Why Do Stars Twinkle

This is a very curious question by people but this question is very common among young minds. A common explanation is that intensity of light of stars change at every instant. But this is not the reason.

In order to explain this phenomenon, we have to know something about the term 'Refraction'. Refraction is the bending of light when it passes through different medium.

As a result of refraction, when we observe an object in water, there is a slight shift in the position of the object.

Coming back to stars, Earth's atmosphere has different layers and each of these layers have different densities. As we go higher and higher, density decreases.

Due to these difference in densities, all these layers of the Earth's atmosphere act as different media.

When light passes through the atmosphere, it undergoes refraction (bending) many times due to change in medium. Since Earth's atmosphere is changing at every instant due to gases, dust and some other factors, the densities of the layers change periodically.

So each and every time, light takes a different path to reach our eyes and takes lesser or greater time to reach our eyes.

Why doesn't moon twinkle then?
As moon appears larger than stars, the twinkling effect of moon isn't detected much by human eyes. If we consider moon as a collection of many small particles, the individual particles twinkle but when we see it altogether, the twinkling effect of all these particles cancel and thus moon doesn't twinkle.