Lecture 3d. Ptolemy & Epicycles
1. Nature was thought to use ``perfect'' shapes for planets orbits --
namely, circles
2. But planets exhibited
retrograde motion across the sky at times. (from Latin "retro" = backward & "grad" = going)
3. Ptolemy solved this problem by inventing
epicycles
4. But problems arose:
- Ptolemy's predictions were only close to observations, and the configurations of
epicycles and deferents needed continual revision as better data became available
- The fact that Mercury and Venus couldn't be seen at midnight bothered some observers
enough to place them in orbit around the Sun
(while keeping the other planets in orbit around the Earth, with epicycles and all). A hint of
things to come?
- We now know why Mercury and Venus can't be seen at midnight, since they do orbit the
Sun -- but our picture is quite different: