Why do stars twinkle at night but planets shine steadily?
A star is so far away it's effectively a point source; its thin beam gets randomly bent by moving pockets of air of different densities, so its brightness and position flicker — twinkling.
A star is so far away it's effectively a point source; its thin beam gets randomly bent by moving pockets of air of different densities, so its brightness and position flicker — twinkling. A planet is much closer and appears as a tiny disc, i.e. many points. Their individual flickers average out, so the planet's light stays steady.
Key point
Point source (star) twinkles via atmospheric refraction; an extended disc (planet) averages out.
Common mistake
thinking planets don't twinkle because they emit their own steady light.
Memory tip
twinkling = atmospheric refraction of a point source.
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