Isaac Newton first published his Universal Law of Gravity in the journal, Philosophiæ Naturalis Principia Mathematica, in July of 1687. The story goes that one day, Newton was sitting underneath an apple tree when an apple suddenly fell to the ground. He considered the problem with this, wanting to understand why the apple would fall straight down, instead of swinging to the right or left, according to the laws of motion that were known as the time. Newton’s theory explains how gravity is the force by which all the particles of matter in the universe attract to one another, and also explains how strong that attraction, or force, is.
Newton relied on the observations and equations of many great scientists who came before him, especially those of the great mathematician Johannes Kepler. Kepler was a scientists who wanted to understand the movement of the planets across the night sky. Using the hundreds of years of detailed observations left by earlier observers, he was able to work out the exact geometry of these movements. Using the movements of Mars, Kepler was able to calculate that the planets orbited around the sun in elliptical paths, not exact circles, in a predictable way. He developed three laws of planetary motions that are still used today by astronomers to predict the position of the planets based on past data.
However, while Kepler’s laws explained how the planets moved around the sun, and enabled scientists to predict their location, they did not explain why they moved the way they did. Newton was able to provide the missing link with his Universal Law of Gravity, which explained how gravitational force decreases the farther apart two objects are, known mathematically as an inverse square law.