Mean Places

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/node205.html | 92 +++++++++++++++++++++++++++++++++++++++ 1 file changed, 92 insertions(+) create mode 100644 src/slalib/sun67.htx/node205.html (limited to 'src/slalib/sun67.htx/node205.html') diff --git a/src/slalib/sun67.htx/node205.html b/src/slalib/sun67.htx/node205.html new file mode 100644 index 0000000..6da5021 --- /dev/null +++ b/src/slalib/sun67.htx/node205.html @@ -0,0 +1,92 @@ + + + + +Mean Places + + + + + + + + + + + + +

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+Mean Places +

+The main effect of the precession/nutation is a steady increase of about + $50\hspace{-0.05em}^{'\hspace{-0.1em}'}$ /year in the ecliptic longitudes of the stars. It is therefore +essential, when reporting the position of an astronomical target, to +qualify the coordinates with a date, or epoch. +Specifying the epoch ties down the equator and +equinox which define the $[\,\alpha,\delta\,]$ coordinate system that is +being used. + For simplicity, only +the smooth and steady ``general +precession'' part of the complete precession/nutation effect is +included, thereby defining what is called the mean +equator and equinox for the epoch concerned. We say a star +has a mean place of (for example) + $12^{h}\,07^{m}\,58^{s}.09$ $-19^{\circ}\,44^{'}\,37^{''}.1$ ``with respect to the mean equator +and equinox of epoch J2000''. The short way of saying +this is `` $[\,\alpha,\delta\,]$ equinox J2000'' (not `` $[\,\alpha,\delta\,]$ epoch J2000'', +which means something different to do with +proper motion). +

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+Up: EXPLANATION AND EXAMPLES +
+ Previous: SLALIB support for precession and nutation +

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

+ + -- cgit