Stellar Motion Reference Point
Date: Spring 2012
How can scientists conclude that the Sun and other Stars move? I know it seems pretty simple, but what would they use as a reference point?
There are sophisticated methods but a simple one that works for the Sun, Moon and Planets is to observe their motion against the background of stars that do not appear to move at all. High resolution observation of the reference background of stars show that they too move with respect to other stars, but that is a much smaller motion.
You are right in thinking that this is not as straight-forward as it sounds. If you are in a car traveling at 55 miles per hour and alongside you is another car traveling at 55 - it would appear that both of you are not moving - unless you can reference the sidewalk or electrical poles by the side of the road. ... and that is the key here, if you compare an object to yourself - say you and a friend are both on skateboards - and you observe your friend as moving, you can only say that your friend is moving relative to your position. In reality, you could be moving and your friend is not, you are stationary and your friend is moving, or both of you are moving at different speeds. What Einstein said about this is that it does not matter which of these conditions you accept, the math and the conclusions about motions and positions will still come out the same. So one way to answer your question is to say, we can just use Earth as the reference point (say that it is not moving) and any observed motion of other objects are a function of their motion relative to Earth - whatever conclusions we derive would still be the same as if the other objects were not moving and only Earth was moving.
Another way to think about this is from pure math. We know the gravitational forces of the planets, we can observe the planets move (against a background of stars that appear to be in slower motion when compared to the planets), and we can calculate what the effect of all these gravitational forces would be on the Sun, and conclude that the Sun must wobble around as a function of the pull of the planets. So the Sun, our math tells us, must at least wobble. So math is the key here.
Greg (Roberto Gregorius)
Good question. We use the Earth as our only reference point. Once we figure out its path around the Sun, the motions of everything else fall into place.
David H. Levy
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Update: June 2012