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Why do stars twinkle but planets do not?

Class 10 ScienceNCERT chapter: The Human Eye and the Colourful Worldइसे हिंदी में पढ़ें

Exam-ready answer

Stars twinkle because their light undergoes atmospheric refraction, causing apparent changes in position and brightness. Planets do not twinkle because they are closer and act as extended sources; the variations from different points average out, giving a steady appearance.

Explanation

Stars are very far away, so they act as point sources of light. When starlight enters the Earth's atmosphere, it passes through layers of different optical densities. This causes continuous refraction, and the apparent position of the star changes. The amount of light entering the eye also varies, making the star appear bright sometimes and dim at other times—this is twinkling.

Planets are much closer to the Earth than stars. They appear as extended sources, meaning they are a collection of a large number of point sources. The light coming from different points of a planet gets refracted by different amounts. The increase in brightness from one point is compensated by the decrease from another, so the total light entering the eye remains nearly constant. Hence, planets do not twinkle.

Atmospheric refraction is the same phenomenon that causes the apparent flattening of the Sun's disc at sunrise and sunset, and the advance sunrise and delayed sunset. The twinkling of stars is a direct observation of this effect.

Points to remember

  1. Twinkling of stars is due to atmospheric refraction of starlight.
  2. Stars are point sources; planets are extended sources.
  3. Atmospheric refraction causes apparent change in position and brightness.
  4. Planets do not twinkle because variations from different points average out.
  5. The phenomenon also explains advance sunrise and delayed sunset.

Exam tip

Write the definition of atmospheric refraction and clearly state that stars are point sources while planets are extended sources. Draw a simple diagram showing refraction through atmospheric layers.

Questions students also ask

What is atmospheric refraction?
Atmospheric refraction is the bending of light as it passes through the Earth's atmosphere, which has layers of different optical densities. This causes the apparent position of objects to differ from their actual position.
Why does the Sun appear flattened at sunrise and sunset?
The Sun's disc appears flattened because light from the upper and lower parts of the Sun undergoes different amounts of atmospheric refraction. The lower part is refracted more, making the disc appear oval.
Do planets also twinkle when seen from space?
No, planets do not twinkle even when seen from space because there is no atmosphere to cause refraction. Twinkling is an atmospheric effect observed from the Earth's surface.

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