leap second

leap second, a second added to the clock to resolve discrepancies between Coordinated Universal Time (UTC), which is measured by atomic clocks, and astronomical time (UT1), as measured by Earth’s rotation.

Leap seconds are added to keep the difference between UTC and UT1 at less than 0.9 second. They are added at midnight on either June 30 or December 31. When a leap second is added, instead of the sequence of time being 23:59:59 to 00:00:00, the sequence becomes 23:59:59 to 23:59:60 to 00:00:00. The first leap second was on June 30, 1972. There have been 26 leap seconds since then, the most recent on December 31, 2016. The addition of a leap second is decided by the International Earth Rotation and Reference Systems Service (IERS).

Why is there a leap second?

The need for a leap second comes from the slight discrepancy between the length of the second in UTC and UT1. UTC is the international basis of civil and scientific time and is based on measurements of time by atomic clocks. UT1 is derived from measurements of Earth’s rotation corrected for polar motion.

Both UTC and UT1 have the second as their basic unit. However, the second is defined differently in the two systems. In UTC the atomic second was defined in 1967 as 9,192,631,770 periods of the wavelength of the hyperfine spectral transition of the cesium-133 atom. This value was chosen to be close to the UT1 ephemeris second, which was defined in 1956 as 1/31,556,925.9747 of the tropical year 1900. (The tropical year is the time between two vernal equinoxes.) The ephemeris second supplanted the mean solar second, which was 1/86,400 of a mean solar day. Although the atomic second and the ephemeris second are very close in duration, they are not exactly equal, and the ephemeris second is slightly shorter than the atomic second.

The difference between the two seconds meant that UTC and UT1 would eventually desynchronize, much to the exasperation of scientists involved in precision time-keeping. (“In retrospect, it seems almost ridiculous that another astronomical definition of the second was accepted during a period when atomic clocks were already being built,” wrote American metrologist Michael Lombardi.) Earth’s rotation is also slowing from tidal breaking at the rate of about 1.4 milliseconds per day per century, which leads to a further mismatch between UTC and UT1.

Earth speeds up and the negative leap second

However, Earth’s rotation has sped up slightly in the last few decades. Only six leap seconds have been done in the 21st century. The rotation of Earth’s core has slowed, which has resulted in a faster rotation of the crust. The speedup has raised the possibility of the need for a “negative leap second,” in which 23:59:58 would be followed by 00:00:00.

The speedup of Earth’s crust has been counteracted, however, by the effect of global warming, which has melted polar ice and moved more water to the Equator, slowing Earth down. This slowdown has pushed the projected need for a negative leap second to 2029. Without the redistribution of polar water, the negative leap second would have been needed in 2026.

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What is the future of the leap second?

With the growing prominence of computers since the first leap second in 1972 and their reliance on precision timekeeping, the leap second has become a nuisance, particularly for technology companies. Some companies, such as the social networking service Reddit in 2012, have suffered slow performance from software glitches caused by the failure to account for leap seconds. Companies, such as Google, Amazon, and Meta, implement leap seconds by running clocks on their computers slightly slower, “smearing” the second over hours or even an entire day.

The inconvenience of the leap second and the looming possibility of the negative leap second, which has never been done, led the General Conference on Weights and Measures in 2022, to pass a resolution advocating that by 2035 the allowed difference between UT1 and UTC should be raised to be more than 0.9 second such that no change would have to be made “for at least a century.” Such a plan would do away with the leap second.