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

Explanation

1. As a star is far away from the earth, it appears as a point source of light. The density of air decreases with height above the earth's surface. Hence, the refractive index of air also decreases with height. 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.
3. Further, the apparent position varies with time as the medium is not stationary due to mobility of air and change in temperature. When moare light is refracted towards the observer the star appears bright. When less light is refracted towards the observer, the star appears dim.

4. Thus there is fluctuation in the brightness of a star when observed from the earth. This is called twinkling of a star.
5. Compared to stars, planets are relatively cloaser 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 from 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 point does not twinkle.