Science and facts

For the first time in history, Earth may require a negative leap second. Scientists report that our planet’s recent acceleration in rotation speed could soon compel global timekeepers to subtract a second from official time, an unprecedented step never taken before. Since atomic clocks were developed in the 1950s, researchers have measured Earth’s rotation with extreme precision. They found that a day is not always exactly 24 hours. Its length fluctuates by tiny amounts, usually milliseconds, as the planet’s spin speeds up or slows down. These variations are imperceptible to humans but critical for modern technology. GPS satellites, telecommunications, financial markets, and computer networks all depend on ultra-precise timing. For decades, Earth’s long-term trend was a gradual slowing, largely due to tidal friction caused by the Moon’s gravity. To keep atomic time (UTC) aligned with solar time, officials have added 27 positive leap seconds since 1972. Recently, however, Earth has been spinning noticeably faster. July 5, 2024, marked the shortest day ever recorded, completing its rotation 1.66 milliseconds early. Several other days in recent years have come close to this record. Much of this acceleration is attributed to dynamic processes in Earth’s liquid outer core, where shifts in mass distribution can alter rotation speed, analogous to a figure skater spinning faster by drawing in their arms. If the current trend persists, the world could face its first negative leap second, in which one second is removed rather than added to official time. We are not on the verge of losing a second immediately. Many experts anticipate that leap seconds will be phased out entirely by international agreement before a negative one becomes necessary. [Agnew, D. C. A global timekeeping problem postponed by global warming. Nature 628, 333–336 (2024). DOI: 10.1038/s41586-024-07170-0] Science and facts💡