A day seems like the simplest unit imaginable: Earth turns once, then the next day begins. That picture is good enough for a calendar. It is not good enough for a precise clock. According to the U.S. Naval Observatory, a full rotation of Earth sometimes takes a few milliseconds more or less than 86,400 seconds.
How do we measure an irregular rotation?
The time scale called UT1 describes Earth's observed angle of rotation. Very Long Baseline Interferometry does this by pointing radio telescopes at distant quasars at the same time. These observations determine daily how Earth is oriented against an almost fixed celestial reference.
The variations are not an error in a single instrument. Tides and exchanges of angular momentum with the atmosphere alter the rotation rate. Over longer periods, tidal friction also slows the planet. Earth is a moving system of solid matter, water and air, not a rigid balance wheel.
The SI second is no longer derived from the mean solar day. The International Bureau of Weights and Measures defines it through exactly 9,192,631,770 periods of a particular radiation of the caesium-133 atom. Atomic clocks use this definition to realise a uniform time scale whose rate does not change with each wind or tidal effect.
Our civil time, UTC, connects the two worlds. It runs at the same rate as International Atomic Time, TAI, but differs from it by a whole number of seconds. Leap seconds are intended to prevent UTC and UT1 from separating by 0.9 seconds or more. Noon therefore remains approximately linked to Earth's position relative to the Sun, while each second remains atomically uniform.
Will there be a leap second in 2026?
No. The current IERS Bulletin C 72, dated July 6, 2026, states that no leap second will be introduced at the end of December 2026. The offset between TAI and UTC remains 37 seconds until further notice. This does not mean that Earth has begun to rotate uniformly. It means the observed difference does not currently require another step in civil time.
Leap seconds are difficult for digital infrastructure because a minute can suddenly contain 61 seconds. In 2022, the General Conference on Weights and Measures therefore decided that the permitted difference between UT1 and UTC will be increased by or before 2035. The exact implementation continues to be prepared internationally.
Earth will remain our astronomical reference, but it is not our most precise timekeeper. Modern time works precisely because we observe the planet's rotation without entrusting every individual second to it.
Sources
U.S. Naval Observatory, UT1-UTC (VLBI) and The IAU Resolutions on Astronomical Reference Systems, Time Scales, and Earth Rotation Models
Bureau International des Poids et Mesures, SI base unit: second
International Earth Rotation and Reference Systems Service, Bulletin C 72 (July 6, 2026)
General Conference on Weights and Measures, Resolution 4: On the use and future development of UTC (2022)