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Building on the Work of Others Building on the Work of Others During the 1500s, a Danish scientist named Tycho Brahe (Tooko Bra-ha) carefully observed the planets. For more than 25 years, Brahe studied how the planets moved through the sky. Based on his observations, Brahe concluded that both the sun and the moon orbit Earth. Of course, Brahe was wrong about the sun orbiting Earth. However, Brahe was correct about one thing. He concluded that other planets in the solar system orbit the sun. This was an amazing discovery when you consider that Brahe died before the invention of the telescope. Even without a telescope, Brahe was able to make observations that opened the universe to scientists. One of these scientists was a German scientist named Johannes Kepler. Kepler had been Brahe’s assistant. However, the two men did not get along very well. Brahe was concerned that Kepler might one day become more famous. So Brahe did not share most of his observations with Kepler. Tycho Brahe was the scientist who first concluded that the planets orbit the sun. At that time, scientists thought that the orbits of the planets were circles. Brahe’s observations on Mars did not fit with this idea. Rather than struggle to make sense out of his observations, Brahe decided to give this problem to Kepler to keep him busy. This was one of the few pieces of information that Brahe shared with Kepler. However, this was all Kepler needed. Like Brahe, Kepler also thought that the orbits of planets were perfect circles. Yet, Brahe’s observations on Mars also made no sense to Kepler. Rather than ignore them, Kepler kept working on the problem. Kepler discovered that Mars did not orbit the sun in a circle. Rather, its orbit was more like an ellipse, or stretched out circle. Kepler was the first to discover that the orbits of planets are ellipses as they travel around the sun. He went on to make more discoveries. Just as Brahe feared, Kepler did become more famous. Johannes Kepler discovered the elliptical orbits of the planets. Discovery Education Science © 2007 Discovery Communications, LLC Page 1 of 2 Building on the Work of Others However, Kepler could not explain what holds the planets in their orbits. An English scientist provided the answer. His name was Isaac Newton. Newton showed that the same force that controls the motion of objects on Earth also controls the motion of objects in space. This force is gravity. All the planets are moving through space. Some move faster than others. Because of their motion, you would think that planets would fly off into space in a straight line rather than orbiting the sun. Newton realized that gravity is the reason why planets do not fly off into space. Gravity is a force of attraction. The sun has a gravitational pull on the planets. The sun’s gravitational pull attracts planets to the sun. So why don’t the planets fall into the sun? While gravity is pulling the planets down, the planets are also moving sideways. They move so fast sideways that they miss the sun. If they moved slower, they might crash into the sun. If they moved faster they would fly off into space. It is a delicate balance! Sir Isaac Newton found that the planets revolve around the sun due to gravity. Kepler explained what Brahe had first observed. Newton explained what Kepler had first discovered. As you can see, scientists often build upon the work of others. Discovery Education Science © 2007 Discovery Communications, LLC Page 2 of 2