3rd Kepler's law

[aphar]

I could never remember what was squared and what was cubed in the 3rd Kepler's law, I always had to derive it anew.

Now I finally have a mnemonic.

• Distance to the Moon is approximately 10 times the radius of the Geostationary orbit (which is just over the length of the equator)


• square root of 10 is just over 3


• 3 cubed is 27


• which is just under the lunar month (29.5 days)

So it's the semi-major axis cubed and period squared.