Weight is given by the formula W = mg, where m is mass (constant) and g is the acceleration due to gravity. The value of g varies slightly across the Earth's surface. The Earth is not a perfect sphere; it bulges at the equator and is flattened at the poles. This means points at the poles are closer to the Earth's center than points at the equator. Gravitational force is inversely proportional to the square of the distance, so gravity is stronger where the distance is smaller (at the poles). Additionally, the Earth's rotation creates a centrifugal force that opposes gravity, and this force is maximum at the equator and zero at the poles. Both the shape effect and the rotation effect cause the effective gravitational acceleration (g) to be maximum at the poles and minimum at the equator.