A formula for the potential due to a distribution of mass or charge at an external point: the potential V at a point P resulting from a distribution of mass or positive charge centered about a point O is V = (kM/r ) + (k /2r³)(A + B + C - 3I ) + O(1/r⁴), where r is the distance from O to P, k is the gravitational or electrostatic constant, M is the total mass or charge, A, B, and C are the principal moments about O, I is the moment about OP, and O(1/r⁴) is a quantity that falls off at least as rapidly as 1/r⁴.