On the morning of April 15, 136 B.C.E., residents of the ancient city of Babylon, in what is now Iraq, experienced quite a sight. An hour and a half after sunrise, the moon moved across the face of the sun, blotting it out so only the golden halo of its atmosphere was visible. The sky went dark.

“Venus, Mercury and the Normal Stars were visible; Jupiter and Mars, which were in their period of disappearance, were visible in that eclipse,” a Babylonian astrologer inscribed in cuneiform, on a clay tablet now stored in the British Museum.

(British Museum)

To measure the eclipse’s duration, the astrologer would have used a cylindrical vessel called a clepsydra, or a water clock, whose slow draining marked the passage of time. “It moved from SW to NE. 35 us [time duration] for obscuration and clearing up. In that eclipse, north wind which...” The rest of the observation is on a missing chunk of tablet and was lost to history, but the astrologer may have been noting that the wind shifted direction, which happens during a total eclipse.

This account, one of the most precise eclipse records from antiquity, is more than a souvenir from the sands of Babylon. In a remarkable example of science being practiced across millennia, today’s astronomers are using it to show that the Earth’s rotation is slowing down and the day is getting shorter—but not as much as we might expect.

By measuring the motion of the Earth around the sun and the moon around the Earth, we can compute where and when eclipses should have been visible throughout time.

“But then when we look at the eclipses, we find they are offset from that. We get this discrepancy,” says Leslie Morrison, an astronomer retired from the Royal Observatory Greenwich in London.

If the rate of Earth’s rotation was the same back then as it is now, the eclipse path on April 15, 136 B.C.E. would have been far to the west of Babylon, slicing up through the Mediterranean and into northern Italy. If the Earth’s rotation was slowing at the rate we would expect based on the tidal influence of the moon, it would have fallen far to the east, over what is now Afghanistan. But we can be sure, based on this cuneiform account, that it was over Babylon, the walled city of Nebuchadnezzar and Hammurabi.

The discrepancy means the Earth’s rotation has slowed since then. In a new research paper, Morrison and colleagues explain that since the first-ever recorded eclipse observation, in 720 B.C.E., the Earth’s rotation has slowed by about six hours. That’s not much change in nearly 2,740 years, but it adds up, Morrison says.

“Nearly a million days have elapsed since then and now, and by the time you get back through a million days of small changes, it comes to several hours,” he says.

Morrison and colleagues analyzed the timing and location of eclipses from Babylon, China, Greece, Arab dominions in the Middle East, and medieval Europe. The record is astonishing, covering nearly three millennia on three continents. It includes translations from ancient lunar calendars to today’s Western calendar, a mind-boggling degree of scholarship encompassing archaeology, astronomy and history.

Curiously, there are no precise records from Mesoamerica or ancient Egypt, despite the importance of the sun and eclipses in those cultures. Morrison says this is because eclipse records from those civilizations are incomplete, either lacking dates or specific locations.

The oldest surviving eclipse observation in human history is from July 17, 709 B.C.E., in the ancient Lu capital of Qufu, China, in what is now Shandong Province. “The sun was eclipsed; it was total,” an astronomer wrote in the Chunqiu, or the Annals of Spring and Autumn.

The Earth's rotation has been slowing far longer than humans have been trying to record it. Our planet formed from smaller, fast-rotating crumbs and dust in the infant solar system, and these “planetisimals” gave it its original spin. After the moon formed from a chunk of the Earth, it (and to a lesser extent, the sun) has been pulling on our planet. The daily rise and fall of the oceans and the crust causes drag that slows down its rotational momentum.

Because we know the rate at which the moon is flying away from us, we can calculate the effect the tides would have on Earth’s day. That would be a change of 2.3 milliseconds per century, Morrison says.

But the team’s careful eclipse history shows it is actually changing by 1.8 milliseconds per century. This means there’s something else going on. Polar ice caps, glaciers, changing ocean depths, and interactions between Earth’s molten core and its mantle must also be involved. There is some evidence for an oscillation every 1,500 years, but this is still unclear.

One major factor is the slow spring-back of crust that was weighed down by gigantic ice sheets during the last ice age. The crust is bouncing back at high latitudes, but shrinking inward at lower latitudes. Like an ice skater bringing his arms to his chest to spin faster, this shift in mass is counteracting the slowdown of Earth’s day, Morrison says.

Glacial melting due to human activity might contribute to this, too, he adds.

Now that the team has figured out the rate of change, they might be able to go through some of those imprecise eclipse records—including Mesoamerican, Egyptian, and medieval European records—and figure out when and where eclipses took place.

“We can now ask historians, ‘Look, this eclipse went bang through Cairo on that day, isn’t there any record? Or maybe in Thebes?’ That must have made an enormous impression on ancient Egyptians,” Morrison says.

Ancient accounts of eclipses are imperfect, but they are by far the longest record we have, he adds. We have only been watching the sky with telescopes since 1609, and we’ve only been measuring time according to the wiggle of atoms since 1962. Neither of those are long enough to show millennia-long changes in the length of the day.

Until the late 18th century, the basic unit of time was the apparent solar day, which measured the interval between two transits of the sun across the same meridian. But this daily trip can vary, because the Earth’s orbit is elliptical and because it’s tilted on its rotation axis. The apparent solar day can vary by as much as 30 seconds every 24 hours.

The advent of pendulum clocks heralded the arrival of mean time, which still kept time according to the solar day but with the variations in Earth’s tilt and orbit averaged out. Greenwich Mean Time was established in 1884. Today, atomic clocks define one second as “9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom.” To keep in tune with this number, humans add a leap second to our year every few months or so. The next leap second will come on New Year’s Eve, at midnight Greenwich Mean Time, which is now called Universal Time.

Almost makes one long for the days of the clepsydra.