Report of the IAU Working Group on Coordinated Universal Time (UTC)
Executive Summary
The IAU Working Group on the Redefinition of Coordinated Universal Time (UTC), after
considering issues that have the potential to affect the astronomical community, is unable to reacha conclusive recommendation regarding a proposal to eliminate occasional one-second
adjustments to UTC. Opinions exist that both support and oppose the proposal, and there is no
possibility of reaching an undivided opinion. The Working Group understands that the majority ofastronomers lack strong opinions on the subject. Relevant arguments are outlined in this report.
Past correspondence between the IAU and the Radiocommunication Sector of the International
Telecommunication Union (ITU-R) on this issue, as well as past formal IAU actions regarding
Universal Time, are also included.
Consequently, the Working Group recommends that the IAU respond to the ITU-R by stating that
the IAU is not in a position to formulate a conclusive opinion regarding any change in the definitionof Coordinated Universal Time. Nevertheless, in the event of the deletion of future leap seconds
the name of the scale should no longer reference the astronomical time scale “Universal Time” toavoid technical confusion, and a time interval of at least five years between adoption andimplementation should be allowed.
Specifically, it recommends that:
1. The IAU express to the ITU-R its gratitude for being asked for its opinion
2. The IAU express to the ITU-R
a. that, as most astronomers are not affected directly and are accustomed to thecurrent definition of UTC, they lack an opinion on a possible change,b. that, considering the diverse interests of a small number of specialized
astronomers, consensus concerning a redefinition of UTC among them is unlikely,c. that, considering operational astronomical applications primarily, the IAU canneither favor nor oppose the deletion of leap seconds from UTC,d. that the word “universal”, and by extension the abbreviation “UT”, is appropriate
only for a time scale that is linked to the rotation of the Earth, and would no longer
be appropriate if leap seconds were to cease,e. that, if a continuous reference timescale is adopted, at least 5 years of lead time isrequired for re-education and changes to legacy software and data storage formats,f. that a different name be considered for a new time scale,g. that the IAU continue to be represented in future discussions relating to time scales.
3. The IAU urge its members to develop astronomical software that requires precise Earthorientation information to use Earth orientation data provided by the International Earth
Rotation and Reference Systems Service (IERS).
4. The IAU request that the IERS investigate more widely distributed and technologicallyuseful means of providing Earth orientation information.
With the acceptance of this report, the Working Group sees no further need for its continued
existence.

