1. Galileo, Tycho, and Kepler

2. Galileo is considered the father of modern physics, and even modern science. He performed a variety of experiments, such as: Dropping balls to measure gravity Rolling balls to examine inertia Observing the sky through a telescope Galileo Galilei (1564-1642)

3. What Galileo Saw An imperfect Sun (sunspots)

4. What Galileo Saw An imperfect Sun (sunspots) A Moon with mountains and craters

5. What Galileo Saw An imperfect Sun (sunspots) A Moon with mountains and craters The “ears of Saturn”

6. What Galileo Saw An imperfect Sun (sunspots) A Moon with mountains and craters The “ears of Saturn” Four moons orbiting Jupiter

7. What Galileo Saw An imperfect Sun (sunspots) A Moon with mountains and craters The “ears of Saturn” Four moons orbiting Jupiter The phases of Venus

8. Tycho Brahe (without a telescope) made extremely accurate measurements of the positions of the stars and planets over the course of 20 years. Tycho Brahe (1546-1601)

9. Tycho believed that the other planets orbit the Sun, and the Sun and those planets together orbit the Earth. He hoped that his measurements of the motions of the planets would confirm this model of the solar system.

10. Johannes Kepler (1571-1630) Tycho hired a young mathematician, Johannes Kepler, to show that his measurements of the planets’ motions could be reproduced by his preferred model for the solar system. Kepler tried to do this, but failed. After Tycho died, Kepler continued to search for a model that would match the observed motions of the planets. He eventually succeeded with a variation of Copernicus’ model in which all of the planets, including Earth, orbit the Sun.

11. Kepler’s Laws 1)Planets move in ellipses with the Sun at one focus 3)The period, P, of an orbit (in years) and its radius, a (in units of the Earth-Sun distance) are related by P 2 = a 3 These laws perfectly predicted the positions of the planets, but they were just math, and Kepler didn’t have a physical understanding of why they were true. 2) Planets sweep out equal areas in equal times as they orbit (as a result, planets farther from the Sun move slower) http://astro.unl.edu/naap/pos/animations/kepler.html