Isaac Newton
by
Harold Richman
One day while Isaac Newton was sitting under an apple tree, an apple fell and hit him on the head. This is the story that everyone has heard and which, we are told, led him to discover the Laws of Motion that govern the movement of the planets and the Theory of Universal Gravitation.Due to the Great Plague in 1660, the University of Cambridge closed for two years. Newton, who had just graduated, was forced to stay home instead of continuing his studies. It was during this time while walking through his garden that he saw an apple fall to the ground. How he was able to use this simple event to develop his theory of gravity has always been a mystery to me. However, recently I read an article written by Voltaire in 1733 that outlines Newton's thoughts at the time.
Voltaire is the well-known French writer and philosopher. Due to his outspoken views, in 1726 he was forcibly exiled to England where he spent the next three years. Newton died in 1727 so Voltaire would have been familiar with the many discoveries made by him. Voltaire was also acquainted with Newton's niece, Catherine Barton. Newton was a bachelor and she had agreed to manage his London home; therefore she would have been familiar with the apple story, which she related to Voltaire. Voltaire explains Newton's train of thought as follows.
An apple falls to the ground because it is attracted by the earth. If an apple falls from a tree that is perhaps twenty feet high, Newton reasoned, an apple would also fall from a height of the tallest building and also from a height of the tallest mountain. He then performed a thought experiment just as Einstein did when he was developing his theory of Relativity. Newton imagined an apple falling from a height as high as the moon. It would still fall to earth which led him to the conclusion that the force of gravity extended out into space and does not apply only to objects on earth.
Next he proposed that the moon would also fall to earth at the same rate as the apple, but of course the moon does not reach the earth. The difference between the moon and the apple is that the moon is moving in a circular orbit around the earth. As it falls, it is also moving horizontally so that it never reaches the earth.
This can be understood if you consider a ball tied to a string. The ball is twirled around until it is travelling around in a circle. The ball has centrifugal force pulling it away from your hand but the string is pulling it toward your hand with the same force. Newton knew that the moon had centrifugal force because of its circular orbit, and he surmised that the attraction between the earth and the moon, the force of gravity, was what was keeping the moon in orbit around the earth.
Newton then made the intuitive assumption that the force of gravity must apply to all the planets and the sun. The force of attraction will vary according to the mass of the two bodies and the distance between them. By studying the planets, comparing their distance from the sun, and the time for one revolution about the sun, he was able to prove that the force of attraction decreases as the square of the distance and not in direct proportion to the distance, as had been previously been proposed.
Using his new formulae, he calculated the moon's orbit. The calculated figure, in Newton's words, was pretty nearly to the actual orbit. He was unable to explain the discrepancy and, for a while, thought that there may be another force involved in addition to gravity. In 1670 a French astronomer, Jean Picard, made a more accurate calculation of the diameter of the earth, which was 15% larger than the figure Newton had used. When Newton inserted the new figure, his formulae produced the exact answer.
In Newton's time there were only eight known planets. In 1905 an American astronomer, Percival Lowell, studied the orbits of the two outermost planets, Neptune and Uranus. He found some discrepancies that he predicted were caused by an unknown planet further out. As Newton's Theory of Gravity explained, the force of gravity between two bodies will vary inversely as the square of the distance between them. This means that the force of attraction between two bodies one mile apart will be one-quarter as much if they are two miles apart. The force of gravity decreases very rapidly as the distance increases. That is why only the orbits of Uranus and Neptune were affected by the unknown planet.
The planet, Pluto, the unknown planet, was discovered in 1930, twenty-five years after Lowell's prediction and fourteen years after he died. However, it seems only fitting that Pluto was discovered by the Observatory that Lowell himself had built in Flagstaff, Arizona.