Sidereal Time: GMST, LAST

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/node220.html | 96 +++++++++++++++++++++++++++++++++++++++ 1 file changed, 96 insertions(+) create mode 100644 src/slalib/sun67.htx/node220.html (limited to 'src/slalib/sun67.htx/node220.html') diff --git a/src/slalib/sun67.htx/node220.html b/src/slalib/sun67.htx/node220.html new file mode 100644 index 0000000..84db99f --- /dev/null +++ b/src/slalib/sun67.htx/node220.html @@ -0,0 +1,96 @@ + + + + +Sidereal Time: GMST, LAST + + + + + + + + + + + + +

+ +

+
+ Next: Dynamical Time: TT, TDB +
+Up: Timescales +
+ Previous: Universal Time: UTC, UT1 +

+Sidereal Time: GMST, LAST +

+Sidereal Time is the ``time of day'' relative to the +stars rather than to the Sun. After +one sidereal day the stars come back to the same place in the +sky, apart from sub-arcsecond precession effects. Because the Earth +rotates faster relative to the stars than to the Sun by one day +per year, the sidereal second is shorter than the solar +second; the ratio is about 0.9973. +

+The Greenwich Mean Sidereal Time GMST is +linked to UT1 by a numerical formula which +is implemented in the SLALIB routines +sla_GMST +and +sla_GMSTA. +There are, of course, no leap seconds in GMST, but the second +changes in length along with the UT1 second, and also varies +over long periods of time because of slow changes in the Earth's +orbit. This makes the timescale unsuitable for everything except +predicting the apparent directions of celestial sources. +

+The Local Apparent Sidereal Time LAST is the apparent right +ascension of the local meridian, from which the hour angle of any +star can be determined knowing its $\alpha$ . It can be obtained from the +GMST by adding the east longitude (corrected for polar motion +in precise work) and the equation of the equinoxes. The +latter, already described, is an aspect of the nutation effect +and can be predicted by calling the SLALIB routine +sla_EQEQX +or, neglecting certain very small terms, by calling +sla_NUTC +and using the expression $\Delta\psi\cos\epsilon$ .

+ +

+
+ Next: Dynamical Time: TT, TDB +
+Up: Timescales +
+ Previous: Universal Time: UTC, UT1 +

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

+ + -- cgit