# Kepler’s Laws Of Planetary Motion

by on August 28, 2010

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Kepler's Laws

Kepler’s Laws Of Planetary Motion were written by Johannes Kepler between 1609 and 1619. He based his first two laws(published in 1609) on the observations of Danish astronomer Tycho Brahe. All of these laws give approximate descriptions of the motion of the planets around the Sun.

There are three Kepler laws: The orbit of every planet is an ellipse with the Sun at one of the two foci, A line joining a planet and the Sun sweeps out equal areas during equal intervals of time, and the square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit(this last was published in 1619). Kepler’s laws challenged the two major theories of the day. First, it challenged geocentrism by proving that the Earth orbited the Sun. Secondly, it challenged the theories of Copernicus. While Copernicus favored the heliocentric view, he thought that orbits were circular and had epicycles. These laws combined with the mathematical equations of Sir Isaac Newton are the foundation of modern astronomy.

Kepler was considered to be quite the radical. He challenged the notion that orbits were circles, yet a circle is a form of ellipse. Most of the planets follow orbits of low eccentricity, so they can be closely approximated to a circle. The result is that it is not immediately evident that orbits are elliptical. Detailed calculations by Kepler, for the orbit of the planet Mars, indicated its elliptical shape. From that work he inferred that other heavenly bodies have elliptical orbits, also. In this way he allowed for highly eccentric orbits.

Kepler’s third law is currently being used to estimate the distance of an exoplanet(planet outside our solar system) to its star. This estimate is then used to determine if the planet may be within the habitable zone of that star. This information is being used to determine which planets may be Earth-like and; therefore, habitable for humans or other intelligent life based on our known model.

Kepler’s Laws Of Planetary Motion, like many new ideas based on solid science, were revolutionary an controversial when they were first introduced. Since the conclusions could be reproduced and were based on acknowledged science the laws were soon adopted and stand today, though somewhat changed to better fit current observations. Kepler is a true founding father of one of our modern sciences.

We have written many articles about Kepler’s Laws of Planetary Motion for Universe Today. Here’s an article about Kepler’s Laws, and here’s an article about the History of Astronomy.

If you’d like more info on Kepler’s Laws, check out NASA’s Imagine the Universe: Kepler’s Laws, and here’s a link to NASA Science’s Understanding Solar System Dynamics: Orbits and Kepler’s Laws.

We’ve also recorded an entire episode of Astronomy Cast all about Kepler’s Laws of Planetary Motion. Listen here, Episode 189: Johannes Kepler and His Laws of Planetary Motion.

Source: NASA

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