ancient astronomers By tisen18 276 BCE eratosthenes sieve of erathosthenes 160 claudius ptolemy geocentric model 1473 nicholaus copernicus heliocentrism Dec 14, 1546 tycho brahe tychonic system 1571 johannes kepler kelpers law of planetary motion 1643 sir issac newton law of gravity 1879 albert einstein theory of relativity