Earth's rotation is slowing down, ancient records from Babylon, China, Greece and Arab civilisations have shown. By looking at documented eclipses dating back to 720 BCE, scientists have shown our days are getting longer by almost two thousandths of a second every hundred years.
Scientists have long known Earth's rotation is slowing down. Early on in our planet's formation, the days are estimated to have been just six hours long. Around 200 million years ago – when dinosaurs lived – days were about 23 hours long. And 200 million years from now, days will be 25 hours long.
In a bid to understand exactly how much Earth's rotation has varied over the last few thousand years, scientists from the University of Durham and the HM Nautical Almanac Office looked back at 2,700 years-worth of records to get a better understanding of this slowing.
Starting with ancient Babylonian tablets, the team looked at how this ancient civilisation measured the timings of the start and end of eclipses – pinpointing the locations. They used texts from China, Greece, Medieval Europe and Arab countries to produce "the most comprehensive list of observations to-date to the study of the variability of the Earth's rate of rotation since 720 BC".
They then compared these records with a computer model that showed when and where eclipses would have taken place thousands of years ago if Earth's rotation had never changed. Findings, published in the Proceedings of the Royal Society A, showed that the Earth's spin is slowing by approximately 1.8 milliseconds per century.
This slowing, is – in part – the result of tidal forces. Earth spins faster than the Moon orbits it. This means the tidal bulge travels ahead of the Moon. Gravitational forces from the Moon pull back on this tidal bulge, causing the rotation to slow. Previously, astronomers had calculated that this would cause Earth's spin to slow by 2.3 milliseconds per century – faster than the results showed.
"Assuming that the measurement of tidal braking in the Earth–Moon–Sun system is secure, our main conclusion is that this mechanism alone does not account for the observed deceleration in rotation over the past 2,700 years," the team wrote.
This finding, they say, has important implications for future studies into Earth's rotation and the geological processes involved. "Our measurements of the Earth's rotation for the period 720 BC to AD 2015 set firm boundaries for future work on post-glacial rebound and core–mantle coupling which are invoked to explain the departures from tidal friction," the team concludes.