Looking at the sky at night, the stars seem to twinkle, changing in size, brightness,
and sometimes even color. This occurrence is caused by the earth’s atmosphere, when the
density of air changes in the line of sight between the viewer and space. This always
happens because weather and wind are constantly moving in the air.
The atmosphere also acts like a prism, bending the colors of starlight by changing
degrees. Refraction makes colors follow different paths, and each is bent and moved by
the atmosphere independent of its neighbors, creating the shimmering, colorful images we
associate with stars.
Stars also look brighter on cold nights because cold, dense air is more stable and
uniform, letting light pass through with less distortion. Another interesting note -
try looking at dim stars out of the corners of your eyes. This is not a natural
phenomenon, but rather a characteristic of the human eye. The cells that receive light
at the back of your eye are place in such a way that distant, dim objects are better
seen when they are not looked at directly.