– Earth’s spin is slow­ing

Sci­en­tists used an­cient records to cal­cu­late the slow­down BELINDA SMITH re­ports.

Cosmos - - Digest -

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.

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