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Longer Days as Earth's Rotation Winds Down

14 December 2016

Astronomers from Durham University in north east England and the U.K.’s Nautical Almanac Office have found that every century the day lasts two milliseconds longer. After compiling 3,000 years of astronomical records they found that Earth’s rotation winds down to produce this time delay.

The group studied historical accounts of astronomer’s records of eclipses and other astronomical events from 720 B.C. to 2015 A.C. They compiled records form Babylonians clay tablets, ancient Greek texts, such as Ptolemy’s Almagest, documents from ancient China and Arab astronomers’ documents. To determine how the rotation of the Earth has changed over the last 2,735 years, the team of researchers used computer models to calculate where and when ancient people would have seen those past events assuming that the rotation of the Earth had remained steady.

“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.”

Earth was formed by a spinning cloud of gas and dust over four billion years ago. At that time its rotation speed was enough to produce a day of only six hours. Then an object the size of Mars impinged on the Earth and broke off the matter that became the moon. This event caused the Earth’s rotation to slow down by one-fourth to the actual 24 hours day.

It is well-known that Earth’s rotation is slowing down due to many reasons, but probably the most important is the effect of the moon on tidal waves caused by its gravity. “The heaping up of water drags on the Earth as it spins underneath,” said Morrison. As Earth’s rotation slows, the moon’s orbit grows by about 4 cm a year.

Tidal braking is not the only force that has changed the rotation of the Earth. Its rotation would have slowed down more had it not been for climate changes that happened millions of years ago. Only after the last ice age, and due to the heating of the Earth land masses that were buried under the frozen water, did it spring back. This phenomenon produced a less oblate Earth, so it started to spin faster as its poles became less compressed. The change in the Earth shape produced a faster rotation, in the same way a spinning ice skater speeds up when he pulls in his arms. In this case climate changes provided a slowing of Earth’s rotational speed, but these changes are not guaranteed going forward.

Don’t hold your breath for the day will be 25 hours—you would have to wait for about two million centuries!

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