# Physics Guru’s notepad: how Newton discovered gravity

Even if Newton did observe a falling apple in 1666, it’s highly unlikely that he developed his theory of gravity right there on the spot. At some point, though – and this is what fits in nicely with the apple story – he came to realise that the same force which governs the acceleration of objects towards the ground also reaches far out into space. His universal law of gravitation was published some 20 years after the alleged apple incident. In Philosophiæ Naturalis Principia Mathematica (1687), Newton explains that the gravitational force between two objects is inversely proportional to the square of the distance between them, and directly proportional to the product of their masses. In other words, if you double the distance between two objects you reduce the gravitational force by a factor of four; triple the distance and you reduce it by a factor of nine. Using this law, Newton was able to show that the orbits of the planets around the Sun – and the Moon around the Earth – can be explained by the bodies’ mutual gravitational attraction. It’s the very same effect that pulls a falling apple towards the ground.