# earth to moon distance scale

**URL:** <https://boards.straightdope.com/t/earth-to-moon-distance-scale/492299>\
**Category:** Miscellaneous and Personal Stuff I Must Share\
**Created:** [April 7, 2009, 8:46pm UTC](https://boards.straightdope.com/t/earth-to-moon-distance-scale/492299 "2009-04-07T20:46:25Z")\
**Posts on this page:** 2\
**Page:** 3

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**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\
**Post date:** [April 10, 2009, 12:20am UTC](https://boards.straightdope.com/t/earth-to-moon-distance-scale/492299/41 "2009-04-10T00:20:50Z")

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Quoth **KidScruffy** :

> [@](#):
>
> Slight hijack, but the bottom picture is the one that surprised me. The moon’s diameter is about 1/4 that of the earth…but I remembering hearing that about its mass, not diameter. So, it was much smaller in comparison than I expected.

You’re probably thinking of surface gravity: The Moon’s surface gravity is about 1/6 that of Earth. While gravity does depend on the object’s mass, it also depends on distance, and since the Moon has a smaller radius, you’re closer to its center when you stand on its surface. A spherical object’s surface gravity works out to be proportional to its diameter times its density, so the fact that the diameter-fraction is relatively close to the surface-gravity-fraction is not a coincidence: It just means that the Earth and the Moon are close to the same density, but the Earth is a little denser.

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**Author:** ![Carnacki](https://avatars.discourse-cdn.com/v4/letter/c/b5e925/32.png) [@Carnacki](https://boards.straightdope.com/u/Carnacki)\
**Post date:** [April 10, 2009, 4:01pm UTC](https://boards.straightdope.com/t/earth-to-moon-distance-scale/492299/42 "2009-04-10T16:01:57Z")

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> [@Chronos](#):
>
> Quoth **KidScruffy** :You’re probably thinking of surface gravity: The Moon’s surface gravity is about 1/6 that of Earth. While gravity does depend on the object’s mass, it also depends on distance, and since the Moon has a smaller radius, you’re closer to its center when you stand on its surface. A spherical object’s surface gravity works out to be proportional to its diameter times its density, so the fact that the diameter-fraction is relatively close to the surface-gravity-fraction is not a coincidence: It just means that the Earth and the Moon are close to the same density, but the Earth is a little denser.

Thanks, I think that’s it exactly. I remember learning as a kid that your weight on the moon is 1/6 your weight on the earth, which I somehow turned into 1/4. And then knowing that gravity depends on mass, I guess I figured the moon was 1/6 (1/4) the mass of the earth…and somewhere along the line, mass became volume (I guess I assumed the densities were the same).

Sounds like a stupid logical progression now, but I’m sure it made sense as a kid, and then it just got ingrained in my melon - hence my surprise at the picture.

Anyhow, ignorance fought - thanks!

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