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News Navigator: What is a leap second?

On Jan. 1, a leap second will be added between 8:59:59 a.m. and 9 a.m. Japanese time (12 a.m. Coordinated Universal Time (UTC)), the 27th time this will have been done since 1972. The Mainichi answers common questions readers may have about leap seconds.

    Question: Why do we add leap seconds?

    Answer: Earth's time was originally astronomical time, based on the Earth's axial rotation and its orbit around the Sun. One rotation was measured as one day, divided into hours, minutes and seconds. Today, our time is based on International Atomic Time using cesium atomic clocks, which are said to be so accurate they would only be off by a second after hundreds of thousands of years had passed. The Earth's speed of rotation is not constant, and differences thus occur between astronomical time and atomic time. In order to keep the difference between UTC and astronomical time from exceeding 0.9 seconds, a leap second is added to UTC once every few years.

    Q: How much difference is there between astronomical time and International Atomic Time?

    A: Over the 59 years since atomic clocks began being used in January 1958 until now, astronomical time has fallen 37 seconds behind atomic time. If the difference was ignored, after tens of thousands of years, UTC could show noon while outside it would be night. Since there are irregularities in the speed of Earth's rotation, leap seconds cannot just be added at regular intervals. An organization called the International Earth Rotation and Reference Systems Service announces half a year in advance when a leap second is going to be added. The speed of the Earth's rotation is thought to be on the decrease in the long term, but it may also increase from the influence of ocean currents or monsoons. So far, though, seconds have only been added, never subtracted.

    Q: Are there any problems caused by adding leap seconds?

    A: In addition to the work needed to calculate when they are needed, there are concerns about whether computers will properly recognize leap seconds. The last time a leap second was added, in 2015, there weren't any major problems, but the time before that, in 2012, computer issues caused delays in airplane takeoffs and landings. Some people say that the practice of adding leap seconds isn't worth the effort and risks, and last year there was an international meeting on the matter. However, participants decided to continue the practice for the time, saying that research is necessary, and agreed to revisit the issue in 2023. (Answers by Ryo Watanabe, Science & Environment News Department)

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