system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How did people on the Apollo missions knew the specifics of the moon?

I'm reading a lot about the beginnings of space exploration, and especially the early missions from soviet Russia and USA with the moon landing goal. But what I read from now is that most of the "complicated" things were about launching and space manoeuvers (like the CSM and LM docking process). Most of the missions before Apollo 11 even just circled around the moon without landing, and I can't figure how they knew they would have to adapt to an environment that different from space.

How did the engineers from NASA (and also soviet Russia, they must have thought about it too) knew what "being" on the moon would be like? What it would need to descend/ascend, gravity, environment, temperatures...

Did they have precise data of it or was it more about safety and redundancy?

Also, irrelevant but I don't know how people lost interest that quickly for space exploration (Apollo 12 and 13 weren't even widely followed until the Apollo 13 incident).
gpt
By knowing the mass and size of the moon, we can what sort of gravity to expect, whether or not there is enough gravity for an atmosphere (there isn't), and without an atmosphere we can determine the temperature of the moon by observing its spectrum from Earth.  

And we can determine the mass and size of the moon by observing how objects in space move around the moon, and how the moon moves around the Earth.  

Think of it this way - if you are in the desert and see a small city in the distance - you might not know if it's small and close, or large and far away.  So you move closer to it and make another measurement, and now you can determine whether it's small or far away.