Ancient astronomers looked at the sky in wonderment, made observations, and recorded them for posterity.  Little did they know thousands of years later, modern scientists would analyze those measurements and it would help them to calculate a more precise measurement of the change in the Earth’s rotation over time.

But, that is exactly what has happened, according to an article on csmonitor.com.  A new study published in the Proceedings of the Royal Society, says researchers looked at hundreds of solar and lunar eclipse records, some actually carved into clay tablets, and others recorded by ancient Chinese, to make a calculation that the speed of the Earth’s rotation has slowed by 1.8 milliseconds per century.

Previous research estimated the change to be 2.3 milliseconds per century, possibly caused by tidal friction as a by-product of the moon’s pull on our oceans.  The new rate is a half second off from the former, but scientists admit they can’t say exactly why the discrepancy exists.

Because of the well-known and documented paths the celestial bodies take across the heavens, solar and lunar eclipses can be calculated and astronomers know when they will occur.  While the complete process of a solar eclipse can take over an hour, the total part only lasts a few minutes.

Richard Stephenson, a now-retired astronomer from the University of Durham and lead author on the study, and his team, were able to find a handful of ancient recordings that were precise enough to measure the differences between the calculated times and the actual times of the events, which led to the determination of the slowing rotation.

Dozens of the texts that were used were recorded between 720 BC and 10 BC, by the Chinese, but other texts from Islamic astronomers dated between 800 AD and 1000 were also included in the research.

“In 90 percent of the cases in the Chinese and Babylonian records, their dates reproduce exactly to the date of the calculated eclipse,” commented Stephenson.  “This gives us a lot of confidence in our data set.”

Stephenson continued, “People recording these things never had the slightest notion that what they were doing would lead to people in our generation actually studying changes in the Earth’s spin.”