Leap Second: Life and Death Story


Approximately every four years, a day is added to the end of February, a convention known as a leap year. The practice was established by Julius Caesar over 2,000 years ago and was modified in the 16th century by Pope Gregory XIII, the very people who bequeathed the Julian and Gregorian calendars to us.

This extra day serves to bring the 365-day calendar year into line with the time it actually takes the Earth to orbit the Sun, which is almost a quarter of a day longer. It helps to prevent the seasons from coming to no longer correspond to the calendar with the increasing shift.

Humanity is also striving to bring order to the other end of the scale of time. Because for a few decades now, this unit of measurement called the second has been causing us a lot of trouble.

Traditionally, the second was defined in astronomical terms as 1/86,400 of the mean solar day (the time it takes Earth to spin). But in 1967, metrologists started measuring time with atomic clocks. The official duration of the base unit, the second, has been set at 9,192,631,770 vibrations of a cesium-133 atom. And a day is made up of 86,400 of these seconds.

The catch: Earth’s rotation slows down slightly from year to year, causing the astronomical second to gradually lengthen relative to the atomic second. To compensate, beginning in 1972, metrologists began to occasionally add an extra second — called a “leap” — to the end of an atomic day. Every time atomic time is one full second ahead of astronomical time, it stops one second to allow Earth to catch up. Ten leap seconds were added to International Atomic Time in 1972, and 27 more have been added since.

Adding that second is no small feat. Because the Earth’s rotation is slightly erratic, the leap second is both irregular and unpredictable. These characteristics already made its addition complex 50 years ago. Nowadays, the operation is a technical nightmare, as precise timing is an integral part of the company’s hyper-computerized infrastructure.

This article is originally published on lactualite.com


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