WASHINGTON — Wednesday will be slightly shorter than usual thanks to our planet’s 

Earth is projected to finish a complete rotation on July 9 approximately 1.30 milliseconds shorter than the usual 86,400 seconds that make up a 24-hour day, based on predictions from astronomical observations.

Though this minute time variation is imperceptible to humans, it forms part of a larger trend that has puzzled scientists: after millennia of slowing down, Earth has been rotating faster in recent years, frequently setting records for its shortest days.

Prior to 2020, the shortest day ever measured by atomic clocks was 1.05 milliseconds under the standard duration. Post-2020, Earth has surpassed that mark every year by about half a millisecond. The shortest recorded day was on July 5, 2024, when Earth finished its rotation 1.66 milliseconds quicker than usual.

The phenomenon can only be measured using atomic clocks, which are precise enough to detect variations of thousandths of a second. A millisecond equals 0.001 seconds.

Scientists measure these tiny daily variations as “length of day,” or LOD, representing how many milliseconds above or below 86,400 seconds Earth takes to complete one rotation relative to the sun.

The moon’s position influences these short-term variations. Earth spins faster when the moon is positioned far north or south of the planet’s equator. Two other upcoming dates this year, July 22 and August 5, are also expected to produce similarly brief days, with August 5 expected to be the shortest, losing 1.51 milliseconds. 

Long-term changes in the planet’s spin speed largely stem from Earth’s core acting in unpredictable ways, researchers say. 

The acceleration trend may be reaching its peak. Leonid Zotov, an expert in Earth rotation at Moscow State University, predicted last year that Earth would begin decelerating, though that hasn’t happened yet.

“I think we have reached the minimum,” he told TimeandDate.com. “Sooner or later, Earth will decelerate.”