Ancient astronomers help prove that Earth’s spin is slowing

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 day over the course of a century, according to a report published Wednesday 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 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 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.