Formulated in 1687, Newton’s Law of Universal Gravitation was a turning point in physic. While the legend of the apple falling on his head is an exaggeration of the truth, Newton did have a brilliant insight: that every object in the universe attracts every other object. The force of attraction between two objects depends on only two things: the mass of the objects, and the distance between them. So, more massive objects exert a stronger force, while more distant objects exert a weaker force. Newton was able to formulate a simple equation to describe this, pictured above: force is equal to Newton’s gravitational constant, multiplied by masses of the objects, then divided by the square of the distance between the objects. What’s remarkable is that the law truly is universal—not only can it predict how things move here on Earth, but it can also the movements of the moon, planets, stars and even galaxies millions of lightyears away. Newton believed that the movement of every object in our universe could be predicted, but we know now that while his theory generally holds true, it is not precise. Einstein’s theory of general relativity had to step in to fill the holes.