Universal Time: UT, UTC

Universal Time, UT, or more specifically UT1, is in effect mean solar time and is really an expression of Earth rotation rather than a measure of time. Originally defined in terms of a point in the sky called ``the fictitious mean Sun'', UT is now defined through its relationship with Earth rotation angle (formerly sidereal time). Because the Earth's rotation rate is slightly irregular and gradually decreasing,⁵the UT second is not precisely matched to the SI second. This makes UT itself unsuitable for use as a time scale.

That role is instead taken by Coordinated Universal Time, UTC, which is clock-based and is the foundation of civil timekeeping. Most time zones differ from UTC by an integer number of hours, though a few (e.g. parts of Canada and Australia) differ by $n+0.5$ hours. Since its introduction, UTC has been kept roughly in step with UT by a variety of adjustments that are agreed in advance and then carried out in a coordinated manner by the timekeeping communities of different countries--hence the name. Though rate changes were used in the past, nowadays all such adjustments are made by occasionally inserting a whole second. This procedure is called a leap second. Because the day length is now slightly longer than 86400 SI seconds, a leap second amounts to stopping the UTC clock for a second to let the Earth catch up.

You need UT1 in order to point a telescope or antenna at a clestial target. To obtain it starting from UTC, you have to look up the value of UT1$-$UTC for the date concerned in tables published by the International Earth Rotation and reference frames Service; this quantity, kept in the range $\pm$ $0^{\rm s}\hspace{-0.3em}.9$ by means of leap seconds, is then added to the UTC. The quantity UT1$-$UTC, which typically changes by of order 1 ms per day, can be obtained only by observation (VLBI using extragalactic radio sources), though seasonal trends are well known and the IERS listings are able to predict some way into the future with adequate accuracy for pointing telescopes.

UTC leap seconds are introduced as necessary, usually at the end of December or June. Because on the average the solar day is slightly longer than the nominal 86,400 SI seconds, leap seconds are always positive; however, provision exists for negative leap seconds if needed. The form of a leap second can be seen from the following description of the end of June 1994:

			UTC	UT1$-$UTC	UT1
1994	June	30	23 59 58	$-0.218$	23 59 57.782
			23 59 59	$-0.218$	23 59 58.782
			23 59 60	$-0.218$	23 59 59.782
	July	1	00 00 00	$+0.782$	00 00 00.782
			00 00 01	$+0.782$	00 00 01.782

Note that UTC has to be expressed as hours, minutes and seconds (or at least in seconds for a given date) if leap seconds are to be taken into account in the correct manner. It is improper to express a UTC as a Julian Date, for example, because there will be an ambiguity during a leap second (in the above example, 1994 June 30 $23^{h}\,59^{m}\,60^{s}.0$ and 1994 July 1 $00^{h}\,00^{m}\,00^{s}.0$ would both come out as MJD 49534.00000). Although in the vast majority of cases this won't matter, there are potential problems in on-line data acquisition systems and in applications involving taking the difference between two times. Note that although the functions sla_DAT and sla_DTT expect UTC in the form of an MJD, the meaning here is really a whole-number date rather than a time. Though the functions will accept a fractional part and will almost always function correctly, on a day which ends with a leap second incorrect results would be obtained during the leap second itself because by then the MJD would have moved into the next day.