How old was Isaac Newton when in 1666 he formulated the theory of universal gravity?

English mathematician Isaac Newton first formulated the theory of universal gravity in 1665 or 1666, after watching an apple fall to the ground from a tree outside his home. Born in 1643, in Lincolnshire, England, Newton was 23 years old at the time. Due to the plague ravishing the country, the cities were in lockdown, so Newton left university to spend time at his family home in the countryside.

When Newton saw the apple fall from the tree, he questioned why the apple fell straight down, rather than sideways or even upward. Newton theorised that the force that made the apple fall is the same as the force that keeps the moon and planets in their orbits.

Expanding upon his new theory, Newton developed three laws of motion to describe how forces affect different objects. 'Inertia' causes an object to stay still. 'Acceleration' increases the speed of a moving object, but not the direction. The third law, 'Action and Reaction', declares that for every action, there is an equal and opposite reaction. For example, if two objects collide, their speed or direction will alter.

Although he was 23 when he first developed his theories, Newton did not publish his work until 1687. The book is called 'Philosophiae Naturalis Principia Mathematica' (Mathematical Principles of Natural Philosophy), which is usually shortened to 'Principia'.

In 1727, Sir Isaac Newton passed away in his sleep at the age of 84.