  Accretion of gas onto a central supermassive black hole is generally accepted
to be the source of the emitted energy in active galactic nuclei.The broad
emission lines we observe in their optical spectra are probably formed in the
wind of an accretion disk at distances of light days to light years from the
central black hole. The variable fraction of the emission lines originates at
typical distances of only 1 to 50 light days from the central supermassive
black hole. We derived a central black hole mass of M_orbital = 1.8 +/-0.4 x
10^7 M_sun in the Seyfert galaxy Mrk110 assuming the broad emission lines are
generated in gas clouds orbiting within an accretion disk. This figure depends
on the inclination angle of the accretion disk. Here we report on the detection
of gravitational redshifted emission in the variable fraction of the broad
emission lines. We derive a central black hole mass of M_grav = 14.0 +/-3.0 x
10^7 M_sun. These measurements are independent on the orientation of the
accretion disk. The comparison of both black hole mass estimates allows to
determine the projection of the central accretion disk angle i to 21 +/-5 deg.
in Mrk110 and therefore the orientation of the spin axis of the central black
hole.
