The observations of ancient astronomers from the time of the Babylonians onwards have revealed our modern days are not getting quite as long as they should be.

Key points: The Earth's spin is slowing down and days are getting longer over time

The Earth's spin is slowing down and days are getting longer over time Analysis of ancient observations indicates days are getting longer by 1.7 milliseconds per century

Analysis of ancient observations indicates days are getting longer by 1.7 milliseconds per century This differs to calculations based on tidal friction, which indicate days are getting longer by 2.3 milliseconds per century

Scientists have long known the Earth's spin is slowing down — and thus the days are getting longer — because of the dragging effect of the oceans as they are pulled on by the Sun and Moon, called tidal friction.

But a study of astronomical records from 720 BC to AD 2015, published in Proceedings of the Royal Society A, has uncovered a small but significant discrepancy between modern calculations and ancient observations.

It seems the Earth isn't slowing down as much as the calculations based on tidal friction would suggest.

The study — the most comprehensive analysis of ancient eclipse records gathered to date — brought together astronomers, geophysicists, historians and archaeologists.

"We use the gravitational theories of the motion of the Earth about the Sun and the Moon around the Earth, so we can compute where and when eclipses should have occurred on the Earth," astronomer and co-author Dr Leslie Morrison said.

The researchers analysed observations of eclipses recorded on clay tablets by the Babylonians, as well as data from previous studies of ancient Chinese, Greek and Arab, and medieval European astronomical records.

Babylonian clay tablets records lunar eclipses ( Supplied: British Museum (CC BY-NC-SA 4.0) )

They also analysed records of the movement of the moon in front of significant stars made since the invention of the telescope.

While tidal friction should lead to an increase in day length of 2.3 milliseconds per century, the records pointed to an increase in day length of 1.7 milliseconds per century.

"From these observations, we're able to distinguish the fact that the tidal deceleration is actually too much," Dr Morrison said.

"What we actually observe is something a bit less than that so there's something acting against that deceleration — a slight acceleration."

Implications for Earth's geophysics

Dr Morrison said there are a few possible geophysical mechanisms that might explain the discrepancy, but the best candidate is the change in the Earth's shape due to the formation of glaciers during the last ice age.

"Like a ballet dancer that takes her arms in, she speeds up, so if you take the weight off the poles of the Earth then you get this slight speeding up," Dr Morrison said.

The effect could also be the result of friction between the Earth's iron core and the rocky mantle that surrounds it.

Astronomer Fred Watson from the Australian Astronomical Observatory said the discovery has greater implications for geophysics than for time-keeping.

"For the last 50-60 years we've known how the Earth's rotation is behaving because we have atomic clocks that have very accurate time keeping, but this is more about physics of the Earth itself," Professor Watson said.

"Working out what's going on inside the Earth is astonishingly difficult."

He said the study was an example of elegant science that used every source of information — both modern and ancient — that it could lay its hands on.

A similar approach has been taken by geophysicists using the magnetic signatures preserved in ancient clay pots to study how the earth's magnetic field has changed.