Question

Planets do not twinkle because:

• A. They are nearer to Earth and hence we receive a greater amount of light
• B. They are very far away resulting in decreased intensity of light
• C. They emit light of constant intensity
• D. Their distance from Earth does not change with time

Hint:

Think about the difference between a point source and an extended source of light passing through a turbulent atmosphere.

Answer 1

The Correct Option is A

We work through each option using the basic physics of scintillation, meaning the rapid brightness changes caused by a turbulent atmosphere.

1. Saying planets are far away with decreased intensity is factually backwards, planets are actually much closer to Earth than stars, not farther.
2. Saying planets emit light of constant intensity ignores the fact that planets do not produce their own visible light at all, they merely reflect sunlight falling on them.
3. Saying the distance from Earth does not change with time is incorrect, planetary distances do shift slightly as both Earth and the planet orbit the Sun, and this is unrelated to the twinkling question anyway.
4. Because planets are nearer to Earth, they appear as small discs rather than pinpoint sources like stars. The atmospheric turbulence affects the many point-like parts of this disc slightly differently, and these small effects average out across the disc, so the light reaching the eye stays close to steady, meaning the planet does not visibly twinkle.

So option A correctly explains why planets do not twinkle.