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.