Re: How does the earth & other planets be held up by the Sun's gravity?

Dear Melanie,

Isaac Newton observed that every bit of mass in the universe attracts every other bit of mass with the force called gravity back in the 17th century. He explained how planets remain in orbit this way.

Imagine a cannon on a mountain top. Fire the cannon and the ball sails quite a ways before gravity pulls it down to earth. Stuff more gun powder in the cannon and the ball flies even further before it crashes to the ground. Now imagine firing the cannonball fast enough (18 or 19 thousand miles an hour!) so that it flies beyond the horizon as the earth's gravity makes it fall. It is in orbit.

Our earth is whizzing around the sun at approximately 66,000 miles per hour in our orbit. We are "falling around the sun". Go to http://www.jpl.nasa.gov/basics/bsf3-4.html for an animated tutorial from NASA's Jet Propulsion Laboratory.

As for proving the theory, you've struck right to the heart of science. Theories can never be proved true in any absolute sense; we just see if they work. And it only takes one exception to destroy a scientific theory. Newton's theory of universal gravitation accurately described the earth's orbit around the sun, the moon orbiting earth and even double stars orbiting each other. But his theory failed! It didn't work for the planet Mercury. Einstein's theory of relativity augments Newton's for fast moving objects near great masses. So far Einstein seems correct, but his theory may be replaced someday because new discoveries won't fit. That's the excitement of science.