500 BC Pythagoras recognizes that the Earth is round 400 BC Eudoxus makes the models of the spheres 400 BC Aristotle- everything in the heavens is perfect; planets, paths unchanging 1543 AD Copernicus says the sun is central to the solar system 1577 Tycho Brahe finds comet to be part of heavens, not just weather 1609 Kepler figures out the squashed circle shape of planets' path through space 1632 Galileo, using telescopes, makes arguments proving the sun is central 1655 Christian Huygens says Saturn has moons and rings 1687 Newton finds spectrum of white light and works out the theory of gravity, predicts return of Halley's comet (in 1759 comet returns right on schedule) 1781 William Herschel discovers Uranus 1846 Neptune is discovered 1864 Giovanni Donati looks at light from comet with spectroscope 1957 First satellite launched from Earth ( Russian satellite) 1969 First man walks on the moon 1990 Hubble Telescope launched but doesn't work right ( fixed in 1993 ) 2006 Pluto is demoted to a dwarf planet by the Astronomical Union More dwarf planets are discovered in the Kuiper Belt past Neptune