Isaac Newton was born on 4 January 1643 in Woolsthorpe Castle in Great Britain. Newton became a mathematician, physicist, and astronomer, and is now world-renowned as a scientist who helped us understand the universe with his discoveries that became the basis of many scientific principles.

Newton published his insights in three well-known volumes which were named together *Philosophae Naturalis Principia Mathematica* (Mathematical principles of natural philosophy), is often called simple *Principle*, which is, by all accounts, a masterpiece. In this paper, Newton presents his own *three laws of motion*, which today form the foundation of classical celestial mechanics. *Principle* also presents Newton’s discoveries on gravity.

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**Newton’s three laws of motion.** They are invited *laws*, but they are really descriptions of the fundamental truths about our physical universe.

1. A stationary object will remain in a state of rest unless it is acted upon by an external force. An object in motion continues to move at the same speed and in the same direction, unless an external force acts on it. This law is often called *law of inertia*. Click here to read more about Newton’s first law of motion.

2. When a force acts on a mass, *acceleration* are produced. The greater the mass of the object that is accelerating, the greater the amount of force required 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 discoveries on gravity.** Remember the story of the apple that fell on Newton’s head? Although not necessarily accurate in all its details, Newton apparently noticed an apple falling from a tree and began to think that, in order to fall to the ground, it was an apple *accelerated* from scratch when he hung on a tree.

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

Newton’s great discovery was that the force of gravity does not extend only to the tops of apple trees. For example, if an apple tree were as high as a mountain, an apple would still fall. The power would still work. Newton’s insight was that the force of gravity extends far beyond … to the moon. He recognized that the Moon’s orbit around the Earth was due to the force of gravity.

Indeed, the force of gravity extends through space. Today, physicists refer to Newton’s ideas about gravity as the universal law of gravity.

Others who followed Newton — especially Albert Einstein — perfected our understanding of gravity. The most accurate description of gravity today can be found in Einstein’s general theory of relativity, which claims this *gravity is due to the curvature of space-time*.

Fascinated by Newton’s discoveries about gravity? Watch this 15-minute video:

If Newton had given only his three laws of motion and his understanding of universal gravitation, we would remember him as one of the world’s greatest scientists. But Newton didn’t stop there. He also built one of the first practical reflective telescopes, contributed to the invention of calculus, and researched how white light could be dissected into a range of colors by a prism, laying the groundwork for much of modern astronomy.

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

I don’t know how the world might seem to me, but I seem to have been to myself just like a boy playing on the seashore and turning in and finding a smooth pebble or a nicer shell than an ordinary one, while the great ocean of truth lay all undiscovered before me.

One unusual fact about Isaac Newton is that you can say he had two birthdays ten days apart. You may have seen Newton’s birthday earlier on December 25, 1642. That reference is starting to change, and now it’s more common to see Newton’s birthday on January 4, 1643. The difference is that when Newton was born, England was in the middle of a 150-year calendar. different from the rest of Europe. The rest of the continent has already adopted the Gregorian calendar, which is the same calendar we use today. However, at the time of Newton’s birth, the English were still using the Julian calendar, which was ten days late due to an incorrect method of calculating leap years. (Coincidentally, 1642 was the year Galileo died.)

So Newton himself would say his birthday is December 25th. But everywhere outside of England, he was born on January 4. Read more about Newton’s birthday mismatch.

Bottom line: Isaac Newton could ask for two dates of birth, but now his birthday is mostly recognized as January 4, 1643. Newton’s work in gravity and the laws of motion form the basis of most of today’s understanding of physics and astronomy.