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Name: Vinod N Thanki
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Question:
What do astronomers use to figure out if an object is a star, planet, asteroid or comet?



Replies:
If you just look at an object in the sky once, it could be hard to tell anything much about them (although astronomers do have techniques called "spectroscopy" that tell something about what an object is made of). But if you watch the object for a while, you can see where it is going, trace out its orbit, see what influence it has on its neighbors, etc., which tells you much about it: stars hardly move at all; planets move in pretty ordinary close-to-circular orbits about the sun, while other things can be seen to orbit around them (which then tells us how massive those planets are) and asteroids tend to be clustered in the asteroid belt or else to have kind of elongated orbits, and comets have really wild orbits, and in particular tend to get close to the sun where they flare up and get bright every once in a while.

A. Smith



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