What Is the Leap Second and Why Have Countries Voted to Scrap It?


Nov 24, 2022


Image Credit: Alamy/Hitesh Sonar For The Swaddle

The time has come for the leap second to go. Scientists and government representatives, in a periodic conference of the Bureau of Weights and Measures (BIPM) — the organization responsible for global timekeeping — held in Versailles, France last week, almost unanimously decided to do away with the leap second, the Agence-France-Presse reported. But what really is the leap second in the first place? And why has the BIPM decided to abandon it?

What Is the Leap Second?

The earth’s rotation and revolution both take a little more than a day and a year respectively, and this extra time is accounted for through extra time recorded in calendars and clocks once every few years. For instance, the earth takes roughly around 365 days and 6 hours to complete one revolution, but the standard year counts only the 365 days. Hence every fourth year, an extra day is added at the end of February to account for those extra six hours of the previous three years and the current year — cumulatively making up a day. This is called a leap year, and the 29th of February is the leap day.

Just like the leap year, the leap second too was devised to account for some extra time that remains undocumented in our standardized methods of keeping time. The earth’s rotation is variable, meaning that while some days last less than 24 hours, others last a fraction more. On average, the difference between these varying rotational rates usually comes around to 24 hours and a few milliseconds. These extra milliseconds make up the cumulative time of a second every few years, and the extra second — denoted as 23:59:60 in clocks and watches before transitioning to 00:00:00 to mark the new day — is known as the leap second. But what purpose does accounting for that extra second serve? After all, isn’t it just one extra second? One answer may lie in understanding the disparities that mark our time recording methods.

How Is Time Recorded?

Historically, a day is comprised of the interval between two successive noons — the time of the day when the sun is directly above one’s head — in a place. This is known today as the apparent solar day, as it is calculated based on the apparent position of the sun in the sky. The length of the apparent solar day varies throughout the year depending on the season. On some months, the apparent solar day may extend beyond 24 hours by a few seconds, while on others it may fall some seconds short of 24 hours. For standardization purposes, an approximation of the apparent solar time known as the mean solar time is used in our clocks to determine local time.

Globally, these local times are measured against what is known as the Coordinated Universal Time (UTC). Falling within one second of the mean solar time at the 0-degree longitude (Greenwich Mean Time), the UTC is maintained by the BIPM and follows the standard of the 365-day year, 24-hour day, and 60-minute hour. A minute is usually 60 seconds, but at times may comprise 59 or 61 — depending on the occasional addition of a leap second. While it falls close to the mean solar time, the current iteration of the UTC is derived from something known as the International Atomic Time (TAI) for the purposes of precision.

The TAI is one of the most precise and accurate measures of time. It is a weighted average of time kept by atomic clocks — clocks that use the frequency of atoms to determine time — around the world.

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Why Countries Are Discontinuing the Leap Second

The mean solar day is a few milliseconds longer than a standard UTC day, which has led to countries adopting the leap second to account for those missed milliseconds cumulatively every now and then. However, this addition of an extra second, especially in the modern digital world, has given rise to a fresh set of problems.

The primary problem caused by this extra second is the disruption of technology services across the world. Telecommunications, satellite navigation, and financial transactions could all suffer due to this addition of an extra second, which usually is announced only 6 months in advance — often leaving organizations with too little time to incorporate this extra second on their softwares.

Telecommunications organizations and financial groups say the continual adding of leap seconds to computers increases the chances of errors being made.” Robin McKie, Science Editor at The Guardian noted a decade ago while discussing the long-standing debate around the use of the leap second. “Precisely timed money transactions could go astray or vehicles could be sent tens of meters out of position if they are a second out in their measurement of time.” In 2012, the addition of a leap second caused glitches in networks around the world, causing sites like Mozilla, Reddit, and LinkedIn to crash. In Australia, confusion arising from the leap second led to around 400 Quantas flights being grounded.

Beyond the technological fallouts of incorporating the leap second, there is the issue that the earth is now increasingly rotating at a faster speed than before, with days on average lasting about 0.5 seconds lesser than 24 hours. If this trend continues, then the use of the leap second will become redundant. In fact, it may even necessitate incorporating a negative leap second. As Peter Whibberly, a senior research scientist at the UK’s National Physical Laboratory told The Telegraph, “it’s also possible that the need for a negative leap second might push the decision towards ending leap seconds for good.”

Keeping these considerations in mind, scientists at the conference in Paris chose to discontinue the leap second from 2035.

What Happens Now?

Scientists are still undecided on the replacement for the leap second. Judah Levine, a physicist at the US National Institute of Standards and Technology, discussed with The Guardian some possible alternatives that could come up once the leap second is discontinued. “The larger value is yet to be determined,” said Levine, but added that the UTC, for now, would be kept as it is since it is run by “a worldwide community effort” through the BIPM. Levine speculated that one possible way to replace the leap second would be by letting a larger discrepancy between the earth’s rotation and atomic time build up and adjusting that through an extra minute every 50 to 100 years, to minimize regular confusion and disruption.


Written By Amlan Sarkar

Amlan Sarkar is a staff writer at TheSwaddle. He writes about the intersection between pop culture and politics. You can reach him on Instagram @amlansarkr.


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