premat: Return the precession matrix needed to go from EQUINOX1 to EQUINOX2
Description
Return the precession matrix needed to go from EQUINOX1 to EQUINOX2
Usage
premat(equinox1, equinox2, fk4=F)
Arguments
equinox1
original equinox of coordinates, scalar
equinox2
equinox of precessed coordinates
fk4
if =TRUE, the FK4 (B1950.0) system will be used; otherwise FK5 (J2000.0) will be used (default = FALSE)
Value
matrix
3 x 3 precession matrix, used to precess equatorial rectangular coordinates
Details
This matrix is used by the functions precess and baryvel to precess astronomical coordinates. The algorithm of this function is obtained from Taff (1983, p.24) for FK4. FK5 constants are obtained from "Astronomical Almanac Explanatory Supplement (1992), page 104, Table 3.211.1.
References
Taff, L. G., 1983, "Computational Spherical Astronomy", Krieger Publ.