Kepler believed the Copernican model and sought to prove that it was correct using Braheís data for the positions of the planets.
He found that
Planets orbit in elliptical paths (not circles!) with the Sun at one focus of the ellipse.
A line from the Sun to a planet will sweep out the same area in a certain time interval, regardless of where the planet is in its path.
The ratio of the (period)2 to (semi-major axis)3 was the same for every planet.
He described the planetsí orbits, but could they be explained? Kepler answered ďWhat?Ē but didnít know ďWhy?Ē
Isaac Newton formulated three laws of motion and a law of gravitation.
This model for understanding motion (how motion is related to forces) and gravitation explained Keplerís three laws.
When ďWhy?Ē matches ďWhat?Ē (theory matches observation), we must reexamine our dearly held beliefs.
This happened again in 1911 with Einsteinís publication of the General Theory of Relativity
an entirely different explanation of gravity
explained phenemena that Newtonís law of gravitation could not explain.
has been verified by experiment to this day
Keplerís first law
planetís orbit the Sun in ellipses, with the Sun at one focus.
the eccentricity of the ellipse, e, tells you how elongated it is.
e=0 is a circle, e<1 for all ellipses
e=0.02 e=0.4 e=0.7
Experiment and theory
eccentricity of the planets
Keplerís second law
The line joining the Sun and a planet sweeps out equal areas in equal time intervals.
As a result, planets move fastest when they are near the Sun (perihelion) and slowest when they are far from the Sun (aphelion).
If it sweeps out equal areas in equal times, does it travel faster or slower when it is far from the Sun?