We know the speed at which the Earth must travel to stay in orbit. We have probably calculated the mass of the Earth. Using some magic formulas we can figure out the gravity of the sun.
maybe so, I might be wrong. But I might be right, there's a lot of things, I bet, that are like how people used to view gravity. People used to think a big rock fell faster then a little rock, nobody thought there was anything wrong with that idea, and it's wrong. There's a lot of things we assume with our calculations in space that I bet are wrong.