  We report direct evidence of a secondary flow excited by the Earth rotation
in a water-filled spherical container spinning at constant rotation rate. This
so-called {\it tilt-over flow} essentially consists in a rotation around an
axis which is slightly tilted with respect to the rotation axis of the sphere.
In the astrophysical context, it corresponds to the flow in the liquid cores of
planets forced by precession of the planet rotation axis, and it has been
proposed to contribute to the generation of planetary magnetic fields. We
detect this weak secondary flow using a particle image velocimetry system
mounted in the rotating frame. This secondary flow consists in a weak rotation,
thousand times smaller than the sphere rotation, around a horizontal axis which
is stationary in the laboratory frame. Its amplitude and orientation are in
quantitative agreement with the theory of the tilt-over flow excited by
precession. These results show that setting a fluid in a perfect solid body
rotation in a laboratory experiment is impossible --- unless tilting the
rotation axis of the experiment parallel to the Earth rotation axis.
