Number 282 (Story #2), August 2, 1996 by Phillip F. Schewe and Ben Stein DAYS WERE ONLY 18 HOURS LONG back in the Proterozoic era, some 900 million years ago. Charles Sonett of the University of Arizona has studied records of ancient tidal deposits preserved in rock strata. Like tree rings, the periodicity of tidal sediments, or tidalites, provide an accounting of ancient times. Sonett's data, collected in Utah, Indiana, Alabama, and Australia, shows that long ago the day was shorter, the year longer, and the moon much closer. Indeed, as the moon recedes from the Earth (at a rate of 3.8 cm/year) it continues to slow Earth's rotation, thus extending the day further. (C.P. Sonett et al., Science, 5 July.)