ISAAC NEWTON

The Man Who Transformed Physics and Astronomy

We live in an expanding cosmos in which objects are constantly in motion. Many of the laws that describe these motions were devised by an Englishman who started out life as a farmer but ended up being one of the most influential scientists of all time. His name was Sir Isaac Newton, and his laws of orbital motion and gravitation are fundamental rules that every astronomer learns. Farming’s loss was science’s gain.

Newton’s Three Laws of Motion

Isaac Newton carefully worked out the math describing an object’s motion in space. On the basis of this, he postulated three laws:

1. An object at rest will stay at rest. If it’s in motion, it will stay in motion unless it’s acted upon by another object or force.
2. Acceleration occurs when a force acts on a mass. The greater the mass, the more force is needed to accelerate the object. Physics and astronomy students learn this as F = ma, where m stands for the amount of mass and a stands for acceleration. In the case of an apple falling to the ground, you plug in the mass of the apple and its acceleration, and that gives you the force (in units called, appropriately, Newtons).
3. For every action there is an equal and opposite reaction. If one ball hits another ball in a bowling alley, the first ball is pushed back in the opposite direction as hard as the second ball is pushed forward.

These may seem like obvious rules to us today, but in Newton’s time they were amazingly new ideas.

Newton and Orbital Motion

Orbital motion is an important part of life in the universe, so let’s take a closer look at it. Orbits illustrate Newton’s First Law of Motion, which states that an object in motion will stay that way unless something acts on it to change the motion. You experience this every day. You see a door about to slam and use your hand to stop the action. Your hand is the force that changes the door’s speed and direction.

The force that changes motion in space is gravity. Physical bodies all have mass. Each mass has an attractive force on other masses. The more massive the body, the stronger the gravitational force. Gravity and velocity keep planets in orbit around the Sun, moons going around planets, and galaxies orbiting other galaxies.

Newton’s Life and Work

Isaac Newton was born in 1642, the same year that Galileo Galilei died. His student days were lackluster, and although he was raised to be a farmer, his talents lay elsewhere. He went to Trinity College, Cambridge, to receive a classical education in logic, ethics, physics, and the works of Aristotle. He left after graduation and spent the next several years working on mathematics. He came up with the basic principles of differential calculus at the same time that, coincidentally, those principles were being established in Germany by Gottfried Leibniz (1646–1716). He also dabbled in optics and spent time working out a mathematical theory of circular motion. By 1667 he was a fellow of Trinity College and was well known for his mathematical ability. He continued to study optics and began working on a telescope of his own design, which used a reflecting mirror instead of a lens. Similar types of telescopes are known today as Newtonian reflectors.

Based on his continuing work in math and optics, Newton was made Lucasian Professor of Mathematics in 1669 (the same chair held today by Stephen Hawking). He published tracts about optics, light, and color and eventually turned his attention to celestial mechanics. His interest was piqued by an exchange of correspondence with Robert Hooke (1635–1703), a fellow scientist. Although offended by some of Hooke’s criticisms of his work, Newton was intrigued enough by the correspondence—plus the appearance of a comet beginning in 1680—to work out the details of planetary orbits. He wrote proofs of his work and published his three universal laws of motion in a book called Principia Mathematica. Its publication made him an international star in scientific circles, and his work on describing motion contributed greatly to the industrial revolution that began during his lifetime.

Sir Isaac Newton’s later life saw him thinking deeply and writing about religious and metaphysical matters. He worked as warden of the Royal Mint for thirty years, beginning in 1696. He also served as President of the Royal Society and was knighted in 1705 by Queen Anne. He died in 1727, leaving behind a tremendous scientific legacy.