What is Kepler's laws?
[[File:Kepler laws diagram.svg|thumb|300px|Figure 1: Illustration of [[Johannes Kepler|Kepler's]] three laws with two planetary orbits.<br> (1) The orbits are ellipses, with focal points ''&fnof;''<sub>1</sub> and ''&fnof;''<sub>2</sub> for the first planet and ''&fnof;''<sub>1</sub> and ''&fnof;''<sub>3</sub> for the second planet. The Sun is placed in focal point ''&fnof;''<sub>1</sub>. <br><br> (2) The two shaded sectors ''A''<sub>1</sub> and ''A''<sub>2</sub> have the same surface area and the time for planet 1 to cover segment ''A''<sub>1</sub> is equal to the time to cover segment ''A''<sub>2</sub>. <br><br> (3) The total orbit times for planet 1 and planet 2 have a ratio ''a''<sub>1</sub><sup>3/2</sup>&nbsp;:&nbsp;''a''<sub>2</sub><sup>3/2</sup>.]] '''Kepler's laws of planetary motion''' are three laws that describe the motion of [[Planet|planets]] around the [[sun]]: #Planets move around the sun in [[Ellipse|elliptic]] [[Orbit|orbits]]. The sun is in one of the two foci of the orbit. #A [[line segment]] joining a planet and the Sun sweeps out equal areas during equal intervals of time. #The square of the [[orbital period]] of a planet is proportional to the cube of the semi-major axis of its orbit. [[Johannes Kepler]] found these laws, between 1609 and 1619. == Comparison to Copernicus == Kepler's laws improve the [[Scientific model|model]] of Copernicus. If the eccentricities of the planetary [[orbit]]s are taken as zero, then Kepler basically agrees with Copernicus: #The planetary orbit is a [[circle]] #The Sun at the center of the orbit #The speed of the planet in the orbit is constant The eccentricities of the orbits of those planets known to Copernicus and Kepler are small, so the rules above give good approximations of planetary motion; but Kepler's laws fit the observations better than Copernicus's. Kepler's corrections are not at all obvious: #The planetary orbit is ''not'' a circle, but an ''ellipse''. #The Sun is ''not'' at the center but at a ''focal point'' of the elliptical orbit. #Neith