[LEAPSECS] Schedule for success

Rob Seaman seaman at noao.edu
Tue Dec 30 22:41:45 EST 2008

M. Warner Losh writes:

> Rob Seaman wrote:


>> BTW, it is unsurprising that people use UTC and GMT

>> interchangeably, since the original CCIR standard said:


>> "GMT may be regarded as the general equivalent of UT."


>> This equivalence predates the ITU, and may well outlive the ITU.




> But UT and UTC aren't the same thing... So this confusion is

> between what we call UT1 and UTC, which differ by hundreds of

> milliseconds most of the time...

An academic point, since we've just thoroughly established that actual
current usage mixes UTC and GMT willy-nilly.

However, I've appended a longer excerpt of the text of ITU-R-TF.
460-4. In addition to the general equivalence of GMT and UT, we also
learn that "Universal Time (UT) is the general designation of time
scales based on the rotation of the Earth."

Different flavors of UT are described that differ only at the level of
hundredths of a second. We learn that leap seconds exist to
"unsure" [sic] approximate agreement of UTC with UT1. (A rather
amusing typo :-) And further, that DUT1 exists to improve that

This is a coherent and close equivalence of 5 different timescales,
plus the general purpose alias "UT", that are all explicitly stated to
reflect the rotation of the Earth.

If we were to actively seek consensus here, a good place to start
would be the word "approximation". Eradicating both leap seconds and
DUT1 would destroy any possible interpretation commensurate with


> A. Universal Time (UT)

> Universal Time (UT) is the general designation of time scales based

> on the rotation of the Earth.


> In applications in which an imprecision of a few hundredths of a

> second cannot be tolerated, it is necessary to specify the form of

> UT which should be used:


> UT0 is the mean solar time of the prime meridian obtained from

> direct astronomical observation;


> UT1 is UT0 corrected for the effects of small movements of the Earth

> relative to the axis of rotation (polar variation);


> UT2 is UT1 corrected for the effects of a small seasonal fluctuation

> in the rate of rotation of the Earth;


> UT1 is used in this document, since it corresponds directly with the

> angular position of the Earth around its axis of diurnal rotation.

> (GMT may be regarded as the general equivalent of UT.)


> Concise definitions of the above terms and the concepts involved are

> available in the glossary of the annual publication, The

> Astronomical Almanac (US Government Printing Office, Washington DC

> and H.M. Stationery Office, London).


> [...]


> C. Coordinated Universal Time (UTC)


> UTC is the time-scale maintained by the BIPM, with assistance from

> the International Earth Rotation Service (IERS), which forms the

> basis of a coordinated dissemination of standard frequencies and

> time signals. It corresponds exactly in rate with TAI but differs

> from it by an integral number of seconds.


> The UTC scale is adjusted by the insertion or deletion of seconds

> (positive or negative leap-seconds) to unsure approximate agreement

> with UT1.


> D. DUT1


> The value of the predicted difference UT1 – UTC, as disseminated

> with the time signals is denoted DUT1; thus DUT1 ≈ UT1 – UTC.

> DUT1 may be regarded as a correction to be added to UTC to obtain a

> better approximation to UT1. [...]

More information about the LEAPSECS mailing list