Nº 7 2013 > The future of time
To abolish or not to abolish the leap second?
Role of the International Earth Rotation and Reference Systems Service
Brian Luzum, Chair, Directing Board, International Earth Rotation and Reference Systems Service
The International Earth Rotation and Reference Systems Service (IERS) has an important role in determining when leap seconds are to be inserted and in announcing the dates for this insertion. In order to understand this role, it is important to know certain features of time.
There are two different kinds of “time”, which in today’s world are related: first, a uniform time, now based on atomic clocks; and second, “time” based on the variable rotation of the Earth. The difference between uniform time and Earth rotation time only became apparent in the 1930s with improvements in clock technology.
Coordinated Universal Time (UTC) is currently the standard for everyday time usage worldwide. In addition, it plays an important role in such diverse applications as communications, computer network synchronization and navigation through global navigation satellite systems (GNSS), for example, the Global Positioning System (GPS). Because of the accuracy of current atomic clocks, UTC is accurate to the scale of several nanoseconds (billionths of a second).
Then and now
Historically, timekeeping was based on the rotation of the Earth. The repetitive passage of astronomical bodies (the Sun, for example) provided a convenient method to mark the passage of time. Time based on observations of the rotation angle of the Earth in a celestial reference system continues to play a role in modern timekeeping.
Today, the measure of the Earth rotation angle is provided by a linear relationship with a time-like quantity called UT1, which is observed using a worldwide network of radio telescopes. Earth rotation data are provided to users in the form of a quantity UT1–UTC.
The rotational speed of the Earth is highly variable as a result of tides, changes in weather, oceans and other geophysical effects. Consequently, the only way to provide this information reliably is to monitor the Earth’s rotation on a regular basis. The Earth’s rotational speed is measured by fixing devices to the surface of the Earth and observing objects in space. Very long baseline interferometry, using radio telescopes to observe distant radio sources called quasars, can measure UT1 to an accuracy of a few tens of microseconds (millionths of a second).
Leap seconds were introduced in 1972 as an attempt to ensure synchronization between clock time and Earth rotation. According to Recommendation ITU-R TF.460-6 on standard-frequency and time-signal emissions:
“A positive or negative leap second should be the last second of a UTC month, but first preference should be given to the end of December and June, and second preference to the end of March and September.”
“A positive leap second begins at 23h 59m 60s and ends at 0h 0m 0s of the first day of the following month. In the case of a negative leap-second, 23h 59m 58s will be followed one second later by 0h 0m 0s of the first day of the following month.”
Because IERS is responsible for monitoring and predicting the quantity UT1–UTC, it provides a vital contribution to the determination of when leap seconds will need to be inserted in order to keep UTC to within 0.9 second of UT1 specified by ITU. In recognition of this, Recommendation ITU-R TF.460-6 stipulates that IERS should decide upon and announce the introduction of a leap second, and that such an announcement should be made at least eight weeks in advance.
Since their inception, there have been 25 leap seconds. Through their implementation as specified in Recommendation ITU-R TF.460-6, leap seconds ensure that the absolute value of the difference between UTC and UT1 never exceeds 0.9 second. In effect, leap seconds allow users to approximate UT1 with UTC to an accuracy of roughly one second. While in the 1970s this level of approximation may have been considered as only a slight loss of accuracy, nowadays the discrepancy is more glaring because, with today’s technology, real-time estimates of the difference between UT1 and UTC can be determined to more than four orders of magnitude better accuracy.
Beyond its important role in determining when leap seconds are to be inserted and in disseminating information regarding leap seconds, IERS provides algorithms that enable users to use Earth-orientation parameters in their operations. These algorithms are developed by experts and tested thoroughly in geodetic and geophysical applications to ensure their quality. The algorithms and associated software are available free of charge through IERS websites for Conventions.
As tasked by the International Astronomical Union (IAU) and the International Union of Geodesy and Geophysics (IUGG), IERS helps to coordinate the regular measurement of all Earth orientation components, including the Earth’s variable rotation. IERS combines these Earth-orientation observations four times per day, providing high-quality predictions for users of parameters of real-time Earth-orientation. All of these data sets are provided to the worldwide community through various computer transfer protocols free of charge.
Announcements of upcoming leap seconds are made through the IERS Bulletin C, which is usually released in January and July, and announces whether it will be necessary to insert a leap second within the next six months. This scheduling meets the “eight weeks in advance” requirement of ITU.
IERS provides the international community with: the International Celestial Reference System and its realization, the International Celestial Reference Frame; the International Terrestrial Reference System and its realization, the International Terrestrial Reference Frame; Earth orientation parameters that are used to transform between the International Celestial Reference Frame and the International Terrestrial Reference Frame; standards, models and constants used in generating and using reference frames and Earth orientation parameters; and geophysical data to study and understand variations in the reference frames and the Earth’s orientation.
In the pipeline
In recognition of rapidly changing technology and in order to meet the emerging needs of its users, IERS plans to create new products, and to move to more modern data file formats to improve the usability of its data. In addition, IERS will investigate the possibility of creating a real-time Earth-orientation parameter transfer protocol. This product would provide UT1 directly to users who currently choose to approximate UT1 using UTC. It would have the advantage of maintaining the same simplicity of implementation that users currently enjoy, while increasing the accuracy of the data by more than four orders of magnitude at no cost to the user.
The IERS commitment
IERS was created in 1987 and began operations on 1 January 1988. It continued much of the work of the Bureau International de l’Heure, which had been established early in the twentieth century. IERS is responsible to the International Astronomical Union and the International Union of Geodesy and Geophysics.
IERS has served the international scientific community for more than 25 years. Recently, IERS has positioned itself to more completely serve the needs of its users whether the current definition of Coordinated Universal Time is retained or whether it is redefined to eliminate leap seconds. Either way, ITU can count on IERS to support its users with the data and software needed. IERS is prepared to meet any future requirements of users by the most convenient means.