Planet orbits as seen from earth
The planets revolve around the sun.
The innermost planets go around the fastest
History of how this model came about.
- Astronomy developed (in many places) from time keeping and prediction
- But it is/was more important: it concerns the whole universe and
mankind's place in it.
- The stars seem relatively simple (although they are not). They seem
to be simply light sources fixed to a giant sphere that rotates around
the earth once a day.
- The sun seems relatively simple too. It seems to be carried around
with the stars once a day; and in addition it moves in a circle around
the ecliptic once per year.
- But the planets are a problem. They don't stay put on the celestial
sphere, and their motions are complicated.
Davison E. Soper, Institute of Theoretical Science,
University of Oregon, Eugene OR 97403 USA