# How do we measure the mass of the moon?

**URL:** <https://boards.straightdope.com/t/how-do-we-measure-the-mass-of-the-moon/540272>\
**Category:** Factual Questions\
**Created:** [May 21, 2010, 4:20pm UTC](https://boards.straightdope.com/t/how-do-we-measure-the-mass-of-the-moon/540272 "2010-05-21T16:20:03Z")\
**Posts on this page:** 3\
**Page:** 2

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**Author:** ![K364](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/k364/32/5_2.png) [@K364](https://boards.straightdope.com/u/K364)\
**Post date:** [May 24, 2010, 5:40pm UTC](https://boards.straightdope.com/t/how-do-we-measure-the-mass-of-the-moon/540272/21 "2010-05-24T17:40:46Z")

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> [@Chronos](#):
>
> That’d only work if the Moon floated. But since it’s more dense than water (unsurprisingly, since it’s basically made of rock), it’d sink, and you’d just get the Moon’s volume (which is easy to calculate anyway), not its mass.

Easy solution for that problem… just fill the bath with Mercury (the liquid metal, not the planet)

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**Author:** ![Malacandra](https://avatars.discourse-cdn.com/v4/letter/m/45deac/32.png) [@Malacandra](https://boards.straightdope.com/u/Malacandra)\
**Post date:** [May 24, 2010, 10:54pm UTC](https://boards.straightdope.com/t/how-do-we-measure-the-mass-of-the-moon/540272/22 "2010-05-24T22:54:01Z")

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> [@K364](#):
>
> Easy solution for that problem… just fill the bath with Mercury (the liquid metal, not the planet)

Yah, but _don’t_ push the moon down to completely submerge it - again, that gives you volume, not mass. Just let it float and weigh the displaced mercury (easiest if you have a nice big eureka can).

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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:** [May 25, 2010, 6:04am UTC](https://boards.straightdope.com/t/how-do-we-measure-the-mass-of-the-moon/540272/23 "2010-05-25T06:04:31Z")

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> [@](#):
>
> Easy solution for that problem… just fill the bath with Mercury (the liquid metal, not the planet)

You could also use the planet, melted down. It’s denser than the Moon, so it would work just as well.

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