On 9 July 2025 the land completed its rotation in 1.3–1.6 milliseconds less in the 24 hours: it was the shortest day ever recorded.
On July 9, 2025 he entered history: The earth completed its rotation in about 1.3–1.6 milliseconds less than the canonical 24 hoursmarking the shortest day ever recorded with modern methods.
But it is not an isolated anomaly: similar phenomena are expected to July 22nd (-1.38 ms) and the August 5th (–1.51 ms). These fluctuations, imperceptible for us, can however Complicate the synchronization of advanced technological systems such as GPSnetworks financial And IT systems Critics.
The role of the moon. The main responsible for this phenomenon is the moon: in these three dates, its “declination” reaches the most more distant from the terrestrial equator—Ing slightly crippling with the sea friction and allowing a slightly faster rotation on the planet.
In long times, we are talking about centuries, the earth slows down due to the tides caused by the moon. This phenomenon reduces the rotation of about 2 ms per century. On short time stairs, and here we are talking about days and months, the effect can be reversed: as happened in July 2025.
How do we measure variations? Since 1960, researchers have used atomic geodesy atomic watches and techniques such as the VLBi ((very long baseline interferometry) to precisely monitor the difference between terrestrial time (UT1) and atomic time (TAI/UTC). Geodesia techniques, such as the VLBi, measure with very high precision the position of points on the earth using distant radio and telescopes for them Observe the same radio sources (like Quasar) in space. calculate movements of the earth’s crust, terrestrial rotation and other fundamental geodesic parameters. A waste greater than ± 0.9 seconds requires the introduction (or subtraction) of a second intercalary. So far, each adjustment has been positive: one second more, precisely because the earth slows down (the last time dates back to 31 December 2016).
Today, however, with the earth that revolves faster, the introduction of the first second negative intercalate is questioned: that is, remove a second from the Utc. According to some models, if the current trend persists, to compensate for these variations, the official duration of the terrestrial day will be updated with the addition of a second, the so -called second intercalate the first “second less” may already be necessary in 2029. However, more recent studies suggest that Today’s acceleration could only be temporary and that we will return to a slowdown trend.
Because it is important. It might seem like a purely physical disquisition, but, in reality, it has concrete repercussions. The main falls on Computer networks and telecommunicationswhich require synchronization to the millisecond.
This phenomenon can create problems for GPS and to the satellite navigation which depend on temporal precision. A second negative intercalary would require global protocol updates, code and hardware of many processes.
