A new method to determine the spin tune is described and tested. In an ideal planar magnetic ring, the spin tune – defined as the number of spin precessions per turn – is given by νs = γ • G (γ is the Lorentz factor, G the magnetic anomaly). For 970MeV/c deuterons coherently recessing with a frequency of ≈120 kHz in the Cooler Synchrotron COSY, the spin tune is deduced from the up-down asymmetry of deuteron carbon scattering. In a time interval of 2.6 s, the spin tune was determined with a precision of the order 10−8, and to 1•10−10 for a continuous 100 s accelerator cycle. This renders the presented method a new precision tool for accelerator physics: controlling the spin motion of particles to high precision is mandatory, in particular, for the measurement of electric dipole moments of charged particles in a storage ring.