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I am interested in descriptions and discussion from the Apollo astronauts.

Almost all animals and many plants are sensitive to the earth's gravitational field. This is important for large mammals and particularly for large vertically-oriented bipeds. It can be quite a challenge to always keep one's center of gravity suitably positioned above one's feet. There are organs in the inner ear called otolith organs that are sensitive to gravitational fields and linear accelerations. However these organs seem primarily sensitive to changes in orientation of fields and perhaps to changes in magnitude of fields but not particularly to gauging the magnitude of static fields. The gravitational field is almost constant everywhere on the Earth, so there is no evolutionary pressure to be able to sense such a thing.

The Moon's gravity is less than 20% of the Earth's gravity. In other words, 80% of normal gravity is missing! That seems like a huge difference. However, astronauts on the Moon would have already experienced several days of zero gravity (plus some sudden bursts of deceleration prior to landing), so any direct comparison to Earth's field would be difficult. Mars' gravity is better at slightly less than 40% of Earth's gravity, but our intrepid astronauts will by then have spent six months of weightlessness plus a few minutes of terror! So Mars would probably feel just like home.

So the question specifically is as follows. When the Apollo astronauts were stationary and not moving around, could they tell they were in a much reduced gravitational field or did everything feel quite normal? Are there any first-hand comments or descriptions addressing this question?

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    $\begingroup$ Considering they were wearing diapers and wrapped tightly in a suit containing tubes with liquid pumped through it removing heat from their bodies, and the whirr of motors and pumps in those 1960's technology suits, "...did everything feel quite normal?" might not be the most precise phrasing. :-) $\endgroup$
    – uhoh
    Commented Mar 7, 2021 at 3:42
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    $\begingroup$ Rhea Seddon describes in her bio standing and walking around in the shuttle middeck in the early part of entry when g's were low. "...I had this funny sensation of being extra light, as if on the surface of the moon." This was an early flight when pressure suits were not worn for entry. She did "giant ballet leaps" Go for Orbit pp. 273-274 $\endgroup$
    – Organic Marble
    Commented Mar 7, 2021 at 4:17
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    $\begingroup$ I was going to provide an 'of course you can tell, don't be silly' comment but actually this is a really interesting question. It's possible it won't have a good answer for some decades until there are people who've lived on the Moon for long enough without needing to be wrapped in a space-suit in a tiny vehicle, but it's interesting. I'm looking forward to someone answering in 30 years (if SE still exists) with 'I've now been working on the Moon for 6 months and I can say ...'. $\endgroup$
    – user21103
    Commented Mar 7, 2021 at 11:36
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    $\begingroup$ @tfb If we don't want to wait that long we could try to ask some of the Apollo astronauts what it felt like inside the LM without wearing their suits. Time is running out but there are still a few alive and kicking... $\endgroup$
    – user2705196
    Commented Mar 7, 2021 at 14:54
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    $\begingroup$ John Young used to say that 1/6 g was way more fun than was zero g... $\endgroup$
    – Digger
    Commented Mar 8, 2021 at 14:36

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When the Apollo astronauts were stationary and not moving around, could they tell they were in a much reduced gravitational field or did everything feel quite normal?

Yes, while their mass would remain the same, the weight of their own bodies, the force the mass of their own bodies is exerting towards the center of the Moon, would be less than on Earth. When standing their legs and hips must exert an equivalent, opposite force. When sitting it's on their hips.

If you've ever gone backpacking, consider what it feels like having a heavy, well-balanced load strapped to you all day. Then you take it off. You feel almost buoyant. Your body has gotten used to pushing against the extra weight (force) all day. Now that it's gone, you can feel the difference.

It would also be easier to hold their arms up, they weigh less and they would feel that. But because their mass is the same it would take the same energy to move them from side-to-side perpendicular to gravity.

Are there any first-hand comments or descriptions addressing this question?

The Apollo transcripts are available to search. Closest I found on a quick search of the Apollo 12 Lunar Module logs is this.

04 19 57 24 CDR -- you know,it is significantly easier just to do anything here. Just like this arm motion is so simple because you're not fighting the rest of your weight like you are there in one g.

If folks find more, edit them into the answer, or add them as a comment.

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    $\begingroup$ @RogerWood The astronauts in the LM did not sleep standing up. Regardless of 1 g or 1/6 g. Standing up for 8 hours is not easy. I know in Apollo 11, the astronauts slept on the engine cover, and on the floor in the crew compartment. It was definitely not comfortable. After Apollo 11, the astronauts slept in hammocks. $\endgroup$
    – Star Man
    Commented Mar 8, 2021 at 0:57
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    $\begingroup$ @RogerWood Vintage Space has a nice video about sleeping on the Moon. There's a drawing of the makeshift Apollo 11 sleeping positions at 2:35. Not standing up, but not comfy. Later missions got hammocks, diagram at 3:35. Her videos and blogs might provide more info. $\endgroup$
    – Schwern
    Commented Mar 8, 2021 at 1:34
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    $\begingroup$ @RogerWood Apollo 15, 16, and 17 were on the surface the longest, three days. I'd suggest reading their LM transcripts for longer term observations. $\endgroup$
    – Schwern
    Commented Mar 8, 2021 at 1:40
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    $\begingroup$ @Schwern Those Apollo LM transcripts are very interesting. I searched various terms but really didn't find anything specific to the 'not moving' situation. This makes me think that it wasn't at the forefront of anyone's thoughts. My guess is that even partial g is enough to give the perception (and comfort) of a distinct up-down direction and the magnitude doesn't matter much. It's probably very difficult to gauge the magnitude anyway in the absence of any recent reference point. $\endgroup$
    – Roger Wood
    Commented Mar 9, 2021 at 3:14
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    $\begingroup$ @RogerWood I'm sure they were very busy. It's possible there's something in later interviews. It's difficult because there's only been a handful of people to experience low-G for extended periods. Mary Roach's Packing For Mars might have something, it's a great read about the human factors of spaceflight. $\endgroup$
    – Schwern
    Commented Mar 9, 2021 at 22:59

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