The heavens change with time. Three thousand years ago, Babylonian observers noted the motions of the Sun, Moon, and planets against the background stars, and Western astronomy (and astrology) was born. Millions of Kuiper Belt Objects, asteroids, including Near-Earth Objects, move slowly across the sky, revealing their existence and orbits, and the architecture of the Solar System. Much further away, stars exhibit slow "proper motion" due to their sideways velocity, and "parallax" as a reflection of Earth's revolution around the Sun. The European satellites Hipparcos and now Gaia have studied these motions in exquisite detail, by measuring precise positions hundreds of times over many years.
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