You’re probably familiar with the leap year. As the earth’s revolution around the sun is not exactly 365 days long, an extra day is added to February every four years to synchronise our date/timekeeping with our planet’s journey through space. Perpetual calendars take this into account when tracking the date, and display February 29^th on a leap year, while simple and annual calendars will require manual adjustments.

What about leap seconds?

The leap year’s erratic cousin occurs irregularly, and owes its origins to increasingly accurate timekeeping by mankind. With the advent of atomic clocks and hyper-accurate timekeeping, the International System of Units revised its definition of a second to be “the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom”. Ignoring the science behind its definition and measurement, the second as a length of time is now a physical constant, and the minute, hour, and day are just multiples of it.

Unfortunately, the “actual day”, which we observe as the earth’s rotation on its axis, is nowhere as regular. The moon’s gravity causes the tides on earth, which has been slowing the latter’s rotation minutely, but perceptibly, over time. Other random events like volcanic eruptions and earthquakes also jolt the planet and affect its speed of rotation. The 2004 Boxing Day earthquake, for example, was estimated to have slowed the earth’s rotation by 2.68 microseconds. How then, does one reconcile a fixed definition of a day with the poorly behaved one that’s being observed in real life?

Enter the leap second. Like the leap year, it aligns the various definitions of timekeeping which have been separately based on atomic clocks and astronomical observation. The insertion of a leap second is usually decided six months in advance, and shows up as the extra second 23:59:60 on either 30 June or 31 December, before the clock begins a new day at 00:00:00. Adjustments have been sporadic and based on necessity – the previous two leap seconds were inserted in 2012 and 2008, and the next is due this June. By 1 July, we’d have had 26 leap seconds since 1972.

Of course, a second may not seem like much, especially in practical terms. Consider this though: satellites travel anywhere from three to seven kilometres per second as they orbit the earth, so a second does make a significant difference for them. GPS satellites, for example, are adjusted for leap seconds to maintain their accuracies. You wouldn’t want to end up on the wrong side of the US-Mexico border because of something as trivial as a second, would you? Closer to home, the leap second has taken over the Y2K bug as the Internet’s new boogieman – 2012’s adjustment crashed websites like Linkedin and Reddit, as their operating systems were not prepped to handle an extra second which came from seemingly nowhere.

Given the underlying reason for the leap second’s existence, adjustments will continue to be necessary. The alternative will be to allow sidereal time (based on the earth’s rotation) and atomic time (based on atomic clocks) to drift apart. Summer in January, anyone?