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We report direct evidence of a secondary flow excited by the Earth rotationin a water-filled spherical container spinning at constant rotation rate. Thisso-called {\it tilt-over flow} essentially consists in a rotation around anaxis 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 ofplanets forced by precession of the planet rotation axis, and it has beenproposed to contribute to the generation of planetary magnetic fields. Wedetect this weak secondary flow using a particle image velocimetry systemmounted in the rotating frame. This secondary flow consists in a weak rotation,thousand times smaller than the sphere rotation, around a horizontal axis whichis stationary in the laboratory frame. Its amplitude and orientation are inquantitative agreement with the theory of the tilt-over flow excited byprecession. These results show that setting a fluid in a perfect solid bodyrotation in a laboratory experiment is impossible --- unless tilting therotation axis of the experiment parallel to the Earth rotation axis.

Earth rotation prevents exact solid body rotation of fluids in the laboratory