‘Twinkle Twinkle Little Star’ is among the first nursery rhymes we learn. We see millions of stars twinkling in the sky during the night. They radiate their light in all directions. Though they appear to be very small, in reality these stars are very big. Most of the stars are many times bigger than our Earth. They look smaller only because they are far off from the Earth. Do you know why stars twinkle? A thick cover of air known as the atmosphere surrounds our earth. There is a huge vacuum beyond this atmosphere. The gases present in the atmosphere are in constant motion. It is because of the movement of gases that 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’. It is because of this refraction, that the light reaching our eyes from the star fluctuates constantly. Due to this fluctuation of light, the stars appear to be twinkling. Now the question arises — Why doesn’t the moon or the other planets twinkle like the stars? The stars are very small in size (apparent size), while planets appear bigger. The star may be considered as a point-size object, while the planets are an extended source or a collection of point-sized sources of light, such that they nullify the twinkling effect. Hence, the angles subtended by the moon, and the planets at our eyes are larger than the angles subtended by the stars. Thus, the turbulent atmosphere is unable to cause the variation in light flux entering the eye. Also because of the larger angles, our eyes are not able to detect the deviation in path of light from the moon and the planets and hence they do not appear to be twinkling.