Estimating the Mass of Earth

Once you know the distance to the moon and the time between full moon phases, you can use a little math to figure out the mass of our Earth!  This basic method is what astronomers use to find the masses of all other objects in our universe! 

From Kepler’s Third Law (balancing the gravitational force and centripetal force in a circular orbit) we have the formula                                                                                                                                                                                                                                                        

V2            GM

-----   =   --------

D              D2

Where V is the orbit speed of the moon, D is its distance from Earth’s center to the Moon’s center, and M is the mass of Earth.

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Thumbnail Image: Moon Orbit 
Credit: NASA / SVS