Sat March 05, 2011 By: Madhav Aggarwal

why do stars twinkle but planets do not?explain?

Expert Reply
Sat March 05, 2011

Dear student

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.

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.

Hope this clarifies your doubt.




Ask the Expert