The time taken by the Earth to rotate fluctuates by milliseconds every day. Experts say that it might be due to atmospheric changes, high and low tides, and likely movements in the Earth's core that are constantly shifting the planet's mass. To keep track of the Earth's vagaries, scientists at the Federal Office for Cartography and Geodesy in Frankfurt, Germany, the technical University of Munich, and the University of Canterbury, New Zealand, have developed the world's largest ring laser gyroscope. The device is similar to the laser gyroscope used in the navigation system of aircraft but it is much more accurate. When it reaches its peak sensitivity, it should be able to detect changes as small as a few milliseconds per day ( Science , Vol 277, No 5331).