1

Members of the Working Group: Felicitas Arias and Dennis McCarthy (Co-chairs), Daniel Gambis,
George Kaplan, Yasuhiro Koyama, Dick Manchester, Robert Nelson (deceased), Masatoshi
Ohishi, Arnold Rots, Rob Seaman, Ken Seidelmann, and Shougang Zhang
Background. The Working Group was formed following the XXVIII IAU General Assembly inBeijing in 2012. Its mission is to prepare a proposed response of the IAU to theRadiocommunication Sector of the International Telecommunication Union (ITU-R) in reply to thatorganization’s request to the IAU for comments regarding the possible redefinition of Coordinated
Universal Time (UTC). The 2015 World Radiocommunication Conference (WRC-15) agenda item
1.14 is scheduled to “consider the feasibility of achieving a continuous reference time scale,whether by the modification of UTC or some other method.”
In 1970 the predecessor organization of the ITU-R (i.e., the International Consultative Committee
for Radio (CCIR)), adopted the current definition of UTC which calls for the insertion of leap
seconds, beginning in 1972, to maintain UTC within 0.9 seconds of UT1, a quantity that measures
the rotation angle of the Earth in the celestial reference system. The modification of this definition
has been under discussion in the ITU-R Study Group 7 for well over a decade and has resulted inmany reports and commentaries. During that time the IAU also considered the issue. Annex 1 isthe Report of the IAU Working Group (2005); Annex 2 is the IAU Letter to ITU-R (2006); Annex 3 isthe IAU Commission 31 Report (2010); Annex 4 is the IAU Response to ITU-R Questionnaire(2011); Annex 5 is a statement by the (then) IAU Secretary General, Ian Corbett, on that responseto the questionnaire (2011); Annex 6 provides a brief history of past IAU General Assemblystatements regarding Universal Time. These documents indicated that establishing a distributed
time scale without leap seconds would have little or no negative operational impact on theastronomical community provided that sufficient lead time was provided in order to accommodatechanges in software and data storage formats. The IAU statements in Annex 6 are consistent withsimilar statements made by the International Meridian Conference in 1884 (Resolution 5), theConsultative Committee on the Definition of the Second (CCDS) in 1974 (CCDS Recommendation
S-1 (1974)), the General Conference of Weights and Measures (CGPM) in 1975 (Resolution 5 of
the 15th CGPM), the International Consultative Committee for Radio (CCIR) in 1975 (CCIR
Recommendation 536), and the World Administrative Radio Conference (WARC) of theInternational Telecommunication Union (ITU) in 1979 (Final Acts of the World Administrative Radio
Conference (WARC-79)). This Working Group has not established the seriousness of negative
impacts, but nonetheless acknowledges that substantial lead time would be required regardless ofthe presumed degree of negative impact.
Discussion. The Working Group has prepared this report to reflect technical concerns ofastronomers regarding a change in the definition of UTC under the assumption that the ITU-R willconsult responsible experts in other fields for their comments.
The astronomical community has many diverse areas of interest and specialties. The large
majority of modern astronomers are generally not concerned, or knowledgeable, about various
aspects of fundamental astronomy, including time scales and reference systems. Consequently
the majority of the IAU membership is not in a position to formulate a conclusive technical opinion
regarding any change in the definition of UTC.
Some IAU members concerned with specialized applications related to navigation, satellite
communication, electronic network synchronization and timing standards laboratories, regard leap
seconds as an unnecessary expense. These adjustments cannot be predicted with certainty, andthey can create confusion for those applications requiring precise time. On the one hand, from anastronomical point of view there appears to be no strong argument for any change to the statusquo, which currently provides a means to obtain low-precision Earth orientation information (1second of time equivalent to 15 arc seconds). On the other hand, since a well-establishedorganization (the International Earth Rotation and Reference Systems Service, IERS) exists within2the IAU reporting Earth orientation information (including predictions) routinely with precision atleast five orders of magnitude better than that available through UTC, there is no longer a need touse UTC as a source of Earth orientation information. Some members dealing with the provision of
precise time through national institutions and needing long-term stable time scales recognize aneed for a uniform time scale and favor a change from the current system that requires occasional
expensive adjustments to timing equipment.
Other astronomers concerned with the definition of UTC include space scientists, almanac
producers, and those involved in the management and operation of observing instrumentationsuch as telescopes, antennas, and instruments. They use software and procedures in which thecurrent definition of UTC is embedded. The elimination of leap seconds does not simplify
requirements for Earth orientation information in these applications. Some astronomical systems,
particularly robotic or automated observatories already fielded or in development, would be
adversely affected by the elimination of leap seconds. Observational data archives also make useof UTC time markers. Any change to the current definition will require expensive searches throughexisting documentation and software to identify and change existing code and data storage
formats to accommodate a new definition. The IAU Working Group urges at least five years leadtime to allow time to make necessary changes if the definition of UTC is changed. Such lead timeis also necessary to inform users of the changes that will be necessary.
Related Issues
Alternate Time Scales No proposed methods for “achieving a continuous reference time scale,whether by the modification of UTC or some other method” have received unanimous support
within the Working Group. A suggestion receiving limited support is the potential international
distribution of two separate standard time scales, one providing time linked to the Earth’s rotation
as is the case now with UTC, and one freed from that constraint (e.g. International Atomic Time
TAI), as multiple scales are already being employed in some astronomical applications. The
working group reached no consensus regarding the desirability of distributing two separate
standard time scales.
Nomenclature Another issue is the name of a re-defined scale. The descriptor “Universal” in theterm “Universal Time” arose in the later part of the 19th century, when the only source of time wasbased on the Earth’s rotation, to refer to the definition of a “universal day” that would be
independent of location. Following developments in timekeeping in the years since, the term
“Universal Time” has been applied to (1) time based on the mean solar day beginning at midnight
along the meridian of Greenwich, as recommended by the IAU in 1928, (2) time scales based on
the rotation of the Earth with respect to fictitious points defined in adopted celestial reference
systems, (3) atomic time scales with internationally adopted adjustments in frequency and epoch
aimed to match the Earth’s rotation, and (4) the atomic time scale encoded to provide Earthrotation information. See Annex 6. The word “Coordinated” was first used in 1960 when U. S. and
U. K. began to “coordinate” adjustments in their clock time scales. In all these applications theword “universal”, and by extension the abbreviation “UT”, has been preserved to describe a timescale that is linked to the rotation of the Earth. There is no compelling technical reason to maintainthe label “Coordinated Universal Time” for a time scale that is not linked to the rotation of the
Earth. The nomenclature issue raised by the proposed change in the definition of ‘Coordinated
Universal Time’ was also discussed in a statement by the ISO’s Technical Committee 37 to theprevious ITU Radio Assembly (January 2012).
Continuous Reference Time Scale. The ITU-R Radiocommunications Sector asked the IAU for
input on a "continuous reference time-scale". Although the IAU WG on UTC does not know of adefinition for that term, the following properties provide a reasonable concept:1) time markers with intervals as uniform as current technology allows2) time markers with values such that the time interval between any two markers is triviallyobtained by subtraction without need for a lookup table
3

