SLALIB support for precession and nutation

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/node204.html | 127 ++++++++++++++++++++++++++++++++++++++ 1 file changed, 127 insertions(+) create mode 100644 src/slalib/sun67.htx/node204.html (limited to 'src/slalib/sun67.htx/node204.html') diff --git a/src/slalib/sun67.htx/node204.html b/src/slalib/sun67.htx/node204.html new file mode 100644 index 0000000..cc880e2 --- /dev/null +++ b/src/slalib/sun67.htx/node204.html @@ -0,0 +1,127 @@ + + + + +SLALIB support for precession and nutation + + + + + + + + + + + +

+ +

+
+ Next: Mean Places +
+Up: Precession and Nutation +
+ Previous: Precession and Nutation +

+SLALIB support for precession and nutation +

+SLALIB offers a choice of three precession models: +

The old Bessel-Newcomb, pre IAU 1976, ``FK4'' model, used for B1950 + star positions and other pre-1984.0 purposes +(sla_PREBN). +
The new Fricke, IAU 1976, ``FK5'' model, used for J2000 star + positions and other post-1984.0 purposes +(sla_PREC). +
A model published by Simon et al. which is more accurate than + the IAU 1976 model and which is suitable for long + periods of time +(sla_PRECL). +

+In each case, the named SLALIB routine generates the $(3\times3)$ precession +matrix for a given start and finish time. For example, +here is the Fortran code for generating the rotation +matrix which describes the precession between the epochs +J2000 and J1985.372 (IAU 1976 model): +

+            DOUBLE PRECISION PMAT(3,3)
+             :
+            CALL sla_PREC(2000D0,1985.372D0,PMAT)
+

+It is instructive to examine the resulting matrix: +

+            +0.9999936402  +0.0032709208  +0.0014214694
+            -0.0032709208  +0.9999946505  -0.0000023247
+            -0.0014214694  -0.0000023248  +0.9999989897
+

+Note that the diagonal elements are close to unity, and the +other elements are small. This shows that over an interval as +short as 15 years the precession isn't going to move a +position vector very far (in this case about $0^{\circ} + \hspace{-0.37em}.\hspace{0.02em}2$ ). +

+For convenience, a direct $[\,\alpha,\delta\,]$ to $[\,\alpha,\delta\,]$ precession routine is +also provided +(sla_PRECES), +suitable for either the old or the new system (but not a +mixture of the two). +

+SLALIB provides only one nutation model, the new, IAU 1980 model, +implemented in the routine +sla_NUTC. +This returns the components of nutation +in longitude and latitude (and also provides the obliquity) from +which a nutation matrix can be generated by calling +sla_DEULER +(and from which the equation of the equinoxes, described +later, can be found). Alternatively, +the nutation matrix can be generated in a single call by using +sla_NUT. +

+A rotation matrix for applying the entire precession/nutation +transformation in one go can be generated by calling +sla_PRENUT. +

+ +

+
+ Next: Mean Places +
+Up: Precession and Nutation +
+ Previous: 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