Observation of radial ring deformations using closed orbits at LEP

With a circumference of 27.7 km the LEP ring is subject to long large geological deformations which must be monitored accurately to guarantee a sufficient precision on the beam energy for the LEP experiments. Changes of the LEP circimference can be extracted from the radial position of the closed orbits with an accurancy better than 0.1 mm. The observed tidal deformations with amplitudes of up to 1 mm were found to be in good agreement with geological models. Besides such periodic effects seasonalcircumference variations of 2 mm are also observed. A summary of measurements collected between 1993 and 1998 is presented.