The non-uniformity of Universal Time is a long-studied phenomenon. If the two points above areproperties of the time scale desired by the ITU-R then no form of Universal Time can satisfy them.
General astronomical software and archival data. The elimination of future leap seconds does
simplify the requirements for general astronomical processing and analysis software. However,
any tools required to handle archival data must continue to accommodate leap seconds; the onlychange in this respect is that the leap second table will not change anymore.
Civil Time and Mean Solar Time. A distributed time scale based only on the SI second, without
leap second adjustments, would slowly diverge from UT1, which is considered to be nominallyequivalent to mean solar time reckoned from midnight on the meridian of Greenwich. Traditionally,mean solar time has been the basis for civil time, and UTC as currently defined has been
considered an acceptably accurate surrogate for civil purposes. Considering the current
deceleration of the rotational speed of the Earth, we might expect the divergence resulting from thedeletion of leap seconds to amount to approximately 2 minutes in 100 years.”
Summary. Most astronomers are not affected directly and are accustomed to the current definitionof UTC; consequently, they likely lack an informed opinion on changing the status quo.
Considering the diverse interests of other specialized astronomers, consensus concerning aredefinition of UTC is unlikely.
Recommendations of the Working Group to the IAU
The Working Group recommends that:
1. The IAU express to the ITU-R its gratitude for being asked for its opinion
2. The IAU express to the ITU-R
a. that, as most astronomers are not affected directly and are accustomed to thecurrent definition of UTC, they lack an opinion on a possible change,b. that, considering the diverse interests of a small number of specialized
astronomers, consensus concerning a redefinition of UTC among them is unlikely,c. that, considering operational astronomical applications primarily, the IAU canneither favor nor oppose the deletion of leap seconds from UTC,d. that the word “universal”, and by extension the abbreviation “UT”, is appropriate
only for a time scale that is linked to the rotation of the Earth, and would no longer
be appropriate if leap seconds were to cease,e. that, if a continuous reference timescale is adopted, at least 5 years of lead time isrequired for re-education and changes to legacy software and data storage formats,f. that a different name be considered for a new time scale,g. that the IAU continue to be represented in future discussions relating to time scales.
3. The IAU urge its members to develop astronomical software that requires precise Earthorientation information to use Earth orientation data provided by the International Earth
Rotation and Reference Systems Service (IERS).
4. The IAU request that the IERS investigate more widely distributed and technologicallyuseful means of providing Earth orientation information.
.

4

Annexes
1. Report of IAU Working Group (2005)
2. IAU Letter to ITU-R (2006)
3. IAU Commission 31 Report (2010)
4. IAU Response to ITU-R Questionnaire (2011)
5. Statement by Ian Corbett Regarding the ITU Questionnaire (2011)
6. References to Universal Time from Past IAU General Assemblies

5Annex 1: Report of IAU Working Group (2005)

6

Annex 1: Report of IAU Working Group (2005)

7

Annex 1: Report of IAU Working Group (2005)

8

Annex 1: Report of IAU Working Group (2005)

9

Annex 2: IAU Letter to ITU-R (2006)10

Annex 2: IAU Letter to ITU-R (2006)11

Annex 3: IAU Commission 31 Report (2010)12

Annex 3: IAU Commission 31 Report (2010)13

Annex 4: IAU Response to ITU-R Questionnaire (2011)14

Annex 5: Statement by Ian Corbett Regarding ITU Questionnaire (2011)

