The Insights of Genius

From: Sir Isaac Newton: The Universal Law of Gravitation

"But as we increase the muzzle velocity for our imaginary cannon, the projectile will travel further and further before returning to earth. Finally, Newton reasoned that if the cannon projected the cannon ball with exactly the right velocity, the projectile would travel completely around the Earth, always falling in the gravitational field but never reaching the Earth, which is curving away at the same rate that the projectile falls. That is, the cannon ball would have been put into orbit around the Earth. Newton concluded that the orbit of the Moon was of exactly the same nature: the Moon continuously "fell" in its path around the Earth because of the acceleration due to gravity, thus producing its orbit."

Newton's thought trail is a mark of his genius. He examines an ordinary occurence- the flight of a projectile and imagines what could happen if velocity were increased. He concludes that there would be a point at which the velocity would produce an orbital effect. In ascertaining this he takes into account the rounded shape of the earth which enables the "continuous fall" concept- the curvature away of the earth matching the rate of the falling projectile. Newton linked his theoretical ponderings to the behavior of real physical entities and in so doing advanced his own comprehension. This subsequently advanced the understanding of humanity; a remarkable consequence of a thought process.

Ever Exacting Theoretical Refinements

With hindsight we sometimes tend to take for granted the brilliance of perceptual breakthroughs. Newton's insightful realization that the force inducing a fruit to fall from a tree was the same as that causing the moon to orbit the earth is an example. A logical continuance was the extrapolation that this force was universal and explained similar phenomenon throughout the universe.

Prior to the realization of Newton's ideas about gravitation Kepler had formulated mathematical descriptions of planetary orbits. Although Kepler did so outside a gravitational framework, his orbital depictions were accurate for solar system models involving planets orbiting the sun. This is due to the fact that in relation to the sun planetary masses are by comparison quite minute. So minute in fact, that the center of the two masses in a planet/sun model lies within the sun itself. Because of this Kepler's third law was a very close approximation of actual planet orbits. Just as Einstein's Theory of General Relativity allowed for more exacting gravitational predictions than Newton's theory, Newton's gravitational theory allowed for more exacting orbital predictions than Kepler.

For two equal celestial bodies the center of mass would lie between them. They would orbit around their common center of mass. While Kepler's laws were a great historic milestone Newton's work furthered our understanding of an underlying force and the manner by which it acted.

The linked webpage Sir Isaac Newton: The Universal Law of Gravitation provides a more complete explanation of Newton's gravitational concepts.