Read the following and answer the questions that follow:
Priya and her friends are on a stargazing trip on a clear night. Priya notices that stars near the horizon seem to twinkle more vigorously than those directly overhead. Her friend Arun points out that a bright object he identified as the planet Jupiter does not twinkle at all. Later that evening, they notice the full Moon also does not twinkle.
(i) Why do stars twinkle? Name the phenomenon responsible. [1]
(ii) Why do planets like Jupiter not twinkle even though they are also observed through the earth's atmosphere? [1]
(iii) Priya wonders why stars near the horizon twinkle more than those directly overhead. Provide a scientific explanation. [2]
Generated by claude-sonnet-4-6 · 2026-06-26 01:09 · grounding rag
Model Answer
(i) Stars twinkle due to atmospheric refraction of starlight. Since stars are very distant point-sized sources, the continuously changing physical conditions of the atmosphere cause the path of light to vary, making the star appear to flicker (sometimes brighter, sometimes fainter).
(ii) Planets are much closer to Earth and appear as extended sources (not point-sized). A planet can be considered a collection of many point-sized sources; the variations in light from all these individual sources average out to zero, nullifying the twinkling effect.
(iii) When a star is near the horizon, its light passes through a thicker, denser layer of the atmosphere compared to stars overhead. More atmospheric layers mean greater and more variable refraction, causing stronger fluctuations in the apparent position and brightness — hence more vigorous twinkling.
Source: Chapter 10, Section 10.5 — Atmospheric Refraction
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Explanation
- (i) Always name the phenomenon (atmospheric refraction) and explain the point-source nature of stars — both are needed.
- (ii) The key word examiners look for is "extended source" and the idea that individual variations cancel/average out.
- (iii) This is 2 marks — you need TWO logical steps: (a) thicker atmosphere near horizon, and (b) more refraction/variation = more twinkling. One line alone won't fetch full marks.
- The Moon/Jupiter comparison in the passage is the same extended-source argument — always link back to point-source vs extended-source distinction.