Why Do Stars Twinkle?
At night, we see millions of twinkling stars in the sky. In all directions light is radiated by these stars. Even if they look very small in size however, actually they are very big in size. Most of the stars are several times bigger than our earth. As they are far away from us therefore they look smaller.
Let us study why do stars twinkle?
A thick cover of air which surrounds our earth is known as atmosphere. In the atmosphere there are some gases which move constantly and due to this motion of gases, the density of air is not same all over the atmosphere. In this way the refractive index of air changes from place to place. The light from the stars when it enters the atmosphere, it gets diverged several times from its path before reaching us due to the change of density and consequent change in the refractive index of air. The deviation of light in its passage from one medium to another is known as ‘refraction’. 
Due to this refraction, the light from the stars deviates before reaching our eyes. Because of this deviation the stars appear to be twinkling. Now here is a question, why don’t the sun, the moon and the other planets twinkle like the stars. The sun, moon and the planets are very close to earth as compare to stars and they look bigger than the stars. Therefore the angle subtended by the sun, moon and the other planets at our eyes are larger than that of stars. Due to the largeness of angles, our eyes are unable to notice the variation in the path of light from the moon, the sun and the other planets and therefore they do not appear to be twinkling.
