– Earth’s spin is slowing
Scientists used ancient records to calculate the slowdown BELINDA SMITH reports.
When ancient astronomers carved cuneiform figures recording solar eclipses on to clay tablets, little did they know they were providing baseline data for an epic science experiment.
Some 2,700 years later, a trio of British scientists from the University of Durham and Her Majesty’s Nautical Almanac Office used these and other historic records to calculate that an Earth day grew by an average of just under twothousandths of a second each century. Their findings appeared in the Proceedings of the Royal Society in December 2016.
Earth’s slowdown is due to a transfer of rotational momentum to the moon; it also gives the Moon a little thrust, speeding up its orbit and causing it to recede from Earth by about 4 cm per year. The end result is that the orbits of the two bodies will eventually lock face to face, rotating once every 47 days. The Moon’s distance from the Earth will move from 380,000 km now to about 500,000 km.
Locked orbits are the ultimate fate of every planet and moon; pseudo planet Pluto and its moon Charon, with their smaller masses, are already there. Physics equations tell us Earth will realise this fate billions of years from now.
But the British astronomers wanted something a little more precise. So they spent four decades scouring records of historical eclipses including those of 8th century BCE Babylonians, the 2nd century
Almagest penned by the Greek astronomer Ptolemy, centuries of Chinese dynastic records and observations from Arab astronomers from about 830 CE to 1020 CE.
Some of these observations timed how long it took for the Moon to cross the path of the Sun – a measure of the Earth’s rotation speed. They found that, on average, the length of Earth’s day increased at a rate of 1.8 milliseconds per century – somewhat lower than the predicted value of 2.3 milliseconds. This discrepancy, they suggest, may be due to melting glaciers or fluxes in the deepest layers of the Earth, which mitigate the forces contributing to the slowing of the planet’s spin.
The Moon’s pull is slowing the Earth’s orbit, but not as much as the maths predicts.