Ptolemy wrote a valuable history of previous observations called the "Almagest". He performed many experiments on optics and realized that starlight is refracted in the Earth's atmosphere. However, his system of planetary motions was a major blunder, and unfortunately, it was adopted for nearly 1400 years.
The Earth is round, stationary and very small relative to the celestial sphere of the sky.
The stars are fixed points of light inside the celestial sphere.
Day and night result from the rotation of the entire celestial system around the fixed, nonrotating Earth.
The planets moved on small circular paths, called epicycles. The centers of these epicycles move around the earth of other circular paths, called deferents. The combination of the planetary motions along the epicycles and deferents produces the observed wandering of the planets among the stars, including the retrograde motion.
|
|
|
|
| [back to beginning of topic] | [back] | [next] |
|
|
| [back to the topics page] | [back to astro 201 home page] |