We see millions of stars twinkling in the sky during night. They radiate their light in all the directions. Even though they appear very small yet in reality they are very big. Most of the stars are many times bigger than our earth. They look smaller only because of their great distance from the earth. Do you know why the stars twinkle?
Our earth is surrounded by a thick cover of air known as the atmosphere. There is vacuum beyond the atmosphere. The gases present in the atmosphere are in constant motion. Because of the movement of gases the density of air in the atmosphere is not uniform everywhere. As such the refractive index of air varies from place to place. When the light from a star enters our atmosphere, it gets deviated from its path several times before reaching our eyes because of the changing density and the consequent change in the refractive index of the air.
The deviation in the path of light in its passage from one medium to another is called ‘refraction’. Because of this refraction, the light reaching our eyes from the star varies. Due to this variation the stars appear to be twinkling.
Now the question arises: why don’t moon, sun and other planets twinkle like the stars? This is so because compared to the stars, sun, moon and the planets are very near to earth and as such they appear bigger than the stars. Hence the angles subtended by the moon, sun and the planets at our eyes are larger than the angles subtended by the stars. Because of the larger angles, our eyes are not able to detect the deviation in the path of light from the sun, moon and the planets and hence they do not appear to be twinkling.