It built on lessons learned from the many failures of the Ranger program before it. And the Russians lost 12 landers before their first success.
It should also be noted that Luna 9 was a relatively hard landing, not suitable for a crewed spacecraft. Surveyor 1 was the first soft landing, and a prototype for the missions that would follow.
In fact we dont even know of the Apollo 11 LM is still in orbit of the moon or not.
https://www.youtube.com/watch?v=dBHbLV7xEhc
Scott Manley's video also has some links in its description for some of the tools NASA uses for more precise gravitational estimations and whatnot. Its pretty cool.
This is true on earth as well, though proportionately less so we don’t notice it as much. But for example the level of the sea is different at different places (I believe the general sea level of the pacific is 2 m higher than that of the Atlantic).
Whats more is that basically all things in the solar system exert influence on one another. For some things its not a huge influence but even if you are orbiting the Moon, Mercury or Jupiter have an influence on that orbit.
Overall its mind blowing to me that at some point, not too long ago, we had people able to figure out things like the mass of some body they could barely see and deduce how it affects other bodies in the system.
even today with something like Planet Nine/Persephone[1]. We cant see it, its 10x the mass of earth, but based on the mass and trajectories of what we CAN see and how they are being influenced, we know something(s) more are still out there.
This is something even KSP doesnt do well. And its one of those deeply humbling facts for me that I am not that smart. No way i could ever figure out things like the mass of a celestial body, even with the tools I have right now in front of me 2]
[1] https://en.wikipedia.org/wiki/Planet_Nine#History
[2] https://earthsky.org/space/how-do-astronomers-know-the-mass-...
I mean, it's mostly math. Before we had an excess of computer power, it was basically plug-n-chug with a room full of grad students. It's not something that any one impressive human just eyeballs and know's "ah, that planet floats in a bathtub" at first glance.
Which isn't to say there weren't scarily capable people! If you want an example of one of the people that for me inspires the type pf awe you're talking about. Bob Farquhar
https://en.m.wikipedia.org/wiki/Robert_W._Farquhar
That man, had to have had either one of the most beautiful mental processes, or horrifying, in that actually running the kind of math he did was preferable to anything else.