Earth’s Days Are Getting Longer... By Two Milliseconds a Century

Researchers at Durham University, UK, and the UK’s Nautical Almanac Office gathered historical accounts of eclipses and other celestial events from 720 BC to 2015. The oldest records came from Babylonian clay tablets written in cuneiform. Other sources came from Ancient Greece, such as Ptolemy’s second century ‘Almagest’, as well as scripts from China, medieval Europe and the Arab world.

The ancient records captured the times and places that people witnessed various stages of solar and lunar eclipses, whilst sources from 1600 AD onwards described lunar occultations (when the moon passes in front of particular stars and blocked them from view).

The team then used gravitational theories about the movement of Earth around the Sun, and the Moon around Earth, to compute the timing of eclipses of the Moon and Sun over time, as viewed from our planet. The computer model 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,’ commented lead study lead co-author Leslie Morrison. ‘It means the Earth has been varying in its state of rotation.’

Astronomers have known for a long time that the Earth’s spin is gradually slowing down. The main braking effect comes from tides caused by the moon’s gravity. ‘The heaping up of water drags on the Earth as it spins underneath,’ said Morrison. As the Earth’s rotation slows, the moon’s orbit grows by about 4cm a year.

Other than this braking effect from the moon, another factor that impacts the Earth’s rotation is the planet’s altering shape due to the shrinking of the polar ice caps since the last Ice Age. Since the end of the Ice Age, land masses that were once buried under frozen slabs have been unloaded and sprung back into place. This causes the Earth to be less oblate on its axis. Changes in the world’s sea levels and electromagnetic forces between the Earth’s core and its rocky mantle have also had an impact on Earth’s spin as well.

The slowing of the Earth’s orbit is the reason why the world’s timekeepers have to adjust high-precision clocks every few years to ensure they remain in sync with the planet’s rotation.

The full study was recently published in the journal ‘Proceedings of the Royal Society A’.