer and blogger Phil Plait.
But the rule is not hard and fast — planets can appear to twinkle when the atmosphere is unusually turbulent.
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