Contemporary geophysics from Babylonian clay tablets
History and astronomy can be brought to bear on problems in contemporary geophysics. From seemingly crude ancient and medieval observations of eclipses, we show that variations in the length of the day can be traced back over the past 2500 years. The tidal torque exerted by the Moon (and, to a lesser extent, by the Sun) is the dominant mechanism in reducing the Earth's spin. It is known that by this mechanism, the length of the day is increasing by .- 1 + 2 3ms per century (mscy ). By analysing observations of eclipses, we find the actual measured change in the length of the day to be + 1.7mscy , from which we conclude that besides the tidal contribution, there is another long-term component acting to reduce - 1 the length of the day by- 0.6mscy . This component, which is thought to result from the decrease in the Earth's oblateness following the last Ice Age, is consistent with recent measurements made by artificial satellites. - 1
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