2. Dead Guys Continued… So far we have talked about: 1.Aristotle 2.Ptolemy 3.Copernicus Now There's Tycho Brahe

4. So why is Tycho important? 1.He made the most precise observations that had yet been made by devising the best instruments available before the invention of the telescope. 2.His observations of planetary motion, particularly that of Mars, provided the crucial data for later astronomers like Kepler to construct our present model of the solar system. 3.He made observations of a supernova (literally: nova= "new star") in 1572 (we now know that a supernova is an exploding star, not a new star). 4.Brahe made careful observations of a comet in 1577. 5.Brahe did not believe that the stars could possibly be so far away and so concluded that the Earth was the center of the Universe and that Copernicus was wrong. 6.Brahe proposed a model of the Solar System that was intermediate between the Ptolemaic and Copernican models (it had the Earth at the center). It proved to be incorrect, but was the most widely accepted model of the Solar System for a time. Thus, Brahe's ideas about his data were not always correct, but the quality of the observations themselves was central to the development of modern astronomy.

5. Strange goings on in Prague Kepler and Brahe did not get along well. - Brahe gave Kepler the task of understanding the orbit of the planet Mars Unlike Brahe, Kepler believed firmly in the Copernican system. Kepler realized that the orbits of the planets were not the circles demanded by Aristotle and assumed implicitly by Copernicus, but were instead the "flattened circles" that geometers call ellipses.

6. Basic Properties of Ellipses An ellipse has two points called foci (singular = focus) Half of the Major Axis = Semimajor Axis Half the Minor Axis = Semiminor Axis

7. The amount of "flattening" of the ellipse is termed the eccentricity. A circle may be viewed as a special case of an ellipse with zero eccentricity. Eccentricity

8. Kepler’s 1 st Law of Planetary Motion. 1. The orbits of the planets are ellipses, with the Sun at one focus of the ellipse. The Sun is not at the center of the ellipse, but is instead at one focus (generally there is nothing at the other focus of the ellipse). The planet then follows the ellipse in its orbit, which means that the Earth-Sun distance is constantly changing as the planet goes around its orbit.