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We spend most of our lives on the bottom of an ocean of air. This ocean provides us with the oxygen that is necessary to maintain life and even protects us from the dangerous radiation of the Sun, but it is also quite a nuisance when looking at the stars. The ocean of air surrounding the Earth is called the atmosphere and, officially, its upper limit is roughly 10,000 miles above the surface of the Earth.
Although the atmosphere extends several times further than the Earth’s radius, most of our problems are found within its lower level. This is where the weather is and where there is weather, there are clouds. A cloud is public enemy number one to a skygazer. Nothing is more frustrating than looking up in a sky atlas a beautiful object, checking that it will be in perfect position that night, rushing out of the house as soon as the sky grows dark, and then watching a huge cloud role in and block out everything above the horizon. There is no solution to this problem other than to move to the desert or to a mountain peak.
Even if the sky is cloudless, the air itself blocks a good amount of light. The air of the atmosphere scatters the light from the Sun during the day. The white light from the Sun is composed of all the colors of the rainbow. Since the air scatters blue and violet light more effectively than the other colors, we see the sky during the day as blue. The air is most dense at sea level where it is 0.0013 as dense as water. If air had this density throughout the entire atmosphere, we would see nothing beyond the Earth’s atmosphere (if we were not crushed by it first). But for every 3.3 miles you move upward in the atmosphere, the density of the air is halved, so most of the air in the atmosphere is concentrated in the ten miles closest to the Earth’s surface.
Whenever light travels from a material of one density to another material of a different density, the direction of the light slightly changes; the light is refracted. As the light from a star enters the lower atmosphere, it too is refracted and because the atmosphere is not stable, but is constantly shifting and flowing, the refraction is not constant. The continuous shifting of the air in the atmosphere causes stars to “twinkle.” As we look at a star, its light travels to us through a slightly different path every moment. These different paths cause the image of the star to shift slightly and we perceive the flow of star images from slightly different spots in the sky as twinkling.
What is so bad about twinkling stars? If you are looking at two stars that are extremely close together, twinkling will make it difficult for your eyes to separate their images. Even a single star is easier to observe if its image is steady.
The twinkling of stars is always there, but is affected by the current state of the atmosphere. I find that cold winter nights provide the best viewing of the sky (although they present other more down to earth problems). Another factor affecting twinkling is where you are looking in the sky. The light from a star or a planet that is at the zenith has passed through less air than when the object was located on the horizon. The atmosphere acts like a giant lens and refracts the light from objects on the horizon so much that you actually see them displaced from their true position by about the diameter of the Moon. An object is best seen at the zenith, the point directly overhead. At that point light has been minimally refracted and the object will appear steadier and at its real position in the sky.