ISAAC NEWTON, about 300 years ago, theorized how gravity works. He imagined a man throwing an object from the top of an unusually high mountain. If simply dropped, the object would fall, as would an apple, downward to the ground.

If, however, it was thrown forward, it would follow a curved path in falling to the ground. Newton then reasoned that if thrown fast enough, it would circle the earth in an orbit.

From this theorizing, the link between gravity and the movements of the moon and the planets became apparent to him: the moon bound in an orbit around the earth because of the pull of earth’s gravity and the planets kept in their orbits by the sun’s gravity.

A Universal Law

After careful study, Newton formulated a precise mathematical description of this universal law. Simply stated, Newton’s equations said that all objects, small or large, exert a pull on one another, the strength of that pull being dependent on how massive the objects are and on the distance between them.

With some refinements, scientists still use Newton’s basic formulas describing gravity, particularly in planning such space ventures as sending a space probe to encounter Halley’s comet in 1985. In fact, English astronomer Edmond Halley, a colleague of Newton, used Newton’s theories to predict the year when that comet would next appear.

Newton’s discoveries about gravity gave him a glimpse of the order manifest in the universe, an orderliness that arises through intelligent design. But his work was by no means the final word on the subject. At the beginning of this century, scientists came to realize that some aspects of Newton’s theories were inadequate, even inconsistent.

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Newton’s law of gravity asserts that in a vacuum, a feather would fall at the same speed as an apple