Kepler’s Laws of Planetary Motion

Johannes Kepler was a German astronomer that lived from 1571 to 1630, whose contributions have earned him the status of ‘father of modern astronomy’ in the eyes of many. Kepler is known for placing the ellipse in the solar system and is mentioned in the same breath as Galileo, Newton, and Copernicus. In this article, you will learn one of the concepts that garnered such acclaim.

Before speaking on Kepler’s accomplishments, you have to take into consideration the thoughts of Copernicus, who spoke of a theory that involved the universe where the sun was located in the center , called heliocentric. While Copernicus wrote about the theory, it was Kepler that concluded that planetary orbits around the sun must be elliptical. If this were true, then the ancient belief that orbits were round would be shattered. Many believed that orbits were round because it was God’s doing and therefore, the shape had to be perfect (in their eyes).

Kepler spent a great deal of time observing planetary actions and following the work of his mentor, Danish astronomer Tycho Brahe (1546 , 1601). Brahe saw the universe with an Earth that didn’t move, which he believed was at the center of it all. He believed the sun orbited the earth and all the other planets revolved around the sun. Kepler felt that he had to ignore the models that positioned Earth in the middle of the universe.

Kepler became a mathematician, which eventually evolved into astronomy. Early in his career, he delivered horoscopes for a living. He started to study the heavenly bodies more closely. When he realized that the planetary orbits were elliptical, his first law of planetary motion defied the beliefs that preceded him. Ellipses were not respected in religious circles.

The second law took a look at the velocity of a planet , stating that it was greater when it was closer to the sun in its orbit and less when it was farther away. The third law focused on the distance (D) of the planet from the sun. Once you have that figure, you can calculate how long it takes for it to go around the sun. This was called a period (or P).

When making these calculations, D was often expressed in astronomical units and P in years. One astronomical unit is equal to 92.9 million miles, which is the approximate distance between the center of the earth and the center of the sun.

3 Laws of Planetary Motion

Interestingly, Kepler established his laws without ever touching a telescope. His calculations can also be used to find the approximate values for other planets. In regards to planetary motion, Kepler stated that:

1) Each planet in the solar system has an elliptical orbit, with the sun at one of the two foci of the ellipse.

2) A line from the center of the sun to the center of a planet sweeps across equal areas in equal amounts of time. This means that the closer a planet is to the sun, the faster it travels.

3) The cube of every planet’s mean distance from the sun equals the square of the time it takes to revolve around the sun.