![]() ![]() Many ancient cultures were able to deduce-even with crude measuring devices-that 365.25 Earth rotations equal one Earth revolution about the Sun. ![]() The ancients knew that some calendar years must be leap years. The seasons would shift a little each year if calendars were not adjusted regularly. The average duration of a solar day, determined by Earth’s rotation, does not divide neatly into the orbital periods for the Sun and the Moon. These range from abandoning the requirement to preserve knowledge of Earth’s rotation through timekeeping, to whether the word day will mean “one turn of the Earth” versus “794,243,384,928,000 cycles of cesium-133 radiation.” As scientists and engineers engaged in astronautics, space navigation and astronomy, we seek to draw attention to this unprecedented situation.Ī calendar is a system for labeling the sequence of days over long periods in a manner consistent with lunar and solar cycles. The change would bring scientific, technological, legal, philosophical and social implications too. Clocks everywhere-on your wall, wrist, phone and computer-would begin to diverge from the heavens. But the ITU-R proposal would cease issuing leap seconds entirely. Leap seconds have remained a means to an end-that is, to reconcile and combine two entirely different yet useful notions of timekeeping. Adjustments are needed because Earth’s rotation is slightly less regular and a bit slower on average than cesium-133’s quantum-scale rhythms. Since then, leap seconds have occasionally been added to that stream of atomic seconds to keep the signals synchronized with the actual rotation of Earth. But starting in 1972, radio signals began broadcasting atomic seconds. By definition, a SI second is the duration of 9,192,631,770 periods of radiation from the transition between two hyperfine levels of the ground state of the cesium-133 atom, a physical phenomenon distinct from the rotation of Earth.īefore atomic timekeeping, clocks were set to the skies. It is maintained by a continuous count of SI seconds, the metric system’s fundamental unit of duration. It is the basis for legal timekeeping in most of the world, including the United States. ![]() In January 2012, a United Nations–affiliated organization known as the Radiocommunication Sector of the International Telecommunications Union (ITU-R) could permanently break this link by redefining Coordinated Universal Time.Ĭoordinated Universal Time is better known by its international acronym UTC, the modern successor to traditional Greenwich Mean Time. But what was certain then-the necessity of linking civil time to the motion of celestial bodies-may soon be abandoned. See the " Current number of leap seconds" section for the number of leap seconds inserted to date.American astronomers Edgar Woolard and Gerald Clemence published those words in 1966, not that long ago in the history of human timekeeping. Leap seconds are inserted as necessary to keep UTC within 0.9 seconds of the UT1 variant of universal time. The current version of UTC is defined by International Telecommunication Union Recommendation (ITU-R TF.460-6), Standard-frequency and time-signal emissions, and is based on International Atomic Time (TAI) with leap seconds added at irregular intervals to compensate for the accumulated difference between TAI and time measured by Earth's rotation. A decision whether to remove them altogether has been deferred until 2023. This CCIR Recommendation 460 "stated that (a) carrier frequencies and time intervals should be maintained constant and should correspond to the definition of the SI second (b) step adjustments, when necessary, should be exactly 1 s to maintain approximate agreement with Universal Time (UT) and (c) standard signals should contain information on the difference between UTC and UT." Ī number of proposals have been made to replace UTC with a new system that would eliminate leap seconds. This change also adopted leap seconds to simplify future adjustments. The system has been adjusted several times, including a brief period during which the time-coordination radio signals broadcast both UTC and "Stepped Atomic Time (SAT)" before a new UTC was adopted in 1970 and implemented in 1972. UTC was first officially adopted as CCIR Recommendation 374, Standard-Frequency and Time-Signal Emissions, in 1963, but the official abbreviation of UTC and the official English name of Coordinated Universal Time (along with the French equivalent) were not adopted until 1967. The coordination of time and frequency transmissions around the world began on 1 January 1960. ![]()
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