Isaac Newton found out that the strength of the pull of gravity weakens
the farther you get away from an object, in proportion to 1/(r*r),
where r is the distance you are away from the center. The strength of
the gravitational pull is also proportional to the mass of the object.
The sun is really massive and really big. It has a mass of about 2
times ten raised to the thirtieth power kilograms. (that's
2000000000000000000000000000000 kg), and a radius of about 700000
kilometers.
This makes the strength of gravity on the "surface" of the sun
(that is, the photosphere, the shiny part we see), 28 times stronger
than the force of gravity on the surface of the Earth. Out here, at the
distance we orbit the sun, the gravitational pull of the sun is only
0.0006 of the strength of the earth's gravity on the surface of the
earth. But that's enough to pull the entire planet around in a big,
nearly circular orbit, once per year. And the variation in the strength
of the sun's gravitational pull from the part of the earth that faces
towards the sun to the part that faces away is partly responsible for
the tides of the ocean. The moon's gravity plays a somewhat larger role
in the tides. Although it's weaker than the Sun's gravity here, it
varies more from one side of the Earth to another.
Tom (w mike)
(published on 10/22/2007)
