Unit 2: The History
In the last unit, one of the important observations the ancients made (and we can make) while standing on the Earths surface was that Venus and Mercury never get very far from the Sun. This is true is because those two planets are closer to the Sun than Earth is. Therefore, we always see them closer than 90 degrees from the Sun in the sky.
Figuring out what makes the planets wander in our sky
is a puzzle that has interested people for a long time. Several very
different ideas or models for motions of heavenly bodies have been popular
over historical times. In this unit, you'll see three different models
for the solar system. Each one explains what you can see with the naked
eye, including why Venus and Mercury stay close to the Sun in the sky.
Each of these models are also capable of reproducing the retrograde
loops of the planets. With information from telescopes, and a little
help from Newton, we now know that some of these models don't really
describe how the solar system works. However, some of the old models
were pretty sucessful in their day and are still worth studying.