Atomic clocks measure time based on the oscillations of Cesium atoms, and precise astronomical measurements recently revealed that a day on Earth is becoming longer. This change not only impacts how humans measure time but also directly affects critical technologies that support daily life, such as GPS.
Interestingly, the Earth’s rotational speed has been gradually increasing in recent decades. One would think that if the Earth spins faster, a day should be shorter, not longer, right? This paradox has left scientists baffled. In fact, on June 29, 2022, astronomers recorded the shortest day on Earth in the last fifty years, which was precisely 1.59 milliseconds shorter than the standard 24-hour day. News outlets worldwide have recently highlighted that the Earth is experiencing shorter days.
However, since that date, all recorded data indicate that the Earth’s rotation speed is gradually slowing down.
A day on Earth is never exactly 24 hours or 86,400 seconds. There are many reasons why a day can be longer or shorter than 86,400 seconds. Geological changes over millions of years are one example. Additionally, events such as earthquakes or superstorms can also affect the length of a day.
To explain further, tidal forces caused by the Moon create friction effects that have gradually slowed the Earth’s rotation over millions of years. Each century, a day becomes about 2.3 milliseconds longer. For comparison, billions of years ago, a day on Earth was only about 19 hours long, equivalent to roughly 68,400 seconds.
However, in the last 20,000 years, another process has occurred that has increased the Earth’s rotation speed. As the last Ice Age ended, the melting ice sheets at the poles reduced pressure, causing the mantle to shift towards the poles. Similar to a ballet dancer pulling their arms in to spin faster, the Earth’s rotation has been affected as the mantle shifts toward the poles, speeding up the rotation by approximately 0.6 milliseconds each century.
Gradually, the connection between the Earth’s core and surface has also created other effects. Significant earthquakes can shorten a day, but the impact is minimal. For instance, the catastrophic earthquake with a magnitude of 8.9 that struck Japan in March 2011 caused the Earth to spin 1.8 microseconds faster. Similarly, changes in climate or even ocean currents and tides can move massive amounts of water around the planet, affecting the length of a day in different ways—some lengthsening it and others shortening it.
Returning to the issue of days growing longer, the cause remains unknown.
Since the 1960s, when experts operating radio telescopes worldwide discovered how to simultaneously monitor quasars in space, humans have been able to estimate the Earth’s rotational speed accurately. Comparing this estimation with atomic clock operations has indicated that, in recent years, days have been gradually getting shorter.
Interestingly, when we eliminate short-term variables caused by seasons or tides, since 2020, a day on Earth has been lengthening. This phenomenon has not been recorded by scientists in the past fifty years.
Any change must have a specific reason, but scientists have yet to pinpoint the exact cause. It could be the result of consecutive extreme La Niña weather patterns or possibly the accelerating melting of ice sheets at the poles. Scientists are also considering another possibility related to a phenomenon called “Chandler Wobble,” which describes the “wobbling” of the Earth’s rotational axis occurring over 433 days. In recent years, through radio telescope monitoring, the Chandler Wobble has been gradually diminishing, which could be a reason for the slowing rotation speed of the Earth, thereby lengthening the day.
What does this mean for us? Time discrepancies could affect critical applications like GPS. For this reason, every few years, timekeeping organizations add a “leap second” to time measurement systems to keep them in sync with real time on Earth.
If the Earth continues to rotate more slowly and days lengthen, scientists and atomic clock systems may eventually need to add “negative leap seconds” to their timekeeping systems. The consequences of this can be significant. For example, in 2012, adding a leap second to the international timing system caused many services to crash, including Reddit, Gawker, and the Australian airline Qantas. Similarly, in 2017, the content delivery network (CDN) Cloudflare experienced a similar issue with its DNS system after adding a leap second to its global timing system. If negative leap seconds are introduced, the timekeeping code of online services will also need to be adjusted, just as with a standard leap second.
These examples illustrate that the Earth’s rotational speed, whether faster or slower, can significantly impact our daily lives more than we might realize.
