Nº 7 2013 > The future of time
To abolish or not to abolish the leap second?
The International Astronomical Union and Coordinated Universal Time
Blue sky thinking
Mizuhiko Hosokawa, President of IAUs Commission 31 on Time, Japan's National Institute of Information and Communications Technology
The International Astronomical Union (IAU) is an organization of researchers in the field of astronomy and has deep and strong relations with the international time-scale. In fact, one of the most important subjects of fundamental astronomy is the construction of reference frames and time-scales. The present Coordinated Universal Time (UTC) is maintained so that the difference between UTC and a measure of the Earth’s rotation angle called UT1 is always less than 0.9 second. The precise measurement of UT1 requires astronomical observations.
To determine the value of UT1 with respect to UTC, the angle between the International Celestial Reference System and the International Terrestrial Reference System has to be measured. For this purpose, radio signals from quasars which realize the International Celestial Reference Frame are observed by means of very-long-baseline interferometry using radio telescopes whose positions are well determined in the International Terrestrial Reference Frame. Currently, the task of monitoring the value of UT1-UTC is one of the responsibilities of the International Earth Rotation and Reference Systems Service (IERS), established jointly by IAU and the International Union of Geodesy and Geophysics (IUGG). IERS is responsible for decisions on the timing of the leap-second adjustments announced through its Bulletin C.
In the astronomical community, there are strong and differing points of view on the insertion of leap seconds. One is academic and the other is practical.
From the academic point of view, the continuity of reference frames and time-scales is important. The link between UTC and astronomical time related to the rotation angle of the Earth (UT1) is considered relevant in that context.
From the practical point of view, many astronomical observatories use UTC as the basis of time for their observations. To track celestial objects with optical and radio telescopes, precise knowledge of the Earth’s rotation angle in space is required. The approximation of UTC to UT1 is convenient for this purpose, and some software might have to be modified if UTC were to be redefined in such a way that it could differ from UT1 by more than one second. Thus, some astronomers are concerned about a possible redefinition of UTC.
The discussion about redefining UTC is going on in ITU’s Radiocommunication Sector (ITU–R). IAU has provided ITU with opinions and considerations regarding the future of UTC, from the perspective of a scientific organization that is deeply involved in the construction of reference frames and time-scales.
Discussions in IAU from 2000 to 2006
The discussion on the future of UTC and the leap second started at the Special Rapporteur Group, created in 2000 by Working Party 7A (Time Signals and Frequency Standard Emissions) under ITU–R’s Study Group 7 (Scientific services). This Special Rapporteur Group was to conduct comprehensive studies in answer to Study Group 7’s Question 236/7 on the “Future of the UTC Timescale”. The group requested IAU and other international organizations to provide their opinions and suggestions on the matter.
In response to this request, the IAU community carried out extensive studies and discussions on a possible redefinition of UTC. IAU established a working group for this purpose. According to its terms of reference, the working group was to discuss whether there is a requirement for leap seconds, as well as the possibility of inserting leap seconds at pre-determined intervals, and considerations relating to the tolerance limits for UTC-UT1.
After six years of activity, the final report of the working group was produced in 2006. No consensus was reached in the working group on supporting or rejecting a change in the definition of UTC, because of the many pros and cons from the different points of view. There was, however, agreement on one practical request, namely to allow sufficient time before implementing any changes to the definition. A letter from IAU to ITU–R nevertheless stated that the IAU community had not been affected adversely by any problems resulting from the insertion of the leap second on 31 December 2005, although significant investment in personnel time and effort is required to prepare for the insertion of a leap second.
Recent and ongoing IAU activities
The final report of the IAU working group was submitted in 2006 with the expectation that ITU–R and its relevant bodies would find an answer to Question 236/7. However, the Radiocommunication Assembly in 2012 (RA‑12) concluded that additional studies were required and that the issue should be discussed at the next Assembly and World Radiocommunication Conference (WRC‑15) in 2015.
During discussions in RA‑12, it was pointed out that many ITU Member States were not aware of the proposal to suppress leap-second adjustments. This is why RA‑12 decided that all technical options should be fully studied and requested further discussions within the ITU membership and other organizations having an interest in the redefinition of UTC before the next RA‑15 and WRC‑15.
In response to ITU’s request, IAU again established a working group on the redefinition of UTC under its Division on “Fundamental Astronomy”. The working group is considering:
- current requirements for civil time-scales;
- options for satisfying the future requirements for civil time-scales;
- retaining UTC as it exists or distributing a purely atomic time-scale;
- the impact of a continuous time-scale on the work of astronomers;
- whether a new continuous time-scale for dissemination worldwide should be adopted and how this should relate to TAI;
- whether the General Conference on Weights and Measures rather than ITU should decide on reference time-scales;
- alternative means of distributing UT1, UTC and/or a new continuous time-scale.
The working group will submit its findings to ITU representing IAU’s position on the redefinition of UTC. These findings will be submitted early enough so that they can inform discussions in the various countries and communities before the next Radiocommunication Assembly in 2015. Meanwhile, the latest discussion in the working group will be presented to the workshop on the future of the international time-scale, organized by ITU and the International Bureau of Weights and Measures (BIPM) in Geneva on 19–20 September 2013.
This article was prepared with the great help of members of IAU’s working group on the redefinition of UTC.