Number 111 (Story #2), January 21, 1993 by Phillip F. Schewe and Ben Stein CHANGES IN THE EARTH'S ROTATION RATE occur at the level of several parts in 10**8. Using laser ranging (bouncing radar waves off the Moon or satellites) and very long baseline interferometry, length-of-day (LOD) measurements can detect 0.03 msec changes. Jean Dickey, a JPL geophysicist, cites three main types of LOD change: a linear increase owing to tidal dissipation; larger, irregular variations, on the scale of decades, owing to core-mantle interactions; and shorter-term (seasonal) changes from the angular momentum exchange between crust and atmosphere. (Eos, 12 Jan. 1993.)