AS THE well-known Australian philosopher, Kylie Minogue, once pointed out, it can be a source of comfort to remember that, no matter what else is happening, the world still turns. Unfortunately, things are not quite so simple. Thanks to the moon’s gravitational tug, the speed at which Earth spins has been slowing since the satellite’s birth about 4.5bn years ago. Physicists can calculate from first principles how big the effect should be. It turns out that the moon should be adding about 2.3 milliseconds to the length of the day with each century that passes. This means, for instance, that 100m years ago, when dinosaurs ruled Earth, a day was nearer 23 than 24 hours.
But that 2.3 milliseconds is only an average. Geological events within Earth can speed the process up, or slow it down. Tracking changes in day length over time is thus of interest. And that requires data. Thanks to the development of super-accurate atomic clocks in the 1950s, and to laser range-finding equipment left on the moon by the Apollo astronauts, researchers have plenty of such data from the past half-century. But more information is always welcome. And extra data are exactly what a team led by Leslie Morrison, a retired professional astronomer, have just provided. In a study just published in the Proceedings of the Royal Society they use observations made by ancient Chinese, Babylonian, Greek and Arab astronomers to reconstruct the history of Earth’s rotation over the past two and a half millennia.
Plenty of ancient cultures were keen on astronomy, for the patterns of the heavens were the basis of their calendars and timekeeping systems (not to mention their systems of astrology). But Dr Morrison and his colleagues were interested in two particular phenomena: eclipses of the sun and of the moon. A solar eclipse happens when the moon moves in front of the sun, as seen from a particular spot on Earth, and blocks out its light. A lunar eclipse happens when the moon moves behind Earth in such a way that Earth blocks the sunlight that, when reflected from the lunar surface, renders the moon visible.
Eclipses were often viewed as portents (usually bad ones), or as battles between moon or sun gods and the forces that sought to overthrow them. For that reason, records of hundreds of historical eclipses survive to the present. Building on work begun in the 1990s, Dr Morrison and his colleagues collated 424 such observations of 250 eclipses that happened between 720BC and 1600AD. The records were scattered across Babylonian clay tablets preserved at the British Museum, translations of histories of Chinese dynasties and Japanese emperors, records from ancient Greece preserved in works like the “Almagest”, and observations made by Arab astronomers during what was, in Europe, the Dark Ages. (The researchers note that no record survives of an eclipse seen in Europe for almost a thousand years following one recorded in 364AD.)
The idea was to combine those observations with modern computer models of the solar system. These are sophisticated enough to let researchers reconstruct the positions of Earth, moon and sun at any given date in the past, and therefore to work out when and from where any past eclipses should have been visible. That means such models can be used to confirm recorded sightings. But because ancient astronomers reckoned the passage of time by the motion of the heavens, and modern models are based on the unvarying output of atomic clocks, the gradual slowing of Earth’s rotation will produce a disagreement in timing between the two.
Many a mickle makes a muckle
Given the tiny differences in day length involved (a few dozen milliseconds at most) and the primitive equipment (such as water clocks) available to ancient researchers, it might be thought that any difference in timing would be too small to detect. But Dr Morrison points out that this is not actually a problem. The error introduced by Earth’s deceleration is cumulative. Today may be only fractionally longer than a day was 2,700 years ago, when the earliest records that the team looked at were collected, but between then and now almost a million days have elapsed. Each passing day adds any daily discrepancy to the total discrepancy, and those repeated daily discrepancies add up to significant fractions of an hour—periods well within the accuracy available to ancient astronomers.
After crunching the numbers, the team found that the actual rate at which days have been lengthening over the past couple of millennia is 1.8 milliseconds per century, considerably slower than the 2.3 milliseconds predicted. The main reason for the difference, they reckon, is the lingering effect of the most recent ice age, during which the mass of ice at the planet’s poles was sufficient to deform its shape and thus alter its rate of spin. That is not, though, the only thing which is happening. The researchers also found small but cyclical patterns in the rate of change that repeat themselves over decades—as well as intriguing hints of longer cycles with time periods of thousands of years. Exactly what geophysical goings-on such cycles represent is one for the geologists to work out.
Correction (December 9th): A previous version of this piece was entitled “Ancient eclipses show how days are getting shorter”, an error repeated at one point in the piece itself. As the piece explains, they are, of course, getting longer. Apologies.