Ptolemy ( AD ) History of Astronomy - lived in Egypt, at the time a Roman province, where he ruled as a governor - wrote The Almagest, one of first books on astronomy - advocated geocentric universe - Sun, planets, and stars moved on fixed spheres called epicycles Ptolemy

Nicolas Copernicus ( 1473 – 1543 ) Nicolas Copernicus - Polish mathematician and astronomer - advocated heliocentric universe - wrote On the Revolutions of the Celestial Spheres - book not published until after his death

Ptolemian universeCopernican universe

Galileo Galilea ( 1564 – 1642 ) Galileo Galilea - Italian physicist and astronomer - invented the telescope - discovered that Jupiter has moons - observed that Venus has phases, like our Moon - wrote Dialogue Concerning the Two Chief World Systems - convicted of heresy; sentenced to house arrest

Tycho Brahe ( 1546 – 1601 ) Tycho Brahe - Danish astronomer and alchemist - plotted the position of Mars over 25 years - also observed a “new star” in Cassiopeia in 1572; called it a nova; later discovered to be a supernova 7500 light-years from Earth - advocated a “geo-heliocentric” view of the universe

Johannes Kepler ( 1571 – 1630 ) Johannes Kepler - German mathematician and astronomer - assistant of Brahe - used orbit of Mars to develop his Three Laws of Planetary Motion

Kepler’s Laws of Planetary MotionKepler’s Laws of Planetary Motion: (1) The orbits of the planets are ellipses, with the Sun at one focus (2) As the planets orbit the Sun, they “sweep out” equal areas in equal times (3) The ratio of the cube of the orbital radius to the square of the orbital period is a constant short version: orbits are ovals short version: planets move fastest when they are closer to the Sun, slower when they are far away short version: R3R3 T2T2 = constant