Observation of radial ring deformations using closed orbits at LEP

With a circumference of 26.7 km the LEP ring is subject to long range geological deformations which must be monitored accurately to guarantee a sufficient precision on the beam energy for the LEP experiments. Changes of the LEP circumference can be extracted from the radial position of the closed orbits with an accuracy 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 seasonal circumference variations of 2 mm are also observed. A summary of measurements collected between 1993 and 1998 is presented.