Space
A number of opposing forces are subtly making the rotation of our planet both speed up and slow down.
Earth’s slowdown
YOU MAY ALREADY know that the Earth’s rotation is very gradually slowing down.That’s the reason we introduce occasional leap seconds into our day – to keep time as told by the Earth’s rotation exactly in sync with the time given by our super-accurate atomic clocks.
But just how much is the Earth slowing down? That’s a great question, and one that turns out to have an intriguing answer. First of all, though – why is it slowing down at all?
The main culprit is the Moon, which is stealing rotational energy from our planet by means of a process called tidal friction. As its name implies, that’s a braking of the rotating Earth caused by the tides raised by the
Moon’s gravity on the planet’s oceans and crust. In return, the Moon receives a tiny acceleration, which causes it to drift away from us at a speed of about 3.8cm per year.
That drift has been accurately measured over many decades by bouncing laser beams from reflectors left on the lunar surface by Apollo astronauts. And from it, we can calculate the increase in the length of the day caused by the braking. This turns out to be 2.3 milliseconds per century. But now, there’s a puzzle. Using an exquisite combination of archaeology, ancient history and scientific observations since 1600, researchers in the UK have used solar eclipses and lunar occultations (events in which the Moon passes in front of a star or planet) to measure directly the average increase in the length of the day since 720 BCE. And guess what? It’s only 1.8 milliseconds per century.
Does that mean someone has done their sums wrongly? It’s very unlikely, since both these quantities have high measurement accuracy. Rather, it means something else is accelerating the Earth’s rotation by 0.5 milliseconds per century.The researchers point to a slow rebound of the Earth’s crust since the end of the last ice age, plus a poorly understood coupling between the planet’s molten core and its mantle, some 3000km beneath our feet.