# Sir Isaac Newton

# Sir Isaac Newton

**1642-1727**

**English Physicist, Mathematician, and Astronomer**

Isaac Newton's combination of abilities as an experimentalist, theorist, and pure mathematician have never been surpassed, and his *Philosophiae Naturalis Principia Mathematica* (1687) stands as one of the greatest, if not the greatest, scientific work ever written.

Newton was born on December 25, 1642, in Woolsthorpe, England. A premature and sickly baby born after his father's death, Newton spent his youth building mechanical contrivances including water-clocks, a mouse-powered mill, and kites bearing fiery lanterns. He entered Trinity College, Cambridge, in 1661 and earned his bachelors degree in 1665, having earlier that year discovered the binomial theorem. Soon thereafter the university closed because of the plague, and Newton retired to Woolsthorpe for the next 18 months.

This period of seclusion was among the most productive of his life. By November 1665 he had discovered differential calculus and by May of the following year integral calculus. During the intervening months his experiments with prisms had revealed that white light was composed of different colors refracted through characteristic angles. Also in 1666 Newton first considered extending gravity to celestial bodies. He correctly theorized that the rate of fall was proportional to the gravitational force with the force falling off according to the square of the distance from Earth's center. However, disagreements between his calculations and observations led Newton to shelve this work for almost 15 years.

Newton returned to Cambridge in 1666 and received his M.A. in 1668. That same year he built the first reflecting telescope, important because it eliminated chromatic aberration inherent to refractors. In 1669 he became Lucasian Professor of Mathematics. (It is generally thought the incumbent Isaac Barrow [1630-1677] resigned so Newton might have it.) After exhibiting his telescope before the Royal Society, he was elected a fellow in 1672 and shortly thereafter presented his optical experiments underlying the invention. A multiyear controversy with Robert Hooke (1635-1703) ensued whereby Newton refined his corpuscular theory of light. These ideas were eventually presented in the *Optiks* (1703), which dominated optical research for the next century.

A letter from Hooke in 1679 stimulated Newton to renew his work on gravitation. Newton quickly achieved a mature understanding of the dynamical principles involved and deduced that a central gravitational force implied Kepler's equal area law. He further showed that elliptical orbits, with the center of force at a focus, implied an inverse-square law of force.

In 1684 Hooke boasted to Christopher Wren (1632-1723) and Edmond Halley (1656-1742) that he had worked out the laws governing planetary motion. Wren was unimpressed and offered a prize for the correct solution. Halley took the problem to Newton, who informed him he had already solved it. Halley encouraged Newton to renew his work on gravitation and prepare it for publication through the auspices of the Royal Society. Newton complied, but the Society's financial difficulties forced Halley to assume full financial responsibility for the *Principia*, which finally appeared in 1687.

The *Principia* represents the culmination of the Scientific Revolution. Newton synthesized Kepler's laws of planetary motion and Galileo's experimental results on falling bodies by codifying the principles of mechanics and extending their application to celestial phenomena. His three laws of motion clarified the distinction between mass and weight and how they are related under a variety of circumstances, while the law of universal gravitation explained Earth's equatorial
bulge, orbital inequalities, the parabolic orbits of comets, and more.

Newton also conducted comprehensive alchemical experiments and studied the Bible throughout his life, leaving extensive manuscripts on Church history and ancient chronology. His academic output diminished in later years as he accepted various civic positions. He died in London on March 20, 1727.

**STEPHEN D. NORTON**

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**Sir Isaac Newton**