Of all Newton's contributions to science, perhaps his greatest was the law of universal gravitation - a law which enabled him to answer so many of the questions which had puzzled scientists for centuries. His answer to why things happen was based on observation and measurement rather than on the so-called logic favored by the ancient Greeks. Why do all bodies fall to the ground with the same acceleration regardless of their mass?
Why do the planets revolve around the sun and not fly off into space and why do these planets have different speeds which are in some way related to their distances from the sun?
Newton suggested that the pull of the earth extended outwards in space like the pull" of a magnet, but he did not claim that his theory explained why this gravitational pull existed in the first place. He extended this idea further to include every particle of matter in the universe and gave a precise account of the strength of this gravitational pull. Whereas this pull of gravity decreases as the distance from its source increases it never disappears altogether. The space traveler who claims to be outside the earth's field, or to have escaped from the earth's gravity, is no scientist for he implies that the pull s·tops somewhere in space.
The force of gravity depends on the size of a planet. Large planets, made of very heavy materials, have strong gravitational forces. As might be expected, the moon - which is smaller than the earth - has far less pull. The weight of an object (which is its gravitational pull) on the moon is much smaller than on earth.
By using the gravitational law, Newton was also able to show that astronomical observations by Tycho Brahe (1546-1601) and predictions by Brahe's assistant Johann Kepler (1571-1630) about the motion of the planets were correct. This happy marriage of theory and practice confirmed the law of gravitation, which, together with the laws of motion, became the foundation stones of the science of dynamics.
Even though the laws are not completely true when applied to the world of atoms, or when bodies are moving at speeds approaching the speed of light, they are still of tremendous value to scientists and engineers and will continue to be used as basic principles in modern technology.