The functional form of the azimuthal anchoring energy, i.e. the anisotropic part of the interfacial free energy, at the interface between the nematic liquid crystal 4-n-pentyl-4'-cyanobiphenyl and an obliquely evaporated SiO substrate is measured for the first time by using a reflectometric method. The anchoring energy function is obtained by measuring the director rotation on the interface caused by an external magnetic field ranging from 0 to 2.3 T. The dependence of the anchoring energy on the director azimuthal angle is found to be well fitted by the function W(a)(phi) = W(a) sin2 phi in agreement with the predictions of the Berreman model for the anchoring at a grooved interface.