By the weight of the silvery moon.

Scientists at the European Laboratory for Particle Physics (CERN) in Geneva, Switzerland, have unmasked an unlikely culprit - the moon - in accounting for small, but significant fluctuations in the energy of particle beams circulating in the Large Electron Positron (LEP) collider. These tiny energy variations became apparent about two years ago as physicists steadily increased the precision of measurements of the mass of a subatomic particle known as the [Z.sup.0].

At the LEP collider, beams of electrons and positrons travel around an underground ring with a circumference of 26.7 kilometers. A radio frequency system keeps the particles circulating at a constant number of revolutions per second. However, tides induced by the moon's gravitational attraction not only make the oceans bulge, but also deform the Earth's crust. These deformations - which rise and fall over a period of roughly 12 hours - are enough to stretch or shrink the collider's circumference by nearly 1 millimeter. To keep circulating at a constant frequency, particles drift to slightly different paths around the collider at various times, depending on the moon's position. Such minute changes in path alter the beam energy by as much as 10 million electron-volts - enough to make the moon's gravitational effect the cause of the dominant error in determinations of the [Z.sup.0] mass. Thus, to calibrate the beam energy and keep improving measurement precision, it now pays to have an almanac or tide table handy.