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Sir Isaac Newton

"discoverer" of gravity, etc.

Isaac Newton was born at Woolsthorpe, Lincolnshire, England, on December 25, 1642 (January 4, 1643, New Style). He attended King Edward VI Grammar School in Grantham, where he was considered a poor student because he was more interested in making mechanical devices than in studying. His youthful inventions included a small windmill that could grind wheat and corn, a clock run by the force of dropping water, and a sundial. He left school when he was 14 to help his widowed mother manage her farm, but was later sent back because he spent more time reading than doing chores.

Newton entered Trinity College, Cambridge University, in 1661. He showed no exceptional ability during his college career, and was graduated in 1665 with no particular distinction. He returned to Cambridge as a fellow of Trinity College in 1667, became professor of mathematics at Cambridge in 1669, and was elected a Fellow of the Royal Society in 1672.

Sir Isaac Newton

Theories of Motion and Gravitation

Newton's first major achievement was to work out the three laws of motion which now form the basis of all mechanical science. The first law says that an object at rest will remain at rest and a steadily moving object will remain in steady motion unless an outside force acts on it. The second law sets out the definition of the concept of force as something that produces an acceleration in a body. The third law states that for every action there is an equal and opposite reaction, a principle that today finds a practical application in rocket and jet engines.

Newton said the concept of a universal force came to him while he was drinking tea in his garden and saw an apple fall from a tree. For some unknown reason he suddenly found himself wondering why the apple had fallen. Was it perhaps attracted to the earth by some previously unknown force? He suddenly realized that this force -- if it existed -- may very well affect all bodies, including the planets. Using his idea and employing his laws of motion, Newton worked out a theory which shows that all bodies in the universe are attracted to each other by a force varying inversely with the distance between them. He named the new force gravity.

Newton also showed that the gravitational force of the sun keeps the planets in their orbits, just as the gravitational force of the earth attracts the moon and an apple. The falling of an apple seems different from the motion of the moon, because the apple falls straight down to earth, while the moon moves approximately in a circle around the earth, but Newton showed that the moon falls just like the apple. If the moon did not fall constantly toward the earth, it would move in a straight line and fly off at a tangent to its orbit. Newton calculated how much the moon falls in each second and found the distance is 1/3600 of the distance an apple falls in a second. Since the moon is 60 times as far from the earth's center as a falling apple, the force of the earth on an object 60 times as far away as another object is 1/3600.

Woolsthorpe Manor, where Newton is said to have discovered the laws of gravity
Woolsthorpe Manor

The Principia

Newton concluded his first investigations on gravity and motion in 1665 and 1666, but it would be almost twenty years before they were made known to the scientific community at large. He knew that his original theory had been based on an inaccurate measurement of the earth's radius, resulting in significant differences between the theory and the facts. Although he later learned the true value of the earth's size, he made no effort to complete his investigation or to produce a book for publication.

One day in 1684, Edmund Halley, an English astronomer, Robert Hooke, an English scientist, and Christopher Wren, an English architect, were discussing what law of force produced the visible motion of the planets around the sun. Halley went to Cambridge to ask for Newton's opinion, and found Newton in possession of complete proof of the law of gravity. After persuading Newton to publish his findings, Halley paid all the expenses, corrected the proofs, and laid aside his own work to publish Newton's discoveries. Those discoveries were finally published in 1687, in Philosophiae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy). This work, usually called The Principia, is considered one of the greatest single contributions in the history of science. It includes Newton's laws of motion and theory of gravitation, and was the first book to contain a unified system of scientific principles explaining what happens on earth and in the heavens.

Newton's personal copy of The Principia, with notes for the 2nd edition
Newton's personal copy of The Principia


The mathematical exposition accompanying the theories and proofs contained in Principia indicates that in order to formulate his ideas about gravitation, Newton had found it necessary to invent an entirely new mathematical technique, now known as infinitesimal calculus.

Light and Color

Newton's discoveries in optics were equally spectacular, explaining why objects appear to be colored, and laying the foundation for the science of spectrum analysis. This science determines the chemical composition, temperature, and even the speed of such hot, glowing bodies as distant stars, or of an object heated in a laboratory. Newton also discovered that sunlight is a mixture of light of all colors. He passed a beam of sunlight through a glass prism and studied the colors that were produced. He found that a green apple illuminated by sunlight looks green because it largely reflects the green light in the sun and absorbs most of the other colors. If the green apple is lighted by a red light or any color light not containing green, however, it will not appear green. He published the results of his experiments and studies in Opticks, in 1704.

The study of light led Newton to consider constructing a new type of telescope in which a reflecting mirror was used instead of a combination of lenses. His first reflecting telescope, built in 1688, was 6 inches long, and, through it, Newton saw Jupiter's satellites.

replica of Newton's telescope, in the possession of the Royal Society
replica of Newton's telescope

Public Life

Newton became active in public life after the publication of Principia. He became the Cambridge University member of Parliament in 1689 and held his seat until Parliament dissolved the following year. He became Warden of the Mint in 1696, and was appointed Master of the Mint in 1699, a position he held until his death. In 1699, Newton became a member of the Royal Society council and an associate of the French Academy. He was elected to Parliament again from the university in 1701. He left Cambridge and settled permanently in London in 1701. He became president of the Royal Society in 1703 and was re-elected annually until his death. He was knighted by Queen Anne in 1705.

Sir Isaac Newton died on March 20, 1727 (March 27, New Style), and is interred in a magnificent crypt in Westminster Abbey.

Newton's tomb in Westminster Abbey
Newton's tomb in Westminster Abbey

Personal Characteristics

Isaac Newton did not enjoy the scientific arguments that arose from his discoveries, and was so sensitive to criticism that his friends had to plead with him to publish his most valuable discoveries.

As a professor, Newton was very absent-minded.

Newton's personal relationships with other people were often fraught with arguments and unhappiness, and he had few close friends. Of a neurotic and vindictive nature, he had at least two nervous breakdowns. Despite his egotism, he always showed great generosity to his nephews and nieces and to publishers and scientists who helped him in his work.

The World Book Encyclopedia Chicago: World Book-Childcraft International, 1979

Robert Hooke
Westminster Abbey

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This page was last updated on 02/25/2017.