The sands of time and tidal friction


Leslie V. Morrison, Cambridge (UK), F. Richard Stephenson, Durham



Ancient Babylonian clay tablets buried for centuries beneath the sands of the desert are part of an extensive historical archive which contains vital information about the rotation of the Earth. Many are preserved, and using these seemingly crude ancient and medieval observations of eclipses, variations in the Earth's rotation can be traced back over the past 2500 years.

The tidal torque exerted by the Moon (and to a lesser extent, the Sun) is the predominant mechanism in reducing the Earth's spin. It is known that by this mechanism, the length of the day is increasing by 2.3 milliseconds per century.

By analysing observations of eclipses, the actual measured change in the length of the day is +1.7 milliseconds per century, from which it is concluded that besides the tidal contribution there is another long-term component acting to decrease the length of the day by 0.6 milliseconds per century. This component, which is thought to result from the decrease of the Earth's oblateness following the last ice age, is consistent with recent measurements made by artificial satellites.

Bibliographical details:

Leslie V. Morrison, F. Richard Stephenson: The sands of time and tidal friction. In: Peter Brosche, Wolfgang R. Dick, Oliver Schwarz, Roland Wielen (Eds.): The Message of the Angles - Astrometry from 1798 to 1998. Proceedings of the International Spring Meeting of the Astronomische Gesellschaft, Gotha, May 11-15, 1998. (Acta Historica Astronomiae ; 3). Thun ; Frankfurt am Main : Deutsch, 1998, p. 100-113.