Johannes Kepler
Johannes Kepler was a German mathematician and astronomer in the 17th century. While working as a mathematics professor in Graz, he wrote the first public defense of the Copernican model of the solar system. He later moved to Prague, to work with Danish astronomer Tycho Brahe. After Tycho died in 1601, Kepler inherited his post as Imperial Mathematician. Using Tycho research and data, Kepler later devised three laws. The laws didn't explain why the planets behaved the way they did, but were instead a way of generalizing the data and making sense of it. Later in his life he calculated predictions for planetary movement and location, finding when the transits of Venus and Mercury would occur, though he was not alive to witness it. Kepler died in 1630, though his grave was destroyed two years later in the Thirty Year War. |
Kepler's First Law
Kepler's first law said that planets travel in ellipses. The first focus is the sun, and the second is located in empty space. The longer axis is the major axis, and the shorter is the minor axis. The drawing is greatly exaggerated. Most of the planets orbit in nearly circular paths. Earth has a difference of 6000 miles between the closest and farthest, but objects like comets have highly irregular orbits. |
Kepler's Second Law
Kepler's second law, the "equal area law," is about the speed at which planets orbit. The closer an object is to the sun, the faster it moves, so the area that a planet sweeps out(see image) in a certain amount of time is always equal, regardless of which times you choose. Kepler was unable to explained why things moved fast close to the sun; it wasn't until Newton explained gravity that scientists could actually do the math. |
Kepler's Third Law
Kepler's third law relates the distance from the sun to the orbital period (a fancy way of saying year). Although it makes since that the further away something is, the longer the year is going to be, because it has to travel more distance, it's not quite that simple. The square of the orbital period is equal to the cube of the semimajor axis (remember: the major axis is the long direction of the ellipse). P^2=A^3 where P is the period in years and A is the semimajor axis in AU. For example, Jupiter is 5.46 AU away from the sun. Therefore, P^2=5.46^3, or P= 12 years. |
Kepler's laws of planetary motion of the basis of learning astrophysics. At the time that they were written, no one, including Kepler, could explain why things happened the way they did. Kepler could only say that, based on observations lasting multiple decades, that that was the way planets behaved while orbiting. A century later though, everything changed with the Newtonian revolution...