  The solar photosphere is filled by a magnetic field which is tangled on
scales much smaller than the resolution capability of solar telescopes. This
hidden magnetic field can be investigated via the Hanle effect. In 2007 we
started a synoptic program to explore if the magnetic flux of the quiet
photosphere varies with the solar cycle. For this purpose we applied a
differential Hanle effect technique based on observations of scattering
polarization in C$_2$ molecular lines around 514.0 nm, taken generally every
month. Our results now span almost one complete solar cycle.
