Earth's slowing rotation revealed by ancient astronomers

With the help of old eclipse records, researchers have determined a new rate for Earth’s slowing spin. But the astronomers responsible will never know what came of their observations – most of them lived and died thousands of years ago.

The length of an Earth day has grown by 1.8 milliseconds per century, according to a study published Wednesday in the Proceedings of the Royal Society A. To make that determination, researchers studied hundreds of lunar and solar eclipses records, many of which were carved into Babylonian clay tablets or recorded in China’s dynastic histories.

“People recording these things never had the slightest notion that what they were doing would lead to people in our generation actually studying changes in the Earth spin,” lead author Richard Stephenson, a now-retired astronomer from the University of Durham, told The Los Angeles Times. “We are very much at the mercy of these ancient chroniclers and astronomers.”

Researchers have long theorized that tidal friction, a side effect of the moon’s gravitational tug on our oceans, has slowed the Earth’s rotation. But going on those measurements alone, the planet’s spin ought to be slowing by 2.3 milliseconds per century. This new research suggests that the old rate is actually half a millisecond off, though the cause of that discrepancy has not yet been determined.

Solar and lunar eclipses can only occur when the sun, Earth, and moon are aligned. Because these celestial bodies have well-known orbits, astronomers can calculate when eclipses will occur in the future or have occurred in the past. A solar eclipse lasts over an hour, but totality – when the sun is completely blocked by the moon – lasts only a few minutes in any location.

Most of the historical records only mention the hour, if that, but a handful of ancient astronomers were precise enough that Dr. Stephenson and his colleagues could measure the deviation between the calculated time and the observed time and thus determine the planet's slowing rotation.

Some records also mentioned lunar occultations, when the moon appears to move over the face of a star. 

With help from Durham’s Chinese studies department, Dr. Stephenson began studying dynastic texts nearly four decades ago. In that time, he has found dozens of observed eclipses and occultations between BC 720 and AD 1280 that were measured precisely enough to be useful. 

Translations of clay tablets, engraved in cuneiform by Babylonian astronomers between 720 BC and 10 BC, also provided Stephenson and his colleagues with reliable dates and times. Still others came from Islamic texts written between AD 800 and 1000.

Many of these observations relied on crude instruments. Ancient Chinese astronomers, for example, used water clocks, which kept time by dripping water into a container at a constant rate. But though these instruments weren’t terribly precise, the skywatchers who used them were still able to take accurate enough records of cosmic events.

“In 90 percent of the cases in the Chinese and Babylonian records, their dates reproduce exactly to the date of the calculated eclipse,” Stephenson said. “This gives us a lot of confidence in our data set.”

Over the course of its lifetime, the Earth’s spin may have changed multiple times. Another new theory suggests that, after the planetary mass Theia clipped Earth about 4.5 billion years ago, our planet was spinning very quickly as a result of early impacts. But that collision, which produced our moon, slowed the proto-Earth down, as The Christian Science Monitor’s Weston Williams reported:

They calculated that 4.5 billion years of gravitational arm-wrestling between the sun, Earth, and moon has pushed the moon out to its current distance and removed angular momentum from the system.

Those forces also shifted Earth's axis to a more upright position and slowed its rotation to the 24-hour period we know today. The moon, too, has had spin slowed as a result of this gradual change. It now rotates exactly once per revolution, which is why it always shows the same face to the Earth.

Editor's note: A previous version of this story included an incorrect calculation for the cumulative difference in the length of an Earth day.