Are We Losing Time?

The winter solstice occurred Wednesday, December 21, the shortest day of the year in the Northern Hemisphere. It’s not as short as it used to be, however. Physicists have long known that the Earth has been spinning more slowly each year, an effect of the gravitational pull of the moon. It’s not a big change—the length of the day increases only about 2.3 milliseconds each century—but it does add up. During the Cretaceous Period, some 100 million years ago, when the dinosaurs were on their way out and mammals were gaining ground, the day was about 23 hours long.

But new research has shown that the rate of increase isn’t steady; it speeds up and slows down depending on various geological events. A team led by a retired astronomer, Leslie Morrison, pored over thousands of years of historic observations of solar and lunar eclipses from Chinese, Babylonian, Greek, and Arab sources; ancient observers generally assigned great importance to these events and carefully recorded them. The team’s research has just been published in the Proceedings of the Royal Society.

The team compared the recorded observations with current computer models of the solar system, which, as this article notes, “are sophisticated enough to let researchers reconstruct the positions of Earth, moon, and sun at any given date in the past, and therefore to work out when and from where any past eclipses should have been visible. That means such models can be used to confirm recorded sightings. But because ancient astronomers reckoned the passage of time by the motion of the heavens, and modern models are based on the unvarying output of atomic clocks, the gradual slowing of Earth’s rotation will produce a disagreement in timing between the two.”

The upshot is that the lengthening process is slowing down. Over the few thousand years for which observations exist, days have been lengthening by an average of only 1.8 milliseconds per century, versus the 2.3 milliseconds the team expected. They think they’ve discovered cyclical patterns in the rate of change, which account for part of the difference, but there’s a more intriguing reason as well: climate change—and not the kind we usually read about. During the last ice age, they believe, the accumulation of ice at the poles changed the shape of the Earth and affected its rate of spin. It’s just as possible that our future actions can influence not only the temperature of the Earth but also—however infinitesimally—the length of its days.