system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How is gravity calculated for distant planets and moons?

How do we know the gravity for saturn is 107% of earths gravity, or how did we calculate the escape velocity for moon for the Apollo 11 mission.
gpt
Gravity is the same everywhere, there's the old Newton formula to calculate the gravitational force that depends on the masses of the two objects and the distance (radius) between their centers.  There is also a  formula for calculating orbital velocity based on the mass of the planet.  So if you know the orbital period of a moon, and the distance, you can calculate the gravity of the planet.