According to Newton, if there were no force acting on it, moon would not move on a curved path, instead it would travel on a straight path. Based on such observations of the path of the Moon, Newton proposed a formula to describe how this force must behave. In doing so, he imagined that the laws of nature that applied to objects on Earth also applied to heavenly bodies. The law has since been confirmed by experiments carried out between a pair of masses on Earth, and is known as Newton’s universal law of gravitation.

If two masses m₁ and m₂ are a distance r apart, Newton claimed that the gravitational force F between these objects is directly proportional to the product of their masses and is inversely proportional to the square of the distance that separates their centers. The magnitude of the force between the two objects is, then given by

where the proportionality constant G represents the universal gravitational constant (G = 6.67 × 10−11 Nm²/kg²).