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Earth's Days are Getting Longer

September 19, 2022

A multiple exposure photo of the 2017 solar eclipse over Redmond, Oregon

How long is a day on Earth? It may seem like a simple question, but not to those that study the planet’s rotation. It takes about twenty-four hours for the Earth to spin completely on its axis, but that number is gradually climbing as the planet’s rotation slows. The forces which set Earth in its orbit around the sun were powerful, but that doesn't mean they stay the same forever. That means, this simple question changes depending on when you ask it.

Recently, scientists have found an unlikely source for nailing down the length of rotation of the distant past: records of solar eclipses from the Byzantine Empire. Scientists from three major Japanese universities have identified five previously unexamined solar eclipses and compared their timing and the path of totality with previous estimates of past solar eclipses.

These records come from a variety of times across the 4th to 7th centuries, 346, 418, 484, 601, and 693 CE. The results from the study are mostly in line with previous estimates, with some outliers: one account from Constantinople suggests the witness was in the path of totality, counter to previous models of the eclipse.

Revising such records allows scientists to resolve discrepancies between Universal Time (the basic 24-hour model of the day) and Terrestrial Time (time as counted by ultra-accurate atomic clocks). As it turns out, we're gaining a microsecond and a half every century!

This is vital for nailing down an accurate timeline of the Earth’s past, as well as providing a model for how Earth’s rotation may function in the future. Even a small change really can make a difference, especially over the billions of years of Earth's history.