We determined a central black hole mass of $M_{\rm virial}=1.8\pm 0.4\times 10^{7} M_{\odot}$ in the Seyfert galaxy Mrk 110 with reverberation mapping methods – i.e. from integrated optical line profile variations as well as from velocity-delay maps. But this black hole mass depends on the unknown orientation of the central accretion disk. Here we report on the detection of gravitational redshifted emission in the variable fraction of all broad optical emission lines in Mrk 110. We derive a central black hole mass of $M_{\rm grav}=14.0\pm 3.0\times 10^{7} M_{\odot}$ with this method. This value is independent on the orientation of the accretion disk. A comparison of both black hole mass estimates allows to determine the central accretion disk inclination angle i to 21° in Mrk 110, and therefore the orientation of the spin axis of the central black hole.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html