Leap years are well-known: we add an extra day to February every four years … mostly … to keep our calendars in sync with Earth’s orbit around the Sun. But have you heard of leap seconds? Yes, seconds. Unlike the widely recognized concept of leap years, leap seconds represent a more nuanced aspect of timekeeping. However, they are crucial for upholding the precision required by today’s global timekeeping standards. This attention to detail in measuring time, down to the second, sets the stage for understanding the importance of Coordinated Universal Time in our daily lives.
Coordinated Universal Time (UTC) is the cornerstone of global timekeeping, serving as the standard by which the world regulates clocks and time. It combines the accuracy of atomic clocks, which measure time based on the consistent vibration of cesium atoms, with the traditional positioning of the Earth relative to celestial bodies, known as solar time. This dual approach ensures that UTC reflects both the precise, scientific measurement of time and the practical, everyday experience of day and night. UTC’s detailed 24-hour time format includes days, hours, minutes, and seconds. UTC’s accuracy reaches us through internet services, GPS signals, and radio broadcasts, keeping our world perfectly synchronized. For instance, the time you see on your phone is UTC adjusted for your time zone, ensuring accurate local time for everyone, everywhere.
So, how do leap seconds factor into our timekeeping? The Earth’s rotation fluctuates due to gravitational forces from the Moon and the Sun, as well as mass redistribution following seismic events. These changes can slightly speed up or, more typically, slow down the Earth’s rotation, leading to UTC running ahead of solar time. To address a slowing of the rotation, leap seconds are added, essentially allowing our clocks to pause briefly and let the Earth catch up. Without these adjustments, our timekeeping would gradually drift out of sync with the natural day-night cycle.
The International Earth Rotation and Reference Systems Service (IERS) is tasked with deciding when to make leap second adjustments. It uses precise astronomical observations to ensure UTC accurately reflects Earth’s rotation. Unlike the predictable addition of leap days every four years, leap seconds are added only when needed. Since adopting the first leap second in 1972, the IERS has added 27 leap seconds. The most recent one was added on December 31, 2016, at 23:59:60 UTC. And yes, during this adjustment, the clock counted up to 60 seconds before rolling over to 0 instead of the usual rollover at 59 seconds. It should be noted that the system also permits the subtraction of seconds if the Earth’s rotation unexpectedly accelerates, although this has never happened.
While leap seconds might not be as well-known as leap days, they are equally vital to the accuracy of our timekeeping system. These extra seconds ensure our clocks stay aligned with Earth’s slightly irregular rotation. So, the next time you check the time on your phone, consider the intricate science and adjustments that keep every moment precise. It’s a reminder of how closely our lives are intertwined with the vast and dynamic universe, even in something as routine as checking the time.