Unit 2 : Activity 2 : Ptolemaic Model : Copernican Model : Brahe : Kepler : Galileo : Newton : Unit Exam

Unit 2: The History

In the last unit, one of the important observations the ancients made (and we can make) while standing on the Earth’s 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.