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<H3><A NAME="SECTION000515200000000000000">
Universal Time: UTC, UT1</A>
</H3>
<I>Coordinated Universal Time</I> UTC is the basis of civil timekeeping.
Most time zones differ from UTC by an integer number
of hours, though a few (<I>e.g.</I> parts of Canada and Australia) differ
by <I>n</I>+0.5 hours. The UTC second is the same as the SI second,
as for TAI. In the long term, UTC keeps in step with the
Sun. It does so even though the Earth's rotation is slightly
variable (due to large scale movements of water and atmosphere
among other things) by occasionally introducing a <I>leap
second</I>.
<P><I>Universal Time</I> UT, or more specifically UT1,
is in effect the mean solar time. It is continuous
(<I>i.e.</I> there are no leap seconds) but has a variable
rate because of the Earth's non-uniform rotation period. It is
needed for computing the sidereal time, an essential part of
pointing a telescope at a celestial source. To obtain UT1, you
have to look up the value of UT1-UTC for the date concerned
in tables published by the International Earth Rotation
Service; this quantity, kept in the range
<IMG WIDTH="15" HEIGHT="25" ALIGN="MIDDLE" BORDER="0"
SRC="img30.gif"
ALT="$\pm$"><IMG WIDTH="24" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
SRC="img31.gif"
ALT="$0^{\rm s}\hspace{-0.3em}.9$"> by means of UTC leap
seconds, is then added to the UTC. The quantity UT1-UTC,
which typically changes by 1 or 2 ms per day,
can only be obtained by observation, though seasonal trends
are known and the IERS listings are able to predict some way into
the future with adequate accuracy for pointing telescopes.
<P>
UTC leap seconds are introduced as necessary,
usually at the end of December or June.
On the average the solar day is slightly longer
than the nominal 86,400 SI seconds and so 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:
<P><A NAME="27722"> </A>
<TABLE CELLPADDING=3>
<CAPTION><STRONG>Figure 1:</STRONG>
Relationship Between Celestial Coordinates</CAPTION>
<TR VALIGN="TOP"><TD ALIGN="CENTER" NOWRAP> </TD>
<TD ALIGN="LEFT" NOWRAP> </TD>
<TD ALIGN="RIGHT" NOWRAP> </TD>
<TD ALIGN="CENTER" NOWRAP> </TD>
<TD ALIGN="CENTER" NOWRAP> </TD>
<TD ALIGN="CENTER" NOWRAP> </TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="CENTER" NOWRAP> </TD>
<TD ALIGN="LEFT" NOWRAP> </TD>
<TD ALIGN="RIGHT" NOWRAP> </TD>
<TD ALIGN="CENTER" NOWRAP>UTC</TD>
<TD ALIGN="CENTER" NOWRAP>UT1-UTC</TD>
<TD ALIGN="CENTER" NOWRAP>UT1</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="CENTER" NOWRAP> </TD>
<TD ALIGN="LEFT" NOWRAP> </TD>
<TD ALIGN="RIGHT" NOWRAP> </TD>
<TD ALIGN="CENTER" NOWRAP> </TD>
<TD ALIGN="CENTER" NOWRAP> </TD>
<TD ALIGN="CENTER" NOWRAP> </TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="CENTER" NOWRAP>1994</TD>
<TD ALIGN="LEFT" NOWRAP>June</TD>
<TD ALIGN="RIGHT" NOWRAP>30</TD>
<TD ALIGN="CENTER" NOWRAP>23 59 58</TD>
<TD ALIGN="CENTER" NOWRAP>-0.218</TD>
<TD ALIGN="CENTER" NOWRAP>23 59 57.782</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="CENTER" NOWRAP> </TD>
<TD ALIGN="LEFT" NOWRAP> </TD>
<TD ALIGN="RIGHT" NOWRAP> </TD>
<TD ALIGN="CENTER" NOWRAP>23 59 59</TD>
<TD ALIGN="CENTER" NOWRAP>-0.218</TD>
<TD ALIGN="CENTER" NOWRAP>23 59 58.782</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="CENTER" NOWRAP> </TD>
<TD ALIGN="LEFT" NOWRAP> </TD>
<TD ALIGN="RIGHT" NOWRAP> </TD>
<TD ALIGN="CENTER" NOWRAP>23 59 60</TD>
<TD ALIGN="CENTER" NOWRAP>-0.218</TD>
<TD ALIGN="CENTER" NOWRAP>23 59 59.782</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="CENTER" NOWRAP> </TD>
<TD ALIGN="LEFT" NOWRAP>July</TD>
<TD ALIGN="RIGHT" NOWRAP>1</TD>
<TD ALIGN="CENTER" NOWRAP>00 00 00</TD>
<TD ALIGN="CENTER" NOWRAP>+0.782</TD>
<TD ALIGN="CENTER" NOWRAP>00 00 00.782</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="CENTER" NOWRAP> </TD>
<TD ALIGN="LEFT" NOWRAP> </TD>
<TD ALIGN="RIGHT" NOWRAP> </TD>
<TD ALIGN="CENTER" NOWRAP>00 00 01</TD>
<TD ALIGN="CENTER" NOWRAP>+0.782</TD>
<TD ALIGN="CENTER" NOWRAP>00 00 01.782</TD>
</TR>
</TABLE>
<P>
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. 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 <IMG WIDTH="96" HEIGHT="17" ALIGN="BOTTOM" BORDER="0"
SRC="img308.gif"
ALT="$23^{h}\,59^{m}\,60^{s}.0$"> and
1994 July 1 <IMG WIDTH="96" HEIGHT="17" ALIGN="BOTTOM" BORDER="0"
SRC="img309.gif"
ALT="$00^{h}\,00^{m}\,00^{s}.0$"> would <I>both</I> 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 routines
sla_DAT
and
sla_DTT
expect UTC in the form of an MJD, the meaning here is really a
whole-number <I>date</I> rather than a time. Though the routines 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.
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<ADDRESS>
<I>SLALIB --- Positional Astronomy Library<BR>Starlink User Note 67<BR>P. T. Wallace<BR>12 October 1999<BR>E-mail:ptw@star.rl.ac.uk</I>
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