Twinkle little star
On most clear nights when we look up into the sky our eyes can spot almost 6,000 stars. The first thing we notice is their twinkle. However stars are not actually twinkling or changing their brightness. The turbulence or the air movement in the earth’s atmosphere causes the light from the stars to bend as the light travels from these distant stars into our atmosphere and finally down to us on the surface. This phenomenon is called the Atmospheric Distortion of Light.
On this path that starlight takes, some rays come to us in a straight line and others get bent. In fact look at the stars that are closer to the horizon. They appear to twinkle far more than the others. This is because there is a greater amount of atmosphere bet-ween you and the star near the horizon, than you and the star when you look up into the sky. This means if you climb high on a mountain where the air is rarefied or thin the distortion should reduce.
So what about the other celestial bodies in the sky, like planets, why don’t they twinkle? Except for the sun, the stars are very far away from the earth. The planets in our solar system however, are smaller than the stars and much closer to us. Through a telescope, stars appear as points and planets appear as discs. Given the earth’s turbulent atmosphere, the single point star image gets mapped to more than one point in image space or sometimes to no single point at all, giving the star the appearance of being invisible.
All this means is that the light coming from the stars did not fall on your eye but landed somewhere else. The planets on the other hand do not appear as just points in space but as larger objects or discs, so it’s unlikely for their image to ever disappear like the light from a star does. Although each ray from a planet is in fact “twinkling” as much as the ray from the stars, when we observe all the rays from a planet together the “twinkling” effect averages out and we see a steady light.
So this is why we sing, Twinkle, Twinkle little Star and not Twinkle, Twinkle little Mercury!
The writer is a Ph.D. in Molecular Genetics and Genomics and is working on skin cancer at Novartis
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