Earth’s spin is slowing at a rate of 1.8 milliseconds per century

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The latest findings in Earth science are brought to you by ancient astronomers who observed the heavens as much as 2,700 years ago.

Thanks to hundreds of records of lunar and solar eclipses carved in clay tablets and written into dynastic histories, modern scientists have determined that the amount of time it takes for Earth to complete a single rotation on its axis has slowed by 1.8 milliseconds per century, according to a report published Wednesday in Proceedings of the Royal Society A.

It may not sound significant, but over the course of 2½ millenniums, that time discrepancy adds up to about 7 hours.

In other words, if humanity had been measuring time with an atomic clock that started running back in 700 BC, today that clock would read 7 p.m. when the sun is directly overhead rather than noon.

“There is time and then there is how fast the Earth spins,” said Duncan Agnew, a geophysicist at Scripps Institution of Oceanography in San Diego, who was not involved with the work. “Traditionally those things are closely linked, but they are not the same.”

Our earliest ancestors measured time based on the position of celestial bodies in the sky, such as the rising and setting of the sun or the changing shape of the moon. Scientists refer to this as Universal Time, and it is governed by the dynamic gravitational motions of the Earth, moon and sun.

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