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Kepler and Mars--Understanding How Planets Move
By Edna DeVore
Director of Educational Programs, SETI Institute
posted: 07:00 am ET
04 June 2001
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Kepler_and_Mars_010604_MB Mars -- the Red Planet, the god of war, the home of life? Of all the planets in the solar system, Mars ranks first on a short list as a home for life beyond Earth, past or present, in the minds of scientists and science fiction writers alike. Today, we are a bit closer to a definitive answer than we were when ancient people looked up at the ruddy "star" glimmering in the evening sky and called it Mars. The study of Mars led early astronomers to understand how the solar system works, and to develop a model that displaced Earth -- and humans -- from the center of the universe. They made one small step toward our current understanding of the universe. Mars looks red when seen in the sky, and is especially prominent in the summer skies this year. Why? Every two years Earth, with its smaller orbit, catches up and passes by Mars as we both circle the Sun. When we pass by Mars and are between it and the Sun, astronomers call this an opposition. 
Mars at opposition. Credit: NASA/HST On June 13, 2001 Mars will be opposite the Sun in the sky. Mars will rise over the eastern horizon opposite the sunset. A few days later, June 21, we will actually be a bit closer to Mars, the closest since 1988. The result is that Mars appears larger in diameter through telescopes -- it's closer so it looks bigger -- and appears brighter to the naked eye. The reason why Mars isn't closest at opposition, when we are passing it, is because we travel around the Sun in an elliptical orbit, and so does Mars. Indeed, Mars' motion is at the heart of how a mathematician, Johannes Kepler, discovered that the orbits of the planets are not circles, but ellipses, around the Sun. Kepler lived about four centuries ago. He worked as a teacher and mathematician, along with his duties as court astrologer and astronomer. He had both the good fortune and misfortune to work for the last great astronomical observer of the pre-telescope age, Tycho Brahe. Brahe spent years measuring the position of astronomical objects, including Mars, with great care. His goal was to understand the motions of the planets. But Brahe, who lacked the mathematical skills to analyze all the data, hired Kepler. | Moving with the Planets | | You can model the orbits and speed of planets with a classroom lesson from "The Science Detectives." It is based on Kepler's laws and is suitable for students of all ages. Using simple materials (meter sticks, paper plates, chalk, string, and scissors) students construct a model of the planet's orbits and use rolling wheels that measure how many Earth-years it takes for each planet to orbit the Sun. | 
The directions for this fun and instructional outdoor activity can be downloaded from the SETI Institute web site education pages. Simply select the sample lesson for "The Science Detectives". | The relationship between the domineering, party-going astronomer Brahe and the shy, mystical mathematician Kepler was difficult. Brahe finally challenged Kepler to figure out the orbit of Mars, a problem Kepler wrestled with for nearly a decade. Brahe gave Kepler the data for Mars, including the measurements of Mars at successive oppositions over a period of many years. Mars' motion never quite fit that of a circular orbit, nor an orbit of multiple circles upon circles favored by Brahe and other contemporary astronomers. In desperation, Kepler fit the data to an ellipse -- and it worked. Kepler's discovery was based on observational data. Basing his work on Copernicus' then-revolutionary theory that displaced Earth from the center of the solar system (and the universe, too), Kepler's calculations added mathematical credibility to Copernicus' idea that the Sun was actually at the center (actually, at one of the two foci of the ellipse) with Earth and the other planets revolving around it.
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