‘Earth’s day lengthens by two milliseconds a century’(thehindu)

Astronomers who compiled nearly 3,000 years of celestial records have found that with every passing century, the day on Earth lengthens by two milliseconds as the planet’s rotation gradually winds down.

The split second gained since World War I may not seem much, but the time it takes for a sunbeam to travel 600km towards Earth can cost an Olympic gold medal, as the American Tim McKee found out when he lost to Sweden’s Gunnar Larsson in 1972.

For those holding out for a whole extra hour a day, be prepared for a long wait. Barring any change in the rate of slowing down, an Earth day will not last 25 hours for about two million centuries more.

Researchers at Durham University and the U.K.’s Nautical Almanac Office gathered historical accounts of eclipses and other celestial events from 720 BCE to 2015. The oldest records came from Babylonian clay tablets written in cuneiform, with more added from ancient Greek texts, such as Ptolemy’s 2nd century Almagest, and scripts from China, mediaeval Europe and the Arab dominions.

The ancient records captured the times and places that people witnessed various stages of solar and lunar eclipses.

To find out how the Earth’s rotation has varied over the 2,735-year-long period, the researchers compared the historical records with a computer model that calculated where and when people would have seen past events if Earth’s spin had remained constant.

“Even though the observations are crude, we can see a consistent discrepancy between the calculations and where and when the eclipses were actually seen,” said Leslie Morrison, an astronomer on the team. “It means the Earth has been varying in its state of rotation.” Astronomers have long known that Earth’s spin is slowing down. The main braking effect comes from tides caused by the moon’s gravity.

Changes in the world’s sea levels and electromagnetic forces between Earth’s core and its rocky mantle had effects on Earth’s spin too, according to the scientists’ report in Proceedings of the Royal Society . The different forces seem to drive cycles in the Earth’s rotation spanning decades to centuries, with one cycle repeating every 1500 years. — Guardian News and Media Ltd.