“All the datacentres that run the internet, communications and financial transactions, they are based on precise timing. We also need a precise knowledge of time for navigation, and particularly for satellites and spacecraft,” says ETH Zurich Prof. Benedikt Soja. It betrays a fact usually obscured in plain sight: the Earth day’s precise length is anything but fixed and is far from being negligible. Now that researchers inform us July 22 will fall just a millisecond less than the usual 24 hours, the elegant dance of our planet’s spin is more than an interesting fact it’s a foundation of modern life.
More than a century ago, researchers realized Earth’s spin is not precisely locked in place. Influences as diverse as the gravitational pull of the moon, changes in the winds, the molten core of the planet moving, and now the redistribution of water from global warming, all mix and mingle such that each day is ever so slightly different. Dennis McCarthy, the U.S. Naval Observatory takes it down to: “We’ve known about the rotation of the Earth being variable for about a hundred years. This is just one of those little variations that comes along.” But during the past decade or so, the rate and character of the variations have lain open to fresh examination.
Advanced measuring methods have transformed our knowledge of these fluctuations. By aiming radio telescopes at quasars cosmically distant, bright galactic nuclei that are universe-wide stationary beacons astrophysicists are able to monitor the planet’s orientation to an accuracy never before possible. The technique, called Very Long Baseline Interferometry (VLBI), is supplemented by readings from the Global Positioning System (GPS) and other space-based techniques. These instruments supply the International Earth Rotation and Reference Systems Service with observations from all over the world, which are then used to compute the duration of each day. The measurement of the disparity between atomic time and the earth’s movement can thus be made to the extent of microseconds, opening the way to everything from GPS to missile launches to precision farming.
The foundation of contemporary timekeeping is the atomic clock, whose definition of the second is based on the vibration of cesium-133 atoms. The International Atomic Time (TAI) is a weighted mean of over 300 atomic clocks across the world, while Coordinated Universal Time (UTC) is taken from TAI but adjusted for leap seconds to match the Earth’s rotation. As technically defined, “Coordinated Universal Time (UTC) is based on TAI, but it is adjusted by leap seconds to account for the difference between the definition of the second and the rotation of Earth” Added sporadically since 1972, these leap seconds are a practical compromise of the conflict between the obstinate periodicity of atomic vibration and the Earth’s less consistent spin.
But over the past few years, there has been a strange anomaly. While the secular process, caused by tidal friction in the Moon, has been the Earth’s deceleration of rotation increasing the day by about 2.4 milliseconds per century the last ten years have witnessed a “bumper crop” of brief days: July 5, 2024, was 1.65 milliseconds shorter than the standard 86,400 seconds since ancient mechanical clocks were replaced by atomic clocks as the norm in the 1950s. This brings in the unprecedented potential for a “negative leap second,” where a second would be taken away from, instead of added to, UTC. MIT geophysicist Thomas Herring comments, “I think it will be unlikely that a negative leap second will be invoked,” but the mere fact that there is such a possibility opens up a new age in timekeeping.
Emerging above these recent aberrations is a dominant new force: climate change. As ice shelves and glaciers melt with increasing speed, mass in the ocean redistributes from the poles towards the equator, quietly changing the world’s shape shaping it more oblate and decelerating its rotation. Thanks to satellite observation, such shifting of masses and their effect on length of day have become possible to be mapped by scientists using data from GRACE and GRACE-FO missions. The rate of day lengthening caused by climate-driven mass redistribution accelerated to 1.33 milliseconds per century since 2000, a rate higher than the last century, as stated in a recent paper. Greenhouse gas emissions, if uncontrolled, could see this rate reach 2.62 milliseconds per century by 2100, which would be greater than the moon’s tidal force effect as the main cause of long-term variations in a day’s length.
These corrections are not theoretical. Accurate measurement and prediction of Earth’s rotation lie behind global communications synchronization, financial transactions integrity, and navigation system dependability. Fluctuations, though slight, can add up, causing positioning errors in GPS or network issues with data. The delicate ballet between atomic clocks, leap seconds, and astronomical observations maintains society’s digital fabric coordinated with the world of physics.
Earth’s rotation science is therefore a testament to the dynamism of the world and of the human condition in keeping it in sight. Paraphrasing Prof. Soja, “We can really become the dominant drivers of Earth’s rotation, due to human-induced climate change”. That was really surprising, and really an interesting revelation to us. The intersection of celestial mechanics, atomic exactitude, and human control continues to recast the definition of a day a millisecond at a time.
