[paul.fr]

Why do stars appear to "twinkle"?

[another_someone]

Air movement, and differences in air temperature, that causing various refractive effects in the upper atmosphere (a little like the shimmering effect of heat haze at ground level).

[daveshorts]

Yeah, basically light goes through hot air slightly faster than cold air, so if light meets a boundary between the two at an angle the part of the light in the hot air will go faster and if you imagine a car where one side of it is going faster than the other it will tend to turn, this effect is called refraction.

There are lots of small changes in temperature in the atmosphere which will cause the light to bend, these are always changing so where it goes changes all the time, so the image of the star will move slightly, also sometimes the atmosphere will act as a slight convex lens so magnifying the star, making it brighter, and other times it will act as a concave lens making the star less bright, hence the twinkling.

This process is very similar to the sunlight patterns you get under ripples in a swimming pool, in some places the ripples are magnifying the sun and in others making it weaker.

[syhprum]

This effect is much more noticeable when looking at stars as opposed to planets due to the much smaller angle that the former subtend

[daveshorts]

This is because a planet looks much larger in the sky, because it is closer, so it is a bit like lots of stars close together, so all their twinkling averages out, when one part of the planet is twinkling brighter another is twinkling dimmer, making it look a much more constant brightness.