Studying measurements taken of the Earth’s rotation as far back as 720 BC scientists have confirmed prior research that attests the planet is slowing down, suggesting that each day is getting progressively longer: by roughly two thousandths of a second every 100 years.
Obviously, in the span of human existence that is very little—infinitesimal. But these moments do add up and, more importantly, the research team argues that they can no longer support tidal friction can no longer explain the phenomenon.
According to Leslie Morrison of the International Astronomical Union, “Several geophysical factors have also operated over the past 2,700 years.” The study co-author goes on to say, “The reduction in the loading from the polar ice caps following the last Ice Age, which alters the shape of the Earth slightly (and its rotation), the electro-magnetic interaction between the core and mantle of the earth — and changes in mean sea level, which affect the speed of rotation.”
Furthermore, Scripps Institute of Oceanography geophysicist, Duncan Agnew (of San Diego) explains, “There is time and then there is how fast the Earth spins. Traditionally those things are closely linked, but they are not the same.”
While he was not directly involved in this work, the UCSD geophysicist notes, “During that period they were using water clocks to tell time.” These devices, he describes work by constantly dripping water into a vestibule at a steady rate. He continues, “While those are not terribly accurate, if you get 50 observations over a period of time you can get a good average.”
But these observations have been made within ancient communities throughout the globe. Islamic astronomers between AD 800 and AD 1000, for example, also make the same mention as European chronicles (written mostly in Latin).
Stephenson comments, “The Arab timing of solar eclipses in particular is superb,” though he could not necessarily attribute the same to the existing European data.
Still he does remark that we owe a debt of gratitude to ancient skywatchers. Their instruments may have been crude, but they were surprisingly accurate. And Agnew notes the new importance of learning to understand how the changing shape of the planet (following an ice age) affects other things, like global warming: because “major amounts of melting and a large rise in sea level will certainly change the earth’s rotation.”
The results of this study have been published in the journal Proceedings of the Royal Society A.