René Descartes  -  A great French philosopher and mathematician. One of Descartes's theories attempted to explain the workings of the solar system by suggesting that space was filled with infinite, miniscule, invisible particles, whose motion created vortices that whirled the planets around the sun. The theory enjoyed popularity in the 17th century but was eventually displaced by Newton's theory of universal gravitation.

Edmund Halley  -  An astronomer in the Royal Society, and the namesake of Halley's Comet. Halley was a close friend of Newton, and assisted in the publication of the Principia.

Robert Hooke  -  A prominent scientist in the Royal Society, of which he was first secretary, and then president. Hooke became a rival of Newton in the 1670s, when the two men quarreled over their respective theories of light. He later felt that Newton had stolen from him the idea for the mathematics of gravity, and when Newton won fame after the publication of the Principia, Hooke grew bitter and deeply resentful.

Gottfried von Leibniz  -  A brilliant German mathematician, and a contemporary of Newton. The two men developed a bitter feud in the early 1700s over who had first invented calculus.

Hannah Newton  -  Newton's mother. His father died before he was born, and when Isaac was only three Hannah remarried and moved away, leaving the boy to be raised by an uncle. This abandonment must have been traumatic for the young Isaac; many biographers have speculated about its impact on his later psychological make-up-- specifically his fierce insecurity.

Alchemy -   · A medieval pseudoscience that sought to find ways to transmute base elements into valuable ones--for example, to transform lead into gold. Alchemy was one of Newton's favorite pursuits.

Arian -   · Arianism was a Christian heresy, which held that the Trinity (Father, Son and Holy Ghost) was not an accurate description of God, and that Jesus, while the son of God, was not equal with the father. (The heresy took its name from its author, Arius.) Newton, while otherwise devout, came to subscribe to Arian theology.

Calculus -   · The field of mathematics dealing with the calculation of variable quantities, such as weight, distance, or time, using forms of algebraic notation. Work in calculus usually involves the use of curves, calculating their slopes, and the distance under them. Newton was one of the pioneers of calculus, which he called "fluxions," although he must share credit for many of his ideas with Leibniz, who developed similar ideas independently around the same time.

Gravity  -   · The force of attraction between all bodies, which hold planets in their orbits and causes objects to fall toward the earth. In his Principia, Newton provided mathematical proof for a theory of universal gravitation that accounts for the fall of an apple from a tree; the orbits of planets, moons, and comets; indeed, the behavior of all objects in the universe. Contemporaries would call this the "fundamental law" of the universe, and it became the centerpiece of Newtonian physics. See the SparkNote on Gravitation for more information on the physics of gravity.

Optics  -   · The study of light. Newton made remarkable advances in this field: he discovered the spectrum of color that makes up white light; and proposed a "particle theory," holding that light is composed of tiny particles rushing through space at extremely high speeds. See the SparkNote on optics.

Philosophiae Naturalis Principia Mathematica  -   · Newton's scientific masterpiece, published in three volumes in 1687. In it, he offered mathematical proof of his law of universal gravitation, and created his system of the world, which would make him famous as the greatest mind of his age.

Puritans  -   · A strict Protestant branch of the Church of England, the Puritans dominated England in the 1650s during the dictatorship of Oliver Cromwell, imposing their harsh, Old-Testament theology on the nation. Their rule came to an end in 1660, with the Restoration.

Royal Society -   · Founded with Charles II's blessing in July of 1662, the Royal Society was a community of scholars brought together for the purpose of "Improving Natural Knowledge." It included, at its inception, scientists, philosophers, and even poets. Newton became a member in 1672, and eventually became its president in 1703, retaining the post until his death.

The Three Laws of Motion -   · The three laws of motion are the basic principles of Newtonian physics, as set out at the beginning of the Principia. They are as follows:

• 1. Every body continues in its state of rest, or of uniform motion in a straight line, unless it is compelled to change that state by forces impressed upon it.
• 2. The change in motion is proportional to the motive force impressed, and is made in the direction of the straight line in which that force is impressed.
• 3. To every action there is always opposed an equal reaction.

For more information on Newton's laws of motion, click here.