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According to the paper's authors, the historical observations show a consistent discrepancy between the gravitational calculations and where and when the eclipses were actually seen.
"This discrepancy," Morrison explained, "is a measure of how the earth's rotation has been varying since 720 B.C., which is where the extant, reliable and accurate observations of eclipses in ancient civilizations begin."
Nevertheless, Morrison's team and other scientists are keen to understand what is driving variations in Earth's rotation. Our planet's movement, after all, affects nearly every aspect of life, from the seasons to the tides.
Duncan Agnew, a geophysicist from the University of California at San Diego and Scripps Institution of Oceanography, told Seeker he agrees with Morrison that the primary rate differences are "probably caused by Earth's gradual change in shape caused by the end of the Ice Age. Understanding this change in shape is important in measured changes in sea level, something very important in this age of global warming. Should global warming cause major amounts of melting and a large rise in sea level, this will certainly change the earth's rotation, though by an amount that will be very small."