From d54fe7c1f704a63824c5bfa0ece65245572e9b27 Mon Sep 17 00:00:00 2001 From: Joseph Hunkeler Date: Wed, 4 Mar 2015 21:21:30 -0500 Subject: Initial commit --- src/slalib/sun67.htx/node219.html | 184 ++++++++++++++++++++++++++++++++++++++ 1 file changed, 184 insertions(+) create mode 100644 src/slalib/sun67.htx/node219.html (limited to 'src/slalib/sun67.htx/node219.html') diff --git a/src/slalib/sun67.htx/node219.html b/src/slalib/sun67.htx/node219.html new file mode 100644 index 0000000..4214aaa --- /dev/null +++ b/src/slalib/sun67.htx/node219.html @@ -0,0 +1,184 @@ + + + + +Universal Time: UTC, UT1 + + + + + + + + + + + + +
+ +next + +up + +previous +
+ Next: Sidereal Time: GMST, LAST +
+Up: Timescales +
+ Previous: Atomic Time: TAI +

+

+

+Universal Time: UTC, UT1 +

+Coordinated Universal Time UTC is the basis 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. 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 leap +second. +

Universal Time UT, or more specifically UT1, +is in effect the mean solar time. It is continuous +(i.e. 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 +$\pm$$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. +

+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: +

  + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +
Figure 1: +Relationship Between Celestial Coordinates
      
   UTCUT1-UTCUT1
      
1994June3023 59 58-0.21823 59 57.782
   23 59 59-0.21823 59 58.782
   23 59 60-0.21823 59 59.782
 July100 00 00+0.78200 00 00.782
   00 00 01+0.78200 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. 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 routines +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 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. +

+

+ +next + +up + +previous +
+ Next: Sidereal Time: GMST, LAST +
+Up: Timescales +
+ Previous: Atomic Time: TAI +

+

+

+SLALIB --- Positional Astronomy Library
Starlink User Note 67
P. T. Wallace
12 October 1999
E-mail:ptw@star.rl.ac.uk +
+ + -- cgit