# eli5: Gravity on the moon

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Did the first astronauts understand what it felt like to walk on the moon prior to departing? And if so how was it calculated and recreated?

In: Physics

We knew the moon’s mass so we knew on paper what the force of gravity would be, but the 1960s didn’t have a ton of technology to sinulate low gravity on Earth. AFAIK astronauts spent quite a bit of each moon landing figuring out how to move effectively. The “moon hop” as opposed to walking was something figured out on the moon, not in training

We have known about gravity for centuries. The force of gravity can be accurately and easily calculated based on the mass of the bodies involved. We know the mass of the moon, so we know what gravity should be like in its surface.

It is however impossible (so far) to recreate gravity. We can approximate a simulation by acceleration. You can feel something like gravity briefly increase when some elevators start to move; and you can feel it briefly decrease when they decelerate rapidly.

They definitely understood the theory of it. At that point, we understood that gravity was two masses attracting each other. As such, we could estimate how much mass the moon was, and we knew how large it was, so we could estimate its gravity quite well. The Russians had landed probes/spacecraft on the moon prior, and then there were crew fly-by’s of the moon before a crewed landing was attempted.

There are ways to simulate reduced gravity on earth, but none are 100% ideal. Water pools with properly ballasted suits, or airplanes following a parabolic arch can simulate it. But there definitely was some learning to be done by the first crew on the moon.

The fact that there would be lower gravity on the moon’s surface has been known long before sending astronauts or probes to it was anywhere near possible.

You can simply observe the moon figure out its size and the effect of its gravity on the earth and come to right result. Early attempts to figuring out how much mass the moon had weren’t exactly exact. Issac Newton for example grossly underestimated it when he calculated it based on tides, but by the time spaceflight became a thing we had much better observations and could calcuate the moon’s surface gravity relatively exactly.

The low gravity on the moon and mars and how it would feel like was a thing commonly discussed in sci-fi stories at that time.

There isn’t a good way to simulate low gravity on earth.

We can add apparent gravity by linear and rotational acceleration and for example put people into centrifuges to make them feel like they are in higher gravity, but the only way to make lower gravity is to go down towards earth at the right speed in a plane or long elevator and that will eventually end due to the ground getting in the way.

One thing NASA did to train for low-g was putting astronauts in pools of water with their suits to use the buoyancy to simulate weighing less but that is not really a good facsimile.

BY the time people went to the moon for the first time we already had a few years worth of practical experience with zero-g. And it turned out that some of the fanciful notions of what it might do to people had been wrong.

To get to the moon astronauts had to endure quite a bit of travel in zero g and so there was little concern that it would affect them too badly.

So the Neil Armstrong and Buzz aldrin new intellectually what it might feel like to walk on the moon in low gravity. But until they did it no human had ever experienced the real thing, to know what it really felt like.

The ones who came after them could at least ask them to prepare themselves.

There were a host of unmanned missions to the Moon before the Apollo landings. American and Russian.

Some landed, some were orbital missions.

Their plans were worked out in advance from theoreticals, and if the end result varied from theoretical values, adjustments were made to the next.

If the advance values were wrong, missions would have missed the Moon, or landed in wrong places.

By the time Apollo 11 came around, they knew what to expect