HISTORICAL ECLIPSES AND THE VARIABILITY OF THE EARTH'S ROTATION

Abstract

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 eclipse records are preserved from a variety of cultures, and using these seemingly crude ancient and medieval observations, 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 rate of spin. It is known that by this mechanism, the length of the day is increasing by 2.3 ms per century. From analyses of the discordance between observations and calculations of eclipses, the average measured increase in the length of the day is 1.8 ms 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 0.5 ms 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. Besides these secular trends, there is clear evidence of fluctuations of the length of the day on a timescale of centuries, with an amplitude of 4 ms which is similar to that of the decade fluctuations.