# Essay on Johannes Kepler

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Johannes Kepler was a German astronomer and mathematician who lived between 1671-1630. Kepler was a Copernican and initially believed that planets should follow perfectly circular orbits (“Johan Kepler” 1). During this time period, Ptolemy’s geocentric theory of the solar system was accepted. Ptolemy’s theory stated that Earth is at the center of the universe and stationary; closest to Earth is the Moon, and beyond it, expanding towards the outside, are Mercury, Venus, and the Sun in a straight line, followed by Mars, Jupiter, Saturn, and the “fixed stars”. The Ptolemaic system explained the numerous observed motions of the planets as having small spherical orbits called epicycles (“Astronomy” 2). Kepler is best known for introducing three…show more content…
His first law states, “The orbits of the planets are ellipses, with the Sun at one focus of the ellipse.” As shown in Figure 1, The Sun is not at the focus of the ellipse, but is instead at one focus [usually there is nothing at the other focus of the ellipse]. The planet then trails the ellipse in its orbit, which implies that the Earth-Sun distance is continually changing as the planet goes around its orbit. Kepler’s second law states, “The line joining the planet to the Sun sweeps out equal areas in equal times as the planet travels around the ellipse.” As shown in Figure 2, an imaginary line from the center of the sun to the center of a planet sweeps out the same area in a given time. This means that planets move faster when they are closer to the sun. Kepler’s third and final law states, “The time taken by a planet to make one complete trip around the sun is its period. The ratio of the squares of periodic times for two planets is equal to the ratio of the cubes of their mean distances from the sun.” Kepler’s third law indicates that the time taken by a planet to orbit the Sun increases quickly with the radius of its orbit ("Johannes Kepler: The” 1-4). Kepler’s laws challenged Aristotelean and Ptolemaic astronomy. His statement that the Earth