Humans haven't always counted time down to the second. mrs/Moment via Getty ImagesYou’re probably used to the month of February having an extra day every four years, during the leap year. Timekeepers added that extra leap day because Earth completes one orbit around the Sun in about 365 and one-quarter days. If we didn’t add leap days, the equinoxes and solstices that mark the beginning of seasons would occur later, slowly shifting the days on which each season began.For millennia, a day has been defined as one full rotation of the Earth. But the Earth’s rotation isn’t completely uniform, and in order to be more precise, modern timekeeping systems have stopped using the Earth’s rotation as their basis. This switch created a discrepancy between how a single second used to be defined, and how it is defined today.Over the course of several years, modern time systems can become a few seconds out of sync with the Earth’s movement. To solve this issue, timekeepers use what’s called a leap second. Just as leap years keep our calendars in alignment with Earth’s orbit around the Sun, leap seconds have kept our timekeeping aligned with Earth’s daily rotation. But in October 2026, an international organization governing timekeeping may change its strategy for adding leap seconds.As a space scientist, I know how measuring time is fundamental to physics research. The precision of physicists’ measurements has gotten better as the way we measure time has changed.History of secondsThe systems humans use to measure time have evolved over the past few centuries.The origin of the 24-hour day can be traced to the second millennium B.C.E., when ancient Egyptians divided day and night into 12 parts each. Because the length of day and night varies during the year, the length of each hour of daylight and each hour of night did as well. The pendulum clock design patented by Christiaan Huygens in the 17th century. Christiaan Huygens, CC BY The Greek astronomer Hipparchus, who lived in the second century B.C.E., later adopted a system using 24 equal hours for astronomical calculations. The division of an hour into 60 minutes of 60 seconds each also has its roots in astronomy. Division by 60, or a base-60 system, traces back to the Sumerian and Babylonian civilizations of Mesopotamia. The number 60 is particularly convenient because it can be divided up evenly into smaller parts. The Greek astronomer Claudius Ptolemy adopted the base-60 system while measuring the motions of planets around 150 C.E. He divided each angular degree into 60 parts, and each of these into 60 smaller parts. In Latin, these were later called partes minutae primae, or “first small parts,” and partes minutae secundae: “second small parts.” This system eventually led to the words “minute” and “second.” Centuries later, as mechanical clocks became increasingly precise, these astronomical subdivisions were adopted for timekeeping as well. Based on the systems devised by Ptolemy and his successors, a mean solar day was divided into 24 hours, times 60 minutes per hour, times 60 seconds per minute: 86,400 seconds.However, actually measuring time down to the second only became possible after Christiaan Huygens invented the pendulum clock in 1657. In a pendulum clock, a suspended weight swings back and forth at a steady rate. As long as you take the differences in the Earth’s gravitational pull into account, all pendulums of the same length complete a swing in the same length of time. This metric provided a standardized way to measure time. The leap day and the leap secondIn the spirit of the pendulum clock, scientists spent three more centuries developing systems that would measure time precisely and in a standardized way so that all clocks around the world could tell the same time. During this process, the definition of a second changed several times. They finally found a solution in the cesium atom.In 1967, the General Conference on Weights and Measures – an intergovernmental organization that sets measurement standards and has over 100 member nations today – adopted a new definition based on the cesium-133 atom. Cesium-133 atoms behave like extraordinarily precise clocks. When the atom switches between two particular energy states, it produces radiation that oscillates, just like a pendulum, at a remarkably steady rate. Scientists defined 1 second as exactly 9,192,631,770 of these oscillations. Atomic clocks exploit a property of atoms to define a second that’s standard everywhere. Coordinated Universal Time, UTC, is based on atomic clocks that use cesium-133. As a result, UT1, the time scale based on Earth’s rotation, can drift relative to UTC. This divergence reflects both short-term irregularities in how fast Earth rotates, and a very gradual, long-term slowdown of Earth’s rotation, caused primarily by the Moon’s gravitational pull on Earth’s oceans. The difference between the 86,400 atomic seconds metric and a day measured by Earth’s rotation is less than 4 milliseconds. This discrepancy may seem small, but it can accumulate to a significant fraction of a second over the course of a year. Since 1972, timekeepers have added 27 positive leap seconds to UTC, at very irregular intervals. Their aim has been to keep the discrepancy between UTC and UT1 to less than 0.9 seconds. They added the last leap second at the end of 2016. The end of the leap secondAdding a second to clocks every so often can disrupt high-precision computer systems. Many computer operating systems assume that a day is 86,400 seconds long and that time advances in a predictable, monotonically increasing sequence. Adding a leap second violates both of those assumptions. On days when a leap second is added, clocks jump by one second on the designated day. That day is then 86,401 seconds long. In 2012, for example, Reddit experienced an outage after a leap second was added on June 30 of that year. Other systems, including the Linux kernel and some Java-based applications, were also affected. Because of incidents like these, many high-profile technology companies, including Google and Meta, use “smearing,” where they spread the extra second in gradual increments over a long time period, rather than inserting it as a one-second step. This endeavor is not easy, and these companies are strongly advocating for abolishing the leap second.In 2022, the General Conference on Weights and Measures decided the maximum difference between UTC and UT1 should be increased in, or before, 2035. They planned to agree upon a new maximum value at their 2026 meeting.One factor accelerating their decision is that the Earth has experienced a period of unusually rapid rotation since about 2015. During the 2020 calendar year, timekeepers recorded 28 days where the Earth completed its full rotation before the clock hit 86,400 atomic seconds. The shortest day measured in the modern atomic-clock era occurred on June 29, 2022, when the Earth completed its rotation 1.59 milliseconds faster than 86,400 atomic seconds. If Earth keeps up this faster rotation, timekeepers might eventually need to introduce the first-ever negative leap second. There’s no large-scale precedent for how computer systems would handle one, which is why the issue has taken on new urgency.The General Conference on Weights and Measures will meet Oct. 13-15, 2026. It is considering a draft resolution that would make UTC a continuous time scale, without the abrupt addition of leap seconds, beginning on May 20, 2027. The resolution would allow the difference between UTC and UT1 to grow as large as 3,600 seconds, or one hour. This is akin to replacing the leap second with a “leap hour.” The resolution does not specify a schedule for inserting discrete one-hour corrections. A leap hour would not be necessary for centuries. Will I notice the change?The clocks on your smartphones, laptops and other electronic devices seldom, if ever, display seconds. Because of this, the addition of leap seconds has typically gone unnoticed by all but the metrologists responsible for various timekeeping systems. As UTC and UT1 are allowed to diverge, astronomers, Earth scientists and other users of Earth-orientation data will need to account for the growing offset when relating their observations and measurements to civil time. Navigating using the positions of the Sun, the Moon, and the stars requires knowing the precise time when observations are made. Celestial navigators who do this will need to account for the growing difference between UTC and UT1 when determining their precise position on Earth. When you use a compass for navigation, you must take into account the difference between the location of Earth’s magnetic and geographic poles. You can do so by factoring in the magnetic declination of your location, or the angle between the direction the compass is pointing in and true north. Software on smartphones can do this automatically. A similar approach already exists for converting between UTC, UT1 and other time scales in use today. If the draft resolution is adopted in October, keeping track of the differences between these time scales, especially the drift between UTC and UT1, would become a more important part of high-precision timekeeping.By mid-October, the leap second may be a thing of the past. Given the importance that people attach to the regular change of seasons, though, the leap year is not going anywhere.Vahe Peroomian has previously received funding from the National Science Foundation (NSF) and from the National Aeronautics and Space Administration (NASA) for work on space weather and geomagnetic storms.