LONG-TERM FLUCTUATIONS IN THE EARTHS ROTATION - 700-BC TO AD-1990

Records of solar and lunar eclipses in the period 700 BC to AD 1600, o riginating from the ancient and medieval civilizations of Babylon, Chi na, Europe and the Arab world, are amassed and critically appraised fo r their usefulness in answering questions about the long-term variabil ity of the Earth's rate of rotation. Results from previous analyses of lunar occultations in the period AD 1600-1955.5, and from high-precis ion data in AD 1955.5-1990, are included in the dataset considered in this paper. Using the change in the length of the mean solar day (l.o. d.) in units of milliseconds per century (ms cy(-1)) as the measure of acceleration in the rate of rotation, it is found that the l.o.d. has increased by (+1.70 +/- 0.05) ms cy(-1) (= (-4.5 +/- 0.1) x 10(-22) r ad s(-2)) on average over the past 2700 years. Yet an increase of +2.3 +/- 0.1 ms cy(-1) (=(-6.1 +/- 0.4) x 10(-22) rad s(-2)) is expected f rom the tidal braking of the Earth's spin, assuming a value of -26.0'' cy(-2) for the tidal acceleration of the Moon. There is thus an avera ge accelerative component in the Earth's rotation which acts to decrea se the l.o.d. by (-0.6 +/- 0.1) ms cy(-1) (=(+1.6 +/- 0.4) x 10(-22) r ad s(-2)). Moreover, it is shown that besides this accelerative compon ent, there is a fluctuation in the l.o.d. with a semi-amplitude of sim ilar to 4 ms and a period of similar to 1500 yr.