Why Do Stars Twinkle, But Planets Do Not?

To understand planets, we must quickly understand stars.

• Stars are incredibly far away from Earth. Because of this massive distance, a star appears to our eyes as a tiny, single Point-Sized Source of light.
• When this thin ray of starlight enters the Earth's atmosphere, it passes through layers of hot and cold moving air. This moving air bends the light ray back and forth constantly (a phenomenon called Atmospheric Refraction).
• Because the light ray is constantly shifting slightly away from our eye and then back to our eye, the star appears to flicker or 'twinkle'.

Planets do not twinkle because of two main reasons:

A. They are much closer to Earth Even though planets (like Venus or Jupiter) do not produce their own light (they reflect sunlight), they are millions of times closer to Earth than the stars.

B. They act as an Extended Source of Light Because they are so close, a planet does not look like a single 'point' of light. Instead, the human eye sees it as a large disk. In physics, we consider a planet to be a collection of millions of point-sized sources of light grouped together.

The Physics Result: The light from some points on the planet's disk might bend away due to atmospheric refraction, but the light from other points bends towards your eye at the exact same time. The flickering effect of millions of points cancels each other out perfectly. The total amount of light entering your eye remains constant, producing a steady, non-twinkling glow.