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  • 6/20/2013

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?

Since no astronaut while traveling in space has reported seeing stars twinkling, the effect must be atmospheric, i.e. due to air. However, it is not exactly correct to say that it is due to turbulence in the air.

Mere turbulence in the air is just what we call wind. Wind does not make stars twinkle, because light travels at a great speed --- over 1 billion km/h.

What distorts the light coming from a star is temperature variations in the air. As you probably know already, air temperature varies a great deal. It typically decreases by 6.5 degrees Celsius for every kilometre you go up, and this accords with the experience that it feels cooler up in the mountains. Also, on a hot day, you may notice the shimmering waves (thermals) that come off a heated road and make a distant car appear wavy.

But exactly how do temperature variations cause twinkling?

When light enters a transparent medium, such as air, it generally changes direction, i.e. it is scattered. By how much it changes direction, i.e. bent, however, depends on the temperature. Warm air bends light less, while cool air bends more, because in warm air the air molecules are further apart from each other, producing less scattering.

Now any star, except the sun, is so far away that practically, it is sending only a single ray of light towards us. As that ray enters the atmosphere, it is scattered differently as it passes through air of different temperatures. When it is scattered away from us, the star seems to disappear for a moment. When it is scattered into our eyes, it seems to reappear, resulting in a twinkle.

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.




Other links:

What Causes Monsoons to Occur?

Why Do We Only See One Side of the Moon?

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