Question

(a) What is atmospheric refraction? What causes atmospheric refraction? (b) Why do stars twinkle on a clear night? (c) Explain why the planets do not twinkle at night.

Answer 1

Hint: All the questions in our problem are based on the basic principle of atmospheric refraction. We know that our atmosphere does not have a definite or constant density as we go up and above. This leads the atmosphere to become an optical medium with variable refractive indices. This fact will be used to answer all the problems in our question.

Complete answer:
Let us start by explaining the first problem of our question and then we will move on in succession.
(a) Atmospheric refraction is defined as the refraction due to Earth’s atmosphere. Earth has a variable density atmosphere. The density of air near the surface is more and it decreases as we go up. This varying density produces a varying optical medium. So, when light from the sun enters the Earth’s atmosphere, it suffers a slight deviation in its path. This deviation is atmospheric refraction.
The varying density of atmosphere as we go up is the main reason behind atmospheric refraction.
(b) Stars, despite being so large in size, act as a point light source in the night sky. The very faint light coming from these stars suffer atmospheric refraction and due to the ever changing nature of our atmosphere, the resulting light seems to be changing its intensity constantly. This variation in light intensity observed by us is the reason why stars appear to twinkle.
(c) Planets are comparatively large objects when viewed from Earth. This makes the planets a multi-luminous object. So, even if light from some points on the planet appear to be twinkling, the other brighter points cancel it out and it is observed that planets have constant intensity, that is, they do not twinkle.

Note:
Astronomers in olden days used this fact of atmospheric refraction to separate planets from stars in the night sky. Even though the concept of refraction wasn’t established until the time of Newton, astronomers were still able to identify a few planets all by themselves just by separating the non-twinkling stars from twinkling stars.