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Does the gravity of the moon affect us?

Date: Tue Sep 3 08:00:04 1996
Posted by: Yeo Wei Cheng
Grade level: 10-12
School/Organization: Dunman High School
City: Singapore State/Province: NIL
Country: NIL
Area of science: Biophysics


Does the gravity of the moon affect us ( our blood etc. ) as it affects other liquids , like causing the tide to rise and fall ?

This is a good question-- we certainly hear a lot about the Moon affecting people, from making them act a little funny (any hospital staffer knows a full Moon means more business) to turning them into werewolves! But is there really any connection? (Note added July 30, 1999: Bad Reader Justin Anderson pointed out to me that the idea of a full Moon affecting hospital business is an urban legend. I knew that, but didn't know I made the above comment on my own website! I guess I found this out after I wrote this page originally. Anyway, if you want to see more about the urban legend, check out the Urban Legends website.)

I am an astronomer, and not a biophysicist, so while I know about the Moon, I cannot say for certain anything about the human body. However, I can say pretty certainly that the gravity of the Moon has a negligible effect on our bodies. I just did a quick calculation-- the gravity of the Moon is about the same as if you filled a small house with water and stood about a meter away! The Moon is big, but it is far away. It's gravity is weak at that distance, and is completely overwhelmed by the gravity of nearby objects.

Tides are caused by the fact that gravity gets weaker with distance. The Moon pulls harder on one side of the Earth than the other, causing the Earth to stretch a bit. But the Earth is almost 13,000 kilometers across, while the Moon is 400,000 kilometers away! The human body is maybe only 2 meters across, so the difference in gravity from the Moon over a distance of 2 meters is almost zero. The tides on a person from the Moon are vanishingly small.

The Full Moon is very bright, though, and that certainly has an effect on behavior. Many animals (including humans) are affected by the Full Moon. Many animals have reproductive cycles that depend on the lunar phases. Agricultural based civilizations could harvest later into the evening when a Full Moon provided light.

One other thing: although tides from the Moon do not directly affect us, they have a profound affect on the Earth which in turn does affect us. I have a web page with information about tides. Click here to see a more lengthy explanation of tides, and their affect on the Earth.

©2008 Phil Plait. All Rights Reserved.

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