Why do stars twinkle?
John A. Graham, an astronomer with the Carnegie Institution of Washington, explains. Have you ever noticed how a coin at the bottom of a swimming pool seems to wobble from side to side? This phenomenon occurs because the water in the pool bends the path of light from the coin. Similarly, stars twinkle because their light has to pass through several miles of Earth’s atmosphere before it reaches the eye of an observer. It is as if we are looking up at the universe from the bottom of a swimming pool. Our atmosphere is very turbulent, with streams and eddies forming, churning around and dispersing all the time. These disturbances act like lenses and prisms that shift the incoming light from a star from side to side by minute amounts several times a second. For large objects like the moon, these deviations average out. (Through a telescope with high magnification, however, we see shimmering images.) Stars, in contrast, are so far away that they effectively act as point sources, and the ligh
On a clear, dark night, our eyes can see about 6,000 or so stars in the sky. They seem to twinkle, or change their brightness, all the time. In fact, most of the stars are shining with a steady light. The movement of air (sometimes called turbulence) in the atmosphere of Earth causes the starlight to get slightly bent as it travels from the distant star through the atmosphere down to us on the ground. This means that some of the light reaches us directly and some gets bent slightly away. To our eyes, this makes the star seem to twinkle. You will notice that stars closer to the horizon will appear to twinkle more than other stars. This is because there is a lot more atmosphere between you and a star near the horizon than between you and a star higher in the sky. Go out some night soon and have a look! Stars also do not blink their eyes (they don’t have any!), tip their hats, nor take a bow!
Stars twinkle because of turbulence in the Earth’s atmosphere. You can think as the atmosphere being made up of several “layers.” Each layer has a different temperature and density. As the light from a star passes through the atmosphere, it is bent by each layer, and we perceive the twinkling. The bending of the light when it passes from one medium to another, like water to air, or one layer of air to another, is called refraction. Refraction is what makes a straw look bent when you put it in a glass of water. Refraction also makes the stars and the Sun near the horizon look higher in the sky than they really are. In fact, when the Sun is setting, we are seeing the sun’s disk when the Sun is already below the horizon! Image not to scale. You will notice that stars closer to the horizon twinkle more; this is because there is a lot more atmosphere between you and a star near the horizon than between you and a star in the zenith (the point directly overhead) . You will also notice that pl
