Abstract: A slowly turning rotating mass, such as a wind turbine rotor, is balanced in situ by measuring the magnitude of a phenomenon, such as acceleration, created by rotor unbalance at a location on a mass support shaft which is close to the mass. The measurement of the phenomenon is made along a line selected to be insensitive to the bias of gravity on the mass. The measurement is made while the mass turns at its usual operating rate. The structure which supports the shaft is fixed as rigidly as possible in space while measurements of the phenomenon are made.