Historical Developments in Gravity 1543 - Polish astronomer Nicolas Copernicus publishes "de revolutionibus" or "of revolutions of the planets".
1601-1619 - German astronomer Johannes Kepler describes the laws of planetary motion (he didn't know the physics behind his observations"
1660-1680 - English scientist Issac Newton coins F=ma and F=GmM/r²
1798 - English physicist Henry Cavendish measures value for G in the lab


Kepler's Laws 1) The planets move in elliptical orbits with the sun at one focus of the ellipse. a is the "semi-major" axis of the orbit. Here the "eccentricity" of the orbit is e = 1 - b/a.

2) A line from the sun to a planet sweeps out equal areas in equal times. (This means that the planet moves faster when closer to the sun, a consequence of conservation of angular momentum.)

3) The period of the orbit squared is proportional to the semi-major axis of the orbit cubed. The eccentricity does not matter, only a.


Kepler's Laws simulator 1

Kepler's Laws simulator 2

Kepler's Law Applet
Kepler's Law applet