From: Ian Corbett [mailto:icorbett@eso.org]
Sent: Tuesday, January 11, 2011 1:39 PM
To: P. Kenneth Seidelmann
Cc: 'George Wilkins'; 'Leslie Morrison'; 'George Kaplan'
Subject: Re: IAU Procedures re UTC leap second
Dear Ken,
Here is a short statement from me:"Following the IAU response to the ITU-R SG7 questionnaire on a new
definition of Coordinated Universal Time (UTC) without leap seconds it has
become clear that there is a division of opinion with the IAU membership.
Given the strength of views expressed it is unlikely that a consensus IAU
position could be reached.
When the IAU was invited by ITU-R SG7 to complete its questionnaire I
referred the matter to the IAU representative to ITU and then to IAU
Commission 31 "Time". I was told that "Many years ago the IAU made asimilar survey, and sent its view to the ITU, saying that astronomers do NOT
oppose the proposed change." Commission 31, after internal consultation,
replied that they did not oppose the proposed change. I replied accordingly
to ITU-R.
Subsequently I became aware that members of other IAU Commissions shouldhave been consulted and that had that happened they would probably have
opposed the proposed change. An extensive e-mail correspondence ensued
which made it clear that there was no consensus within the IAU. The IAU
representative to ITU was involved in this exchange of views and cantherefore explain the situation to the ITU."Best wishes
Ian

15

Annex 6: References to Universal Time from Past IAU General
Assemblies
Resolution 1 of IAU Commission 4 at the 7th General Assembly in 1948 found on page 4 of
http://iau.org/static/resolutions/IAU1948_French.pdf”La Commission recommande que la désignation 'Temps Universel' (Universal Time;
Weltzeit) soit seule utilisée par les astronomes pour désigner le temps solaire moyen,compté à partir de minuit du méridien de Greenwich. Elle exprime le vœu que cettedésignation remplace aussitôt que possible les autres expressions encore employées.”English Translation by the Working Group:
The commission recommends that the designation 'Universal Time' (Temps Universel;
Weltzeit) only be used by astronomers to designate mean solar time, reckoned frommidnight of the Greenwich meridian. It hopes that that this designation replaces otherexpressions still being used as soon as possible.

The proceedings (1964) of the meeting of IAU Commission 31 found on page 16 of
http://www.iau.org/static/resolutions/IAU1964_French.pdf
1. L'UAI note qu'il serait utile d'exprimer clairement la distinction entre les deux aspects de la
notion de temps, à savoir, l'époque (instant) et l'intervalle de temps, et l'utilité des diverseséchelles de temps.
2. L'époque du T.U. est déterminée par la position angulaire de la Terre autour de son axe;elle est demandée pour divers usages scientifiques et techniques et pour les usages civils,
parfois sans délai.
3. Une unité de temps atomique (T.A.), fondée sur une transition quantique, convient commeunité d'intervalle de temps en physique et est entrée dans l'usage pratique depuis 1955.
L'adoption d'une transition particulière pour la définition de la second physique est de lacompétence de la Conférence Générale des Poids et Mesures.
4. Le T.E. est le temps qui convient à la méchanique céleste dont les travaux immédiatsn'exigent la connaissance ni de l'époque, ni de l'unité d'intervalle de temps.
5. L'UAI reconnait que les physiciens ont besoin de la second de temps atomique, mais insist
sur la nécessité de fournir aux usagers, d'une façon continue et sans délai, l'époque du
T.U.
6. Le besoin existe donc de fournir par des émissions radioélectriques à la fois l'époque duT.U. et l'unité d'intervalle de T.A. C'est ce qui est fait depuis 1959.
7. La méthode pour fournir les deux est possible parce que
a. l'époque du T.U.2 n'a besoin d'être connue sans correction immédiate qu'avec une
tolérance de 0.1 s etb. (b) la fréquence peut être maintenue constante par rapport aux étalons atomiques
pendant des durées d'une ou plusieurs années au moyen d'un décalage defréquence connu.
Dans cette méthode, on maintient la cohérence entre les signaux horaires et la fréquence
de l'onde porteuse.
8. On reconnait que d'autres méthodes de compromis sont possibles. Cependant, le système
actuel semble le mieux adapté à beaucoup d'exigences courantes.
9. On reconnait qu'il serait désirable de transmettre l'époque de T.U.2 et l'unité d'intervalle detemps sans sauts d'époque ni décalages de fréquence.
English Translation by the Working Group:16Annex 6: References to Universal Time from Past IAU General
Assemblies
1. The IAU notes that it would be useful to articulate the distinction between the two aspectsof the concept of time, namely, the epoch (instant) and the time interval, and the utility ofvarious time scales.
2. The epoch of U.T. is determined by the angular position of the Earth about its axis; it isrequired for various scientific and technical purposes and for civil purposes, sometimes
without delay.
3. A unit of atomic time (A.T.), based on a quantum transition, suitable as a physical unit oftime interval has come into practical use since 1955. The adoption of a particular transition
for the definition of the physical second is the responsibility of the General Conference of
Weights and Measures.
4. E.T. is the time that suits celestial mechanics whose immediate work requires neither theknowledge of the epoch, nor the unit of time interval.
5. The IAU recognizes that physicists need the second of atomic time, but insists on the needto provide users, continuously and without delay, the epoch of U.T.
6. Therefore the need exists for radio emissions to provide both the epoch of U.T. and the unitof interval of A.T. This has been done since 1959.
7. The method for providing the two is possible because
a. the epoch of U.T.2 need not be known without immediate correction to within atolerance of 0.1 s andb. frequency can be kept constant with respect to atomic standards for periods of one
or more years by means of a known frequency offset.
Via this method, coherence is maintained between time signals and the frequency of thecarrier wave.
8. We recognize that other methods of compromise are possible. However, the current
system seems best suited to many common requirements.
9. We recognize that it would be desirable to transmit the epoch of U.T.2 and the unit of timeinterval without steps of time nor offsets of frequency.

