Scientists have obtained evidence from ancient Babylonian astronomical records that the speed of Earth’s spin has been slowing gradually over the centuries. The gradual increase in the time it takes for the Earth to do a complete rotation on its axis means that the length of a solar day is getting longer. According to researchers, the on-going lengthening of the 24-hour day could lead to a 25-hour day millions of years in the future.

The time it takes for the Earth to complete a full rotation, and consequently the average day length, has been increasing by about +1.8 milliseconds every century over the past 27 centuries, according to a new study published in the journal Proceedings of the Royal Society A, that examined ancient Babylonian astronomical records dating back to 750 B.C.

Even before scientists were able to access relevant information from ancient astronomical records, they had known that the Earth’s spin, which determines the average day length, was slowing due to friction generated when the Moon’s gravity pulls the planet’s large bodies of water over solid Earth, according to Live Science.

But researchers had previously estimated the rate of slowing of the Earth’s spin at about 2.3 milliseconds per century, a fraction of a millisecond faster than the estimate of 1.8 milliseconds obtained through the evidence of ancient Babylonian records, Phys.org reports.

Although, the difference of.5 milliseconds appears trivial, Leslie Morrison, retired astronomer at the Royal Greenwich Observatory, who has been working on measuring the Earth’s rotation accurately, said that the difference has significant implications for scientists wishing to understand how the Earth’s shape has changed since the last ice age, 12,000 years ago.

In their study published in the journal Proceedings of the Royal Society A, Morrison and his colleagues were able to obtain astronomical records in cuneiform dating back to 720 B.C.

Cuneiform (“wedge-shaped”) is an ancient system of writing inscribed on baked clay tablets that archaeologists first stumbled upon in the 1800s while exploring ancient Babylonian ruins in modern Iraq. A stunning wealth of information about the ancient world became accessible after researchers deciphered the ancient system of writing. The information obtained about ancient Babylonian civilization ranged from daily transaction records kept by merchants, teachers’ classroom arithmetic lessons, state records, religious tablets containing accounts of myths and legends, and records of observations of the stars and planets by ancient astrologers and astronomers.

“There were guys 2,500 years ago putting things in clay tablets. They weren’t thinking later somebody would be writing a paper about Earth’s rotation.”

Ancient Babylonian clay tablet inscribed with cuneiform script [ [Image by Schoyencollection/Public Domain/Wikimedia]

And thanks to the diligence and painstaking efforts of ancient Babylonian astronomers, 21st-century scientists have access to reliable and graphic records of solar eclipses that occurred thousands of years ago. The records have helped scientists to make reliable estimates of the rate of Earth’s rotation about 720 B.C

“The descriptions of a total solar eclipse [by ancient Babylonian astronomers] are so graphic. When the days suddenly turn to night and the stars appear.”

Earth's slowing spin is increasing day length [Image by Vadim Sadovski/Shutterstock]

Morrison and his colleagues were also able to compare the Babylonian records with parallel ancient Chinese, Greek and Arab records.

The researchers observed that when they worked backwards, basing their calculations exclusively on the theory of universal gravitation as it affects the rotation and orbits of the Earth and Moon relative to the Sun, a significant conflict or discrepancy emerged between the researchers’ timing of eclipses, where the eclipses should have been observed and the timing and locations indicated by the ancient Babylonian records.

The discrepancy, according to Morrison and his research colleagues, is the basis for measuring how the Earth’s spin has changed since 720 B.C. and thus the degree of lengthening of the solar day since 720 B.C. when the ancient records were taken.

The team calculated that the solar day has increased by an average of about 1.8 milliseconds per century in the past 27 centuries.

The factors responsible for increasing day length, according to Duncan Agnew, research geophysicist at the University of California in San Diego, include friction generated by the tides due to the Moon’s gravitational pull on the Earth’s oceans, changes in the Earth’s solid mass due to melting polar ice caps since the last ice age and changes in molten iron in the Earth’s core.

According to Agnew, the retreat of ice sheets at the end of the last ice age, 12,000 years ago, has caused a delayed process of rebounding of the Earth’s shape in response to being relieved of the weight of the solid mass of ice. The rebounding and the resulting change in the shape of the Earth has affected the Earth’s spin, just as a figure skater can change the rate of her spin by drawing her arms in or holding them out.

[Featured Image by Vadim Sadovski/Shutterstock]