Slowing of Earth's Spin Revealed in Ancient Astronomers' Tablets

The work of ancient astronomers reveals that the Earth's spin is slowing down — though not as much as scientists believed.

Each century, the length of the solar day, or the time it takes the planet to do a full rotation, grows by 1.8 milliseconds, according to a new study using astronomical observations going back to 750 B.C. Researchers have known that the planet's rotation is slowing because of friction caused by the tides, as water that's being tugged on by the moon's gravity sloshes against the solid Earth. However, measurements of this tidal effect suggest that the planet should be slowing in its rotation by 2.3 milliseconds per century, slightly more than the new research finds.




The difference between 2.3 milliseconds and 1.8 milliseconds over a century may seem trivial, said study researcher Leslie Morrison, who worked at the Royal Greenwich Observatory for nearly 40 years. But those fractions of milliseconds are important for understanding the ways that the Earth has changed shape since the end of the last ice age, about 12,000 years ago, Morrison told Live Science.

Ancient records

Morrison and his colleagues have been working on measuring the Earth's rotation for decades. The new study is perhaps the most comprehensive effort yet, Morrison said, mostly because the ancient Babylonians were so good at keeping records.

By 720 B.C., this civilization, located in what is now Iraq, was keeping records on clay tablets in a writing system called cuneiform. When archaeologists discovered some of these tablets in Babylonian ruins in the 1800s, the language had been lost; it took decades to decipher those original tablets.

Fortunately for modern-day Earth scientists, some of these tablets happened to contain records of eclipses, particularly solar eclipses, when the moon moves between the sun and the Earth, casting a shadow on this planet. These eclipses tended to make an impression on ancient people, Morrison told Live Science. The events also strengthened tides slightly, because the alignment between the Earth, moon and sun meant a stronger pull on the planet and its oceans.

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