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Originally Posted by stingc
vinaur is correct. If one object is much less massive then the other, the precise value of its mass is irrelevant (the big mass still matters).
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Could you elaborate on this point? It sounds to me like you just stated certain conditions based on matter distribution and from those conditions decided that you did not need to know those conditions. It is like saying "If A is greater than B, or if B is greater than A, then you do not need to know their value."
My understanding of gravity is this: If you have two masses they will orbit around each other. If one of the masses is larger, the smaller object will orbit it much more than the larger object orbits the smaller (but it still does a little bit). Planets can be detected around distant stars by watching for this "wobble".
What is a precise value? Is it within 1 trillion earth masses or 1/2? When you accept a loss of precision in your mass measurement you also accept a loss of precision in your orbital calculations. You can ignore some data on the grounds that it will not make a large impact on your calculations, but too many people fail to understand that the universe didn't
forget it was there.