Why do stars twinkle?
Stars twinkle because of turbulence in the atmosphere of the Earth.
As the atmosphere churns, the light from the star is refracted in
different directions. This causes the star's image to change slightly in
brightness and position, hence "twinkle." This is one of the reasons
the Hubble telescope is so successful: in space, there is no atmosphere
to make the stars twinkle, allowing a much better image to be obtained.
Planets do not twinkle the way stars do. In fact, this is a good way
of figuring out if a particular object you see in the sky is a planet or
a star. The reason is that stars are so far away that they are
essentially points of light on the sky, while planets actually have
finite size. The size of a planet on the sky in a sense "averages out"
the turbulent effects of the atmosphere, presenting a relatively stable
image to the eye.
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