Resolutions of IAU Commissions 4 and 31 at the 15th General Assembly in 1973 (on page 20 of)
http://www.iau.org/static/resolutions/IAU1973_French.pdf“Resolution No. 1 by Commissions 4 and 31(Adopted 25 August 1973)
Considering:(a) that a single worldwide coordinated clock time scale based upon the SI second is desirable,(b) that the Coordinated Universal Time system (UTC) makes the International Atomic Time
(TAI), which is based on the SI second, generally available, and(c) that the UTC system provides mean solar time to a precision that is needed for navigationand surveying,
Recommend:
The adoption of the UTC system as the basis for the dissemination of standard time (heure
normale) in all countries.
Resolution No. 2 by Commissions 4 and 31(Adopted 29 August 1973)
Considering:(a) that TAI equalled UT2 (nominally) when UT2 was 1958 January 1d, 0h 0m 0s(b) the necessity of maintaining all conventional calendarial and time designations,(c) the desirability of bringing into accordance ET and TAI, which differ by about 32s;
Recommend:17Annex 6: References to Universal Time from Past IAU General
Assemblies(1) that a change be introduced in the TAI as soon as practicable so that TAI (new) equals
TAI (old) plus 32 seconds precisely,(2) that DTA (DAT) be defined by TAI minus UTC, where DTA is an integral number of seconds,(3) that the CCDS be invited to take the necessary action.
Explanation:(1) TAI is not affected by leap seconds but UTC is.
(2) The value of DTA is determined and announced by the BIH.
(3) UTC is the basis of standard time, the time in common (civil) use, as disseminated byradio time signals.”
Resolution No. 4 by Commissions 4 and 31(Adopted 29 August 1973)
Considering:(a) that present procedures governing the insertion of leap seconds have been variouslyinterpreted and,(b) that if the present trend of the rotation of the earth continues, it will become impossible tomaintain UTC within the present limits by the insertion of leap seconds on two preferreddates only and,(c) that it is important fo many users to have UT1 minus UTC remain within a *fixed* limit;
Recommend:(1) that the *maximum limit* of UT1 minus UTC be set at +/- 0.950 seconds,(2) that the maximum deviation of UT1 from UTC plus DUT1 be +/- 0.100 s,(3) that, when necessary, leap seconds may be introduced at the end of any month, but thatfirst preference be given to the end of June and December, and second preference begiven to the end of March and September.

18