Long-term changes in the rotation rate of the Earth

Abstract

In the last decade, major advances in both observational and theoretical fields have taken place in our understanding of the rotation of the Earth. With the adoption of Atomic Time, the rotation rate can now be measured with unprecedented precision and independently of planetary or lunar motions. Information from earlier observations is also being recovered more accurately: independent analyses and new data from ancient eclipse observations have increased confidence in the controversial results obtained earlier. Perhaps the most outstanding development is the tentative measurement of the mean rotation rate in the remote geological past, made possible by the study of periodicities in the skeletal growth of fossil organisms. The improved data have exposed new geophysical problems and accentuated earlier ones. An unexplained acceleration of the Earth's rotation is confirmed by the re-examination of ancient eclipse records. In addition, Darwin's theory of tidal friction now has the arbitrariness of its time-scale replaced by data from modern astronomical observations so that, in spite of various theoretical refinements, it leads to the unacceptable prediction of a catastrophic period in the history of the Earth-Moon system in the mid-Precambrian. Information from the study of fossil growth increments can make a vital contribution in both these problems. The palaeontological data have been gathered mainly from corals and bivalves, although several other groups with accretionary skeletons provide a few figures and are potentially useful. The unexpected cosmogonic applications of these data have stimulated research into the physiological and particularly the ecological factors controlling incremental growth in these organisms, especially the bivalves. The main problems centre on the recording accuracy and the definition of the growth increments. The difficulty of eliminating irregular interference and of recovering geophysically useful measurements from the fossil record is discussed and we have tried to assess the reliability of the information already derived from this source.