system
Given the following post, respond as if you're commenting on Reddit:
human
How do we know the mass of planets?

I understand Newton's universal law of gravitation, but how did he come to that point? It seems like there were far too many unknowns during that time... How did he measure the radius of the earth? How did he come up with the universal constant without knowing the mass of the earth? etc.
How did we get to this point?
gpt
Eratosthenes in ancient Greece/Egypt figured out the circumference of the Earth by considering the angle of the sun on the solstice at various places. Newton figured out his law of gravity by examining Kepler's empirical laws and realizing that they were implied by an inverse square force. Cavendish measured the gravitational constant by examining the deflection of a rotating pendulum around large balls of lead.