Įventually, this definition too was found to be inadequate for precise time measurements, so in 1967, the SI second was again redefined as 9,192,631,770 periods of the radiation emitted by a caesium-133 atom in the transition between the two hyperfine levels of its ground state. In 1956, a slightly more precise value of 1⁄ 31,556,925.9747 was adopted for the definition of the second by the International Committee for Weights and Measures, and in 1960 by the General Conference on Weights and Measures, becoming a part of the International System of Units (SI). In 1955, considering the tropical year to be more fundamental than the sidereal year, the IAU redefined the second as the fraction 1⁄ 31,556,925.975 of the 1900.0 mean tropical year.
Soon afterwards Simon Newcomb and others discovered that Earth's rotation period varied irregularly, so in 1952, the International Astronomical Union (IAU) defined the second as a fraction of the sidereal year. With this definition, the second was proposed in 1874 as the base unit of time in the CGS system of units. Muslim scholars, including al-Biruni in 1000, subdivided the mean solar day into 24 equinoctial hours, each of which was subdivided sexagesimally, that is into the units of minute, second, third, fourth and fifth, creating the modern second as 1⁄ 60 of 1⁄ 60 of 1⁄ 24 = 1⁄ 86,400 of the mean solar day in the process. Vertical segments correspond to leap seconds.Ībout 140 CE, Ptolemy, the Alexandrian astronomer, sexagesimally subdivided both the mean solar day and the true solar day to at least six places after the sexagesimal point, and he used simple fractions of both the equinoctial hour and the seasonal hour, none of which resemble the modern second. Graph showing the difference between UT1 and UTC. The relevant international standards body has been debating whether or not to continue the practice. This practice has proven disruptive, particularly in the twenty-first century and especially in services that depend on precise timestamping or time-critical process control.
Insertion of each UTC leap second is usually decided about six months in advance by the International Earth Rotation and Reference Systems Service (IERS), to ensure that the difference between the UTC and UT1 readings will never exceed 0.9 seconds. The leap second facility exists to provide this adjustment.īecause the Earth's rotation speed varies in response to climatic and geological events, UTC leap seconds are irregularly spaced and unpredictable. The UTC time standard, widely used for international timekeeping and as the reference for civil time in most countries, uses TAI and consequently would run ahead of observed solar time unless it is reset to UT1 as needed. In the U.S., the leap second took place at 18:59:60 local time on the East Coast, at 15:59:60 local time on the West Coast, and at 13:59:60 local time in Hawaii.Ī leap second is a one- second adjustment that is occasionally applied to Coordinated Universal Time (UTC), to accommodate the difference between precise time ( International Atomic Time (TAI), as measured by atomic clocks) and imprecise observed solar time ( UT1), which varies due to irregularities and long-term slowdown in the Earth's rotation.
gov during the leap second on December 31, 2016.
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