**January 4, 1643.** Isaac Newton, who was born on this date, was an English physicist and mathematician. He’s remembered as one of the world’s greatest scientists, because his insights laid a foundation for our understanding of celestial motion, light and gravity.

Newton’s work in three famous volumes comprise the *Philosophae Naturalis Principia Mathematica* (Mathematical Principles of Natural Philosophy), often referred to as simply *the Principia*. In this work, Newton states his *three laws of motion*, which today form the foundation of classical celestial mechanics. The Principia also lays out Newton’s revelations about gravity.

By all accounts, Newton’s Principia is a masterpiece. Read more about his work by clicking the links below.

Newton’s three Laws of Motion.

Newton’s revelations about gravity.

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**Newton’s three Laws of Motion.** They’re called *laws*, but they are really descriptions of fundamental truths about our physical universe.

1. An object at rest will remain at rest unless acted on by an outside force. An object in motion continues in motion with the same speed and in the same direction unless acted upon by an outside force. This law is often called *the law of inertia*. Click here to read more about Newton’s First Law of Motion.

2. When a force acts on a mass, *acceleration* is produced. The greater the mass of the object being accelerated, the greater the amount of force needed to accelerate the object. Click here to read more about Newton’s Second Law of Motion.

3. For every action there is an equal and opposite reaction. Click here to read more about Newton’s Third Law of Motion.

**Newton’s revelations about gravity.** Remember the story of the apple falling on Newton’s head? While not necessarily true in all its details, Newton apparently observed an apple fall from a tree and began thinking that, in order to fall on the ground, the apple was *accelerated* from zero when it hung on the tree.

According to his Second Law of Motion, acceleration is produced when a force acts on an object. Newton must have thought, what is that force? He came to understand this force as what every school child today knows as *gravity*.

Newton’s great revelation was that the force of gravity doesn’t just extend to the tops of apple trees. If an apple tree were as high as a mountain, for example, the apple would still fall. The force would still be operating. Newton’s insight was that the force of gravity extends much further … to the moon. He recognized that the orbit of the moon around Earth is a consequence of the force of gravity.

Indeed, the force of gravity extends throughout space. Today, physicists refer to Newton’s ideas about gravity as *the universal law of gravitation*. Click here to read more about universal gravitation.

Others who followed Newton – most particularly Albert Einstein – refined our understanding of gravity. The most accurate description of gravity today can be found in Einstein’s general theory of relativity, which asserts that *gravity is a consequence of the curvature of space-time*.

Fascinated by Newton’s revelations about gravity? Check out this 15-minute video:

If Newton had only contributed his three Laws of Motion and his understanding of universal gravitation, we’d have remembered him as one of the world’s greatest scientists. But Newton didn’t stop there. He also built one of the first practical reflecting telescopes, contributed to the invention of calculus, and explored how white light can be broken up into a spectrum of colors by a prism, thereby laying the foundation for much of modern astronomy.

Yet Newton himself knew how much more remained to be discovered. He is known to have said:

I do not know what I may appear to the world, but to myself I seem to have been only like a boy playing on the sea-shore, and diverting myself in now and then finding a smoother pebble or a prettier shell than ordinary, whilst the great ocean of truth lay all undiscovered before me.

By the way, in the past, you often saw Newton’s birthday as December 25, 1642. That is beginning to change, and now we are more and more often seeing Newton’s birthday as January 4, 1643. The difference is due to the fact that, when Newton was born, England was in the midst of a 150-year period of using a different calendar from the rest of Europe. The rest of the continent had already adopted the Gregorian calendar – same calendar we today use. However, the English were still using the Julian calendar, which lagged ten days behind because of a faulty method of accounting for leap years.

So Newton himself would have said his birthday was December 25. But, everywhere else, he was born on January 4. Read more about Newton’s birthday discrepancy.

Bottom line: Isaac Newton was born on January 4, 1643 (December 25, 1642, in the same year that Galileo died, in the old style Gregorian calendar). His work laid much of the foundation for our modern understanding of the physical universe.