Galileo, Tycho, and Kepler

Galileo’s Experiments (1564-1642)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

What Galileo Saw• An imperfect Sun (sunspots)

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

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

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

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

Tycho Brahe Measurements

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’s Geocentric Model

• Earth at the center of the “Universe” (because parallax still was not seen)

• The Sun travels about the Earth in a perfect circle

• The planets move around the Sun in perfect circles

Tycho had no good reason for adopting this model. He just thought it sounded cool.

Kepler’s Analysis

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 rather than Tycho’s.

Kepler’s Laws1) 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 P2 = a3

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)

Kepler’s Laws

http://astro.unl.edu/naap/pos/animations/kepler.html