The Law of Gravitation is an inverse-square law. Look at the formula F_g = G m₁m₂ d² , F is on one side, and d is on the other side squared and on the bottom of the fraction. It means that as the distance between two objects increases, the force of gravity between them decreases by the SQUARE of the distance. Also, if the distance decreases, the force of gravity will increase by the SQUARE of the distance between them. If I double my distance, I have ¼ the force of gravitation, and if I triple it, I have 1/9 the force. If I cut my distance in half, I have 4 times the force of gravitation, and if I cut it to 1/3, I have 9 times the force.

Source: Tidal_Forces, Windows to the Universe