Sun Sentinel Palm Beach Edition
Earth slowing slightly every century
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 La Canada Flintridge 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 as well.
Changes in the amount of polar ice affect the shape of Earth and can affect 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, he 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.”
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 from AD 800 to 1000, as well as from mentions of solar and lunar eclipses in medieval European chronicles, mostly written in Latin.
“The Arab timing of solar eclipses in particular is superb,” Stephenson said. (He could not say the same for the European data.)
Stephenson said he is grateful to all these ancient sky watchers.
“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,” he said. “We are very much at the mercy of these ancient chroniclers and astronomers.”