Unifying Concept:

In previous reading you learned that Ptolemy's geocentric model of the solar system was the accepted truth until the 1600's. By using epicycles he tried to explain retrograde motion in his earth-centered model. Copernican sun-centered model provided a much easier and simpler explanation of the motions which were observed with the celestial bodies. It marked the beginning of the modern understanding of the universe structure.

Another person who was at the heart of the renaissance in astronomy was Tycho Brahe. He made careful observations of the positions of stars and planets during a twenty year period. All of these observations were made without a telescope (it had not been invented) and were the first long-term observations of these objects.

Upon Tycho Brahe's death, Johannes Kepler inherited and studied Tycho Brahe's notebooks. It was because of these notes that Kepler was able to formulate the three laws of planetary motion.

• 1st law of planetary motion states that the planets travel in elliptical orbits (Each planet revolves around the Sun in an elliptical path (348K QT-NASA), with the Sun occupying one of the foci of the ellipse - NASA)
• 2nd law of planetary motion is known as the equal area law (The straight line joining the Sun and a planet sweeps out equal areas (380K QT- NASA) in equal intervals of time (360K QT - NASA.)
• 3rd law of planetary motion is the harmonic law (The squares of the planets' orbital periods (484K QT- NASA) are proportional to the cubes of the semi-major axes of their orbits - NASA).

Kepler's laws apply not just to planets orbiting the Sun, but to all cases in which one celestial body orbits another under the influence of gravitation -- moons orbiting planets, artificial satellites orbiting the Earth and other solar system bodies, and stars orbiting each other (NASA)