Planetary+Motion

https://sciencing.com/two-planets-motion-around-sun-8675709.html http://education.seattlepi.com/planet-stay-orbit-losing-inertia-5918.html

Kepler's three laws of planetary motion.

http://www.1728.org/kepler3a.htm

https://earthobservatory.nasa.gov/Features/OrbitsHistory/page2.php