- NAME
PREMAT
- PURPOSE
Return the precession matrix needed to go from EQUINOX1 to EQUINOX2.
- EXPLANTION
This matrix is used by the procedures PRECESS and BARYVEL to precess
astronomical coordinates
- CALLING SEQUENCE
matrix = PREMAT( equinox1, equinox2, [ /FK4 ] )
- INPUTS
EQUINOX1 - Original equinox of coordinates, numeric scalar.
EQUINOX2 - Equinox of precessed coordinates.
- OUTPUT
matrix - double precision 3 x 3 precession matrix, used to precess
equatorial rectangular coordinates
- OPTIONAL INPUT KEYWORDS
/FK4 - If this keyword is set, the FK4 (B1950.0) system precession
angles are used to compute the precession matrix. The
default is to use FK5 (J2000.0) precession angles
- EXAMPLES
Return the precession matrix from 1950.0 to 1975.0 in the FK4 system
IDL> matrix = PREMAT( 1950.0, 1975.0, /FK4)
- PROCEDURE
FK4 constants from "Computational Spherical Astronomy" by Taff (1983),
p. 24. (FK4). FK5 constants from "Astronomical Almanac Explanatory
Supplement 1992, page 104 Table 3.211.1.
- REVISION HISTORY
Written, Wayne Landsman, HSTX Corporation, June 1994
Converted to IDL V5.0 W. Landsman September 1997
- eturn to caller
Compute 3 rotation angles
Compute 3 rotation angles