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Comets and stars are two entirely different bits of sky phenomena, but they "travel" for exactly the same reasons. Their apparent movement in the sky is due not to their movement but to the movement of the earth beneath them.

Comets are part of the solar system -- relatively tiny chunks of dirty ice that orbit the sun in more or less radical ellipses. The more radical the ellipse, the closer they come to the sun at perihelion (their nearest approach to the sun, as opposed to aphelion, the furthest they get). These are the ones we see, because they get close enough to the sun for the sun's energy to begin sublimating their ice into vapor (the comet's tail). They appear to travel in the sky because of their motion around the sun, because of the earth's rotation, and because of the earth's motion around the sun.

Stars also follow orbits, but in general in our neck of the galactic woods, they orbit the center of our galaxy (the Milky Way), and do so in orbital periods of billions of years. They appear to move in the sky primarily because of the earth's rotation.

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17y ago

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