With a neat labelled diagram, explain twinkling of a star. Also explain why a planet does not twinkle.

Explanation

As a star is far away from the earth , it appears as a point source of light . When starlight enters the earth's atmosphere , it undergoes refraction continuously in the medium with gradually changing refractive index . The bending of starlight occurs towards the normal as it passes from the optically rarer part of the medium to the optically denser part .
(2) Hence , when a star is observed near the horizon , its apparent position is slightly higher than the actual position .

Further , the apparent position varies with time as the medium is not stationary due to mobility of air and change in temperature . When more light is refracted towards the observer , the star appears bright . When less light is refracted towards the observer , the star appears dim .
This there is fluctuation in the brightness of a star when observed from the earth . This is called twinkling of a star .
Compared to stars , planets are relatively closer to the earth . Hence , a planet appears as a collection of a large number of point sources . Due to the changes in the refractive index of air , there is a change in the position and brightness of these point sources . There is an increase in intensity of light coming from some point sources while there is a decrease in intensity of light coming equal number of other point sources , on an average . The average brightness of a planet remain the same . Also , there is no change in the average position of a star . Hence , a planet does not twinkle .