A New Study Says One Earth Day Is Going to Be 25 Hours Long

A New Study Says One Earth Day Is Going to Be 25 Hours Long

global time xl
  • The Earth’s rotation isn’t exactly 24 hours long. The planet instead experiences slight daily differences due to its heterogenous composition.
  • Scientists at the Technical University of Munich (TUM) improved a ring laser designed to measure the Earth’s rotation with hyper-accuracy.
  • With the addition of a corrective algorithm, TUM can now measure Earth’s rotation down to nine decimal points—about a fraction of a millisecond.

How long is a day? It’s a deceptively complicated question. Because the Earth isn’t a solid mass, but a rolling ball of various solids and liquids, the planet’s rotation actually accelerates and decelerates depending on these shifting dynamics. If you’ve ever Googled “leap second” you’ll know the strange temporal hoops humans need to go through to account for these minute changes.

In order to understand these minute changes in the Earth’s rotation, scientists at the Technical University of Munich (TUM) have fine-tuned a machine called a “ring laser” to such a degree that they’ve been able to accurately measure the length of a day like never before—and the implications far exceed adding and subtracting seconds here and there.


“Fluctuations in rotation are not only important for astronomy, we also urgently need them to create accurate climate models and to better understand weather phenomena like El Niño,” Ulrich Schreiber, project lead at the Observatory for TUM, said in a press statement. “And the more precise the data, the more accurate the predictions.”

Located at the Geodetic Observatory Wettzell, this device uses a laser ring gyroscope and a 13.1-foot-wide “racetrack,” all snug inside a pressurized chamber that’s embedded in the ground some 20 feet deep. This careful calibration means that the device’s lasers are only influenced by the slight perturbations of the Earth’s rotation.

The device uses a complicated system of lasers and mirrors to accurately capture the Earth’s rotation—greater differences between two laser frequencies means the Earth is spinning faster. For example, at the equator, the Earth travels 15 degrees every hour. According to TUM’s ring laser, this latitude generates 348.5 Hz. And every day, this number fluctuates by only 1 to 3 millionths of a Hertz (also called a microhertz).

But even with this super-advanced tech, exact measurements are inherently difficult. As TUM explains:


Exact measurement is only possible when the wave forms of the two counter-propagating laser beams are nearly identical. However, the device’s design means a certain amount of asymmetry is always present. Over the last four years geodesists have used a theoretical model for laser oscillations to successfully capture these systematic effects to the extent that they can be precisely calculated over a long period of time and thus can be eliminated from the measurements.

With this corrective algorithm in place, TUM scientists can measure an Earth day all the way to nine decimal places, which works out to about a fraction of a millisecond per day. It turns out that the Earth’s rotation fluctuates by about 6 milliseconds every two weeks.

Over the course of the Earth’s life, the length of the day has only grown longer and longer. When the dinosaurs roamed the Earth, for example, a day was only 23 hours. 1.4 billion years ago, it was only 18 hours and 41 minutes. And in 200 million years, it’ll be 25 hours long. Of course, who knows who (or what) will be around to make those hyper-accurate rotational measurements.

Headshot of Darren Orf

Darren lives in Portland, has a cat, and writes/edits about sci-fi and how our world works. You can find his previous stuff at Gizmodo and Paste if you look hard enough.


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