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SHIRLEY GRIFFITH: Years later, the British astronomer Edmund Halley visited Newton. He said he wanted Newton's help in finding an answer to a problem no one had been able to solve. The question was this: What is the path of a planet going around the sun?

Newton immediately gave Halley the answer: an egg-shaped path called an ellipse.

Halley was surprised. He asked for Newton's proof. Newton no longer had the papers from his earlier work. He was able to recreate them, however. He showed them to Halley. He also showed Halley all his other scientific work.

STEVE EMBER: Halley said Newton's scientific discoveries were the greatest ever made. He urged Newton to share them with the world.

Newton began to write a book that explained what he had done. It was published in sixteen eighty-seven. Newton called his book "The Mathematical Principles of Natural Philosophy." The book is considered the greatest scientific work ever written.

SHIRLEY GRIFFITH: In his book, Newton explains the three natural laws of motion. The first law is that an object not moving remains still. And one that is moving continues to move at an unchanging speed, so long as no outside force influences it.

Objects in space continue to move, because nothing exists in space to stop them.

Newton's second law of motion describes force. It says force equals the mass of an object, multiplied by the change in speed it produces in an object.

His third law says that for every action, there is an equal and opposite reaction.

STEVE EMBER: From these three laws, Newton was able to show how the universe worked. He proved it with easily understood mathematics. Scientists everywhere accepted Newton's ideas.

The leading English poet of Newton's time, Alexander Pope, honored the scientist with these words: "Nature and nature's laws lay hid in night. God said, --'Let Newton be!' - and all was light. "

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SHIRLEY GRIFFITH: This Special English program was written by Marilyn Christiano and Frank Beardsley. This is Shirley Griffith.

STEVE EMBER: And this is Steve Ember. Listen again next week for another Explorations program on the Voice of America.