Whenever there is a discussion of the "greatest names in the history of human thought," Newton is always ranked near, if not at, the top of the list. Since many of Newton's important achievements were in the field of astronomy, he is sometimes called the greatest astronomer who ever lived, as well as being thought the greatest scientist of all time.
Born on Christmas Day, 1642 -- the same year in which Galileo died -- in Woolsthorpe, England, Isaac Newton was raised only by his mother, whose husband had died before Isaac was born.
As a child and as a pupil at the King's School in nearby Grautham, Newton had an intense interest in drawing and in hands-on experiments, copying diagrams and designs out of books and fashioning working models of windmills and other mechanical devices.
Ever absentminded and quite inattentive in class, Newton never stood out as a student. His mother took him out of school as a teenager and made an unsuccessful attempt at starting him in a career of farming. But, after convincing her that his true fascination lay not in farming but in science and mathematics, he returned to school at eighteen: he had been accepted in 1661 at Trinity College, one of England's most prestigious and influential universities.
As in grammar school, Newton completed his studies, receiving a bachelor's degree in 1665, with no special recognition for achievement. Still, encouraged by a professor who saw potential in his thinking, Newton pondered and contemplated many of his days, intellectually isolated.
Returning in 1665 to Woolsthorpe Manor, his boyhood home, to avoid plague in Cambridge, Newton began his two greatest years of discovery. Thoroughly convinced that the motions of celestial bodies were controlled by the same basic laws as those on Earth, Newton probably sat pondering these problems for hours in the countryside, as he said, "always thinking unto them." Newton, himself, claimed that "I was in the prime of my age for invention, and minded mathematics and [natural] philosophy" (what we today call science) during the "plague years."
By 1666, when Newton was only twenty-four years old, he had completed his initial investigations into gravity, concluding that gravitational forces were responsible for the motion of many heavenly bodies. He also proved that the strength of this force depends on the masses of the bodies and on the distance between them. For example, Earth's gravity attracts a rock a great deal more strongly than it does a pebble because the rock has much more mass. Therefore, the weight -- the pull of the Earth on an object -- of the rock is greater than that of the pebble. Though we do not think of the concept of weight in these terms ("if only gravity's pull weren't so strong..."), we use Newtonian theory every time we stand on the scale!
Diagram. The rock, more massive than the pebble, is more strongly pulled towards the earth and thus weighs more. Original diagram by The Online Planetarium Show.
In Philosophiae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy), published in 1687, Newton layed out his theory of gravitational forces and his laws of motion, what we know today as "Newton's laws." Principia was undoubtedly his great master-work and is perhaps the greatest single work in the history of science because it was the first book that attempted to explain motion both on Earth and in other realms of the universe.
Many of Isaac Newton's theories were formulated in their most basic forms by as early as 1669 -- when he was only 27 -- but he was rather unwilling to publish his results and most of his ideas surfaced publicly only many years later. Newton developed many theories in his time, all very important and far-reaching in mathematics, physics and astronomy. We will focus here on his discoveries that were most significant as contributions to astronomy:
In Opticks, published in 1704, late in his scientific career, Newton proposed that white light is not in fact white but in truth a mixture of all colors. Having carefully looked at light's behavior when reflected and refracted -- bent when it passes into a different medium -- Newton designed in 1668 the world's first reflecting telescope. Now a standard for large telescopes in observatories, these principles that originated with Newton in the 17th century were used in designing the Hubble Space Telescope's Optical Telescope Assembly.
Diagram. Reflecting telescope: Light is first reflected off of the larger, concave primary mirror onto the smaller, convex secondary mirror, which in turn reflects it through the eyepiece. The light, which is upside-down by the end of its journey, is focused by the eyepiece.
Spectrum analysis. Newton's contributions are also important because they would lay the foundation for a science that would become known as spectrum analysis, in which light from stars is used to determine their chemical composition, temperature, density, distance from Earth and velocity. This is particularly noteworthy because two spectrographs reside in the focal plane of the Hubble Space Telescope, designed to perform these very tasks.
Copernicus, Galileo and Kepler all swept aside vast areas of misconception about the universe and each identified important facts and laws. Newton's great achievement was in finding order among their seemingly unconnected ideas and in using his theory of gravitation to explain planetary motion in one triumphant theory. By applying his laws of motion and of gravitation to the paths of planets, he demonstrated that the motions of the planets could be precisely pinpointed. Copernicus had shown that planets orbited the sun, Galileo had used the telescope to collect evidence and to defend the Copernican theory and Kepler concluded after painstaking calculation that orbits are elliptical, not circular. Newton made it all fit together.
In 1669, Newton began teaching at Cambridge, giving weekly lectures on mathematical and astronomical subjects. Elected as a member first in 1672, Newton would be active in the Royal Society, becoming a member of the Society's council in 1699 and serving as the body's president from 1701 until his death. He also served outside of science, holding a seat in Parliament in 1689, until Parliament was dissolved one year later, and appointed for life the royal master of the mint. For his great achievement and service, he was knighted by Queen Anne in 1705. Sir Isaac Newton died in 1727 and was the first scientist to be buried in Westminster Abbey.
Isaac Newton was probably the greatest scientist of all time and is considered by many to be the greatest astronomer in history. By collecting the ideas of those before him, Newton created one of science's great unified thoeries and devised laws that sweepingly explained the persistent problem of planetary motion. He contributed immeasurably to our understanding of the universe and is still incredibly influential even today. Yet in the final analysis, we should also recognize that Newton was inspired and greatly aided by the laws, theories and observations of those before him. As he said, "I stand upon the shoulders of the giants of the past."