Calibration of centre-of-mass energies at LEP1 for precise measurements ofZ properties

The determination of the centre-of-mass energies from the LEP1 data for 199 3, 1994 and 1995 is presented. Accurate knowledge of these energies is cruc ial in the measurement of the Z resonance parameters. The improved understa nding of the LEP energy behaviour accumulated during the 1995 energy scan i s detailed, while the 1993 and 1994 measurements are revised. For 1993 thes e supersede the previously published values. Additional instrumentation has allowed the detection of an unexpectedly large energy rise during physics fills. This new effect is accommodated in the modelling of the beam-energy in 1995 and propagated to the 1993 and 1994 energies. New results are repor ted on the magnet temperature behaviour which constitutes one of the major corrections to the average LEP energy. The 1995 energy scan took place in conditions very different from the previ ous years. In particular the interaction-point specific corrections to the centre-of-mass energy in 1995 are more complicated than previously: these a rise from the modified radiofrequency-system configuration and from opposit e-sign vertical dispersion induced by the bunch-train mode of LEP operation . Finally an improved evaluation of the LEP centre-of-mass energy spread is p resented. This significantly improves the precision on the Z width.