Johannes Kepler, Ptolemy and Copernicus

This article explains Keplers thoughts and other great minds in his time as well, including Copernicus and Ptolemy.

Johannes Kepler

(from “The Scientific Revolution” in Holt World History: The Human Journey)

Johannes Kepler was a brilliant mathematician who used models, observation and mathematics to test Copernicus’s heliocentric theory. Some of the ideas on which Copernicus had based his theory were wrong. That slowed Kepler down, but he eventually proved the heliocentric theory correct. Johannes Kepler tried to use mathematics to prove that the planets orbit around the sun. Kepler discovered three “laws” about how planets move in space. These laws are referred to as Kepler’s laws of planetary motion.

Using the observations made by his employer, Tycho Brahe, Kepler found that planets travel around the sun in oval-shaped orbits called ellipses. Kepler’s second discovery was that planets move faster when they are closer to the sun. Kepler’s third law relates to the time it takes for a planet to complete an orbit around the sun.  He published his laws of planetary motion in 1609. It took the work of an Italian scientist, however, to produce clear evidence that the earth moves around the sun.

Kepler: Kepler, or also known as Johannes Kepler, was a brilliant German astronomer who used a variety of estragedies such as mathematics, models, observations to study Copernicus helio-centric theory. Along Keplers observations and studies he proved some ideas wrong about the sun centered theory, but eventually proved it correct. Not only that, but Kepler also used his mathematical skills to prove the fact that the planets orbited around the sun as well. Later on, in the year 1609, Kepler published a book called the Laws of Planetary Motion which basically described the three fundamental laws of the planets movement in space. However, one of Keplers employees named Tycho Brahe held observations, which helped Kepler prove the fact that the movement of the planets traveled around the sun in an oval-shaped orbit also known as ellipses. Not only did the fact of the ellipses came up but also the fact that planets advance faster when they’re nearer to the sun. 


Around the year 1500’s, a Polish scientist and mathematician, also known as Nicolaus Copernicus, was a great man who made great contributions at his time. Along the years, people believed throughout many centuries that Ptolemy theory regarding the earth being in the middle of the universe was incorrect. In the other hand, Copernicus had a different theory which was called the helio-centric theory, regarding the fact that the earth and the other planets revolved around the sun. Therefore, Copernicus soon realized that his theory explained many of the then-known facts about astronomy. Unluckily for Copernicus, when he published his theory in the year 1543, people paid little attention because they said that they could see that the sun and the planets moved around the earth. Not only that, but people also stated that they couldn’t feel the Earth move as well. Through out his observations, Copernicus developed a model which stated that the sun was in the middle of the universe rather than the Earth as Ptolemy said. However, in order to make his theory fit his observations, he added 34 epicycles to his system to make it more specific. 


In the A.D 100’s, a Greek astronomer named Ptolemy stated that the Earth was the center of the universe. Therefore, Ptolemy called it the geo-centric theory. Along his observations, he described the movement of the sun, moon, planets and also the stars through the heavens. The advantage Ptolemy had was that people believed in his theory due to the fact that the helio-centric theory seemed to deny what peoples senses told them. Therefore, Ptolemy geo-centric system/idea explained specifically the movement of the all the planets, stars, sun, and moon and could also predict eclipses and other phenomenon’s in the sky. Not only that, but according to the system, the universe was made up of various spheres within spheres, therefore all having the same center. In other words the center was the Earth as described by Ptolemy.In order to make his system regarding the celestial spheres, Ptolemy added cycles and epicycles, due to the fact that all the things such as planets, stars, moons, ect moved in small circles within their specific spheres. 

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One Response to “Johannes Kepler, Ptolemy and Copernicus”

    On February 6, 2010 at 5:12 pm


    I think Kepler was wrong when he said:planets orbit an elliptical shape,the Sun is at a focus of this ellipse,a ray going from the Sun to the planet sweept out equal area in equal interval of time.
    Please have a time to read my contrary article on Scienceray/2010/02/06.Thanks.

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