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Video:Overview of Kepler's Laws of Planetary Motion

with James Lincoln

Johannes Kepler was an astronomer who developed three laws explaining how the planets move through space. This video from About.com will go over Kepler's Laws of Planetary Motion.See Transcript

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Transcript:Overview of Kepler's Laws of Planetary Motion

My name is James Lincoln. I'm a physics teacher here for About.com.

Kepler's Background

Johannes Kepler was a German mathematician and astronomer, and contemporary and correspondent of Galileo. He worked for Tycho Brahe, who wanted him to prove that the orbits of the planets were circles. This didn't work out; the orbits of the planets were not circles.  

Kepler's 1st Law of Planetary Motion

Kepler showed that they were ellipses, that is, planets go around the sun in the shape of an ellipse. That's Kepler's First Law.K epler mostly proved this by using the planet Mars. See, the planet Mars' orbit is particularly elliptical.  

Kepler's 2nd Law of Planetary Motion

And he also noticed that when it was closer to the sun, it travelled faster. That is: the r x v is equal to the r x v. But he called this Equal Areas in Equal Times. That's Kepler's Second Law.

Kepler's 3rd Law of Planetary Motion

Kepler, being a mathematician, also noticed a very unusual phenomenon. The time squared divided by the distance cubed for all of the planets was the same. A constant value, now called the Kepler Constant, that only a mathematician could love. So strange, the time squared and the distance cubed were always in direct proportion. That's Kepler's Third Law.  

Kepler didn't know why these things were true. He had a lot of explanations, which these days we would call non-physical. Later on, Isaac Newton's Universal Law of Gravitation would explain all of Kepler's laws. 

I'm James Lincoln for About.com. Thanks for watching.

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