Earth's erratic spin has forced computers to swallow 27 leap seconds
Atomic clocks tick with relentless precision, but Earth's rotation slows down unevenly due to tidal friction from the Moon. To keep solar time and atomic clocks aligned, metrologists have inserted 27 leap seconds into official world time since 1972. Because computer operating systems expect time to move forward predictably, these sudden 61-second minutes have repeatedly crashed major web servers and disrupted financial networks, prompting international regulators to vote to phase them out.
Two Clocks, One Planet
Human timekeeping was long governed entirely by the motion of the heavens. A day was simply the duration required for Earth to rotate once on its axis relative to the Sun, and an hour was one twenty-fourth of that period. As mechanical and electronic timepieces advanced, however, scientists discovered that the planet itself is a remarkably inconsistent clock. Earth accelerates and decelerates across days, years, and millennia, buffeted by physical processes within its core, the friction of ocean tides against the seabed, and movements across its atmosphere and ice sheets.
The development of the atomic clock in the mid-twentieth century fundamentally shifted the baseline of measurement. In 1967, the International System of Units (SI) formally redefined the second not by dividing the solar day, but by measuring the fundamental properties of matter: exactly 9,192,631,770 oscillations of the microwave radiation corresponding to the state transition of a cesium-133 atom. For the first time in history, civilization possessed a time standard that was uniform, repeatable, and entirely uncoupled from the variable rotation of the Earth.
This shift produced an inevitable dilemma. International Atomic Time (TAI) accumulated seconds with near-perfect regularity, while astronomical time—designated as UT1, derived from the physical rotation of the Earth—drifted. Because tidal dissipation from the Moon's gravitational pull gradually saps Earth's rotational momentum, the length of a solar day typically exceeds the 86,400 SI seconds measured by atomic devices. Without intervention, atomic noon would gradually drift away from the moment the Sun stood directly overhead.