or an analogy to general relativity, consider that you stretched out a bedsheet or piece of elastic flat, attaching the corners firmly to some secured posts. Now you begin placing things of various weights on the sheet. Where you place something very light, the sheet will curve downward under the weight of it a little bit. If you put something heavy, however, the curvature would be even greater.
Assume there's a heavy object sitting on the sheet and you place a second, lighter, object on the sheet. The curvature created by the heavier object will cause the lighter object to "slip" along the curve toward it, trying to reach a point of equilibrium where it no longer moves. (In this case, of course, there are other considerations -- a ball will roll further than a cube would slide, due to frictional effects and such.)
This is similar to how general relativity explains gravity. The curvature of a light object doesn't affect the heavy object much, but the curvature created by the heavy object is what keeps us from floating off into space. The curvature created by the Earth keeps the moon in orbit, but at the same time the curvature created by the moon is enough to affect the tides.
Sunday, September 5, 2010
Why Do Stars Twinkle?
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
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 planets do not twinkle. Stars are so far away that they appear as points of light, but planets are much closer and their disks can seen through telescopes. The fluctuations in the atmosphere are not large enough to affect the light coming from the planets, the Moon or the Sun.
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
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 planets do not twinkle. Stars are so far away that they appear as points of light, but planets are much closer and their disks can seen through telescopes. The fluctuations in the atmosphere are not large enough to affect the light coming from the planets, the Moon or the Sun.
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