The Atlanta Journal-Constitution
Earth’s spin rate gradually slowing
Rate slowing at rate of 1.8 milliseconds per century.
The latest findings in Earth science are brought to you by ancient astronomers who observed the heavens as much as 2,700 years ago.
Thanks to hundreds of records of lunar and solar eclipses carved in clay tablets and written into dynastic histories, modern scientists have determined that the amount of time it takes for Earth to complete a single rotation on its axis has slowed by 1.8 milliseconds per century, according to a report published in Proceedings of the Royal Society A.
It may not sound significant, but over the course of 2½ millenniums, that time discrepancy adds up to about 7 hours.
In other words, if humanity had been measuring time with an atomic clock that started running back in 700 BC, today that clock would read 7 p.m. when the sun is directly overhead rather than noon.
“There is time and then there is how fast the Earth spins,” said Duncan Agnew, a geophysicist at Scripps Institution of Oceanography in San Diego, who was not involved with the work. “Traditionally those things are closely linked, but they are not the same.”
Our earliest ancestors measured time based on the position of celestial bodies in the sky, such as the rising and setting of the sun or the changing shape of the moon. Scientists refer to this as Universal Time, and it is governed by the dynamic gravitational motions of the Earth, moon and sun.
Terrestrial Time, on the other hand, is measured by clocks and is independent of the laws of physics. Since the 1960s, it has been tracked by exquisitely precise atomic clocks. According to our modern take on Terrestrial Time, there are exactly 86,400 seconds in a day and each second is defined as exactly 9,192,631,770 oscillations of a cesium-133 atom.
But our planet does not keep perfect time, so Universal Time and Terrestrial Time do not always line up.
In our modern world, governed by atomic clocks, the International Earth Rotation and Reference Systems Service calls for a leap second to be added whenever Universal Time is on track to be out of sync with Terrestrial Time by more than 0.9 of a second. Leap seconds traditionally are added on June 30 or Dec. 31. In fact, one will be added this New Year’s Eve.
The Earth’s rotational rate, which determines Universal Time, is affected by many factors.
Large weather systems and atmospheric winds can exert enough force on Earth’s surface to cause it to slow down or speed up by thousandths of a second over a single season. Large volcanoes and earthquakes may also cause Earth to speed up or slow down, but these changes are hard to detect. In 2011, a researcher at the Jet Propulsion Laboratory in California calculated that the magnitude 9 quake that struck off Japan may have shaved about 1.8-millionths of a second off the calendar year.
The dominant force affecting Earth’s spin rate over longer time periods is the interplay of gravity between the oceans and the moon. Scientist have known for decades that this phenomenon is causing Earth to spin more and more slowly. However, there are other, more subtle factors at play on this timescale as well.
Changes in the amount of polar ice affect the shape of Earth and can impact how fast or slow the planet turns on its axis. In addition, movements of molten rock in the planet’s core can affect the speed of its rotation, Agnew explained.
To determine how much all these forces have changed the planet’s spin rate over the centuries, a small group of British scientists took on the Herculean task of gathering hundreds of astronomical records made by ancient scribes from around the world.
The team’s goal was to pinpoint when and where lunar and solar eclipses occurred in antiquity, then compare them to computer models of when and where they should have occurred based on the current rate of Earth’s spin. By measuring the difference between these two sets of data, researchers can determine how much Earth’s rotation rate has changed over time.
It wasn’t easy to assemble. Richard Stephenson, an astronomer who recently retired from Durham University in England, started working on this project 40 years ago when he was a young researcher at the University of Essex.
Because nearby Durham University had a good Chinese studies department, Stephenson began with ancient texts from China. To aid him in his search, he memorized 1,500 Chinese characters — too few to read a newspaper, but enough to help him decipher astronomical records written into dynastic histories. Over the years he was able to find about 50 reliable dates and times of solar and lunar eclipses recorded between AD 434 and AD 1280.
“During that period they were using water clocks to tell time,” he said. These devices work by dripping water into a vessel at a constant rate.
“While those are not terribly accurate, if you get 50 observations over a period of time you can get a good average,” Stephenson said.
A treasure trove of data came from translations of clay tablets written in cuneiform by Babylonian astrologers. Although many of the tablets were damaged in the excavation process, Stephenson found more than 150 useful entries dating from 720 BC to 10 BC.
Other observations came from Islamic astronomers working in the Arab world between AD 800 and 1000, as well as from mentions of solar and lunar eclipses in medieval European chronicles, mostly written in Latin.