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Chapter 29 The Solar System Ch. 29.1 Models of the Solar System Geocentric model—early Greek belief that the Earth was the center of the universe. The sun, planets, stars, etc. all revolved around the earth! Could not explain the occasional backward appearing motion (east to west) of some planets…retrograde motion. Ptolemy proposed epicycles to explain retrograde motion. Copernicus’s Model In 1500’s, Nicholas Copernicus proposed that the sun was the center of the universe…heliocentric model. Earth and other planets revolved around the sun in the same direction, but at different distances and speeds. This explained retrograde motion. https://www.youtube.com/watch?v=1nVSz zYCAYk Kepler’s Laws Tycho Brache used a telescope to make detailed observations of the stars and planets. His assistant, Johannes Kepler, explained Brache’s observations in precise mathematical terms. Kepler’s three laws explain most aspects of planetary motion. Law of Ellipses Each planet orbits the sun in a path called an ellipse (oval shaped). Therefore, a planet is not always the same distance from the sun. Perihelion—the point where a planet is closest to the sun. Aphelion—the point where a planet is farthest from the sun. Astronomical Unit (AU)—The average distance between the earth and the sun. Useful term for comparing other planets’ distances from the sun. Law of Equal Areas Describes the speed at which planets travel at different points in their orbits. Kepler discovered that planets move the fastest when they are closest to the sun. They move the slowest when they are farthest from the sun. The triangular sections formed between the sun and any two points in a planet’s orbit are always equal in area for equal time periods. Law of Periods Describes the relationship between the average distance of a planet from the sun and its orbit period. Orbit period—time it takes for a planet to make one complete revolution around the sun. Cube of average distance is always proportional to the square of the period. K x r3 =p2 r = average distance from the sun. p= orbit period. K is a mathematical constant. Convenient to use astronomical unit for K, so K = 1 If K=1, then r3 =p2 Practice If an asteroid is 4 astronomical units from the earth, what is its orbit period? K x r3 =p2 1 x 43=p2 64=p2 p= 8 years Newton’s Application of Kepler’s Laws Newton explained why the planets move the way they do. He developed the concept of inertia…a moving body will change its direction only if acted on by an outside force. Newton identified the force causing the planets to move in curved paths (instead of straight lines) as gravity!