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Tides: Why are they more influenced by the moon than by the sun?

Date: Mon May 26 20:01:40 1997
Posted by Ian Landry
Grade level: 10-12
School: Cegep de Granby
City: Granby State/Province: Quebec
Country: Canada
Area of science: Astronomy
ID: 864694900.As


Even if we know that the gravitationnal force of the sun is greater than the gravitationnal force of the moon, we know that tides are more influenced by the moon than the sun.Why? The best answer I got is that it's just because the moon is closer to earth than the sun, but how does that make a difference?

The answer you got is correct; the tides from the Moon are larger because it is closer. Tides are caused by the change in gravity as you get closer to or farther from an object. You probably already know that as you get further from an object, the force you feel from gravity decreases. So someone standing on the side of the Earth nearer the Moon feels more gravity from the Moon than a person standing on the opposite side of the Earth. The difference between these two forces is what we call the "tidal force".

Gravity also depends on the mass of the object doing the pulling. The Sun is tremendously more massive than the Moon, but is also much farther away. Out here at the Earth, the numbers work out so that the tides from the Moon, which is smaller but closer, are about twice as strong as the tides from the Sun, which is more massive but farther away.

I have a more detailed explanation of this (along with a lot of other effects of tides) on my homepage. click here to see that page.

©2008 Phil Plait. All Rights Reserved.

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