references - Gravity explored

https://www.khanacademy.org/science/physics/torque-angular-momentum/torque-tutorial/a/rotational-inertia

https://nl.wikipedia.org/wiki/Wetten_van_Kepler

https://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html

https://youtu.be/dO00VLAbZ74

http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit5/gps.html