Why do stars twinkle but planets do not twinkle

CBSE Class 10 Answered

Why do stars twinkle but planets do not twinkle

Asked by Kc4471905 | 01 Mar, 2020, 08:06: PM

Expert Answer

The twinkling of a star is due to atmospheric refraction of starlight. The starlight, on entering the earth’s atmosphere,
undergoes refraction continuously before it reaches the earth. The atmospheric refraction occurs in a medium of gradually
changing refractive index. Since the atmosphere bends starlight towards the normal, the apparent position of the
star is slightly different from its actual position. The star appears slightly higher (above) than its actual position when
viewed near the horizon ( Refer Figure). Further, this apparent position of the star is not stationary, but keeps on changing
slightly, since the physical conditions of the earth’s atmosphere are not stationary due to temperature variation and climatic conditions.

Since the stars are very distant, they approximate point-sized sources of light. As the path of rays of light coming from the star

goes on varying slightly, the apparent position of the star fluctuates and the amount of starlight entering the eye flickers –

the star sometimes appears brighter, and at some other time, fainter, which is the twinkling effect.

The planets are much closer to the earth, and are thus seen as extended sources. If we consider a planet as
a collection of a large number of point-sized sources of light, the total variation in the amount of light entering

our eye from all the individual point-sized sources will average out to zero, thereby nullifying the twinkling effect.

Answered by Thiyagarajan K | 01 Mar, 2020, 11:53: PM