Gravity is the tendency of masses to move toward each other.
The first mathematical formulation of the theory of gravitation was made by Sir Isaac Newton and proved astonishingly accurate. He postulated the force of "universal gravitational attraction".
Newton's theory has now been replaced by Albert Einstein's theory of general relativity but for most purposes dealing with weak gravitational fields (for example, sending rockets to the Moon or around the solar system) Newton's formulae are sufficiently accurate. For this reason Newton's law is often used.
