astrolibR (version 0.1)

precess: Precess coordinates from EQUINOX1 to EQUINOX2

Description

Precess coordinates from EQUINOX1 to EQUINOX2

Usage

precess(ra, dec, equinox1, equinox2, fk4=F, radian=F)

Arguments

ra

Right Ascension, in degrees, scalar or vector

dec

declination, in degrees, scalar or vector

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)

radian

if =TRUE, the input and output RA and DEC vectors are in radians rather than degrees (default = FALSE)

Value

ra

precessed Right Ascension, in degrees, scalar or vector

dec

precessed declination, in degrees, scalar or vector

Details

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.

The accuracy of precession decreases for declination values near 90 degrees. PRECESS should not be used more than 2.5 centuries from 2000 on the FK5 system (1950.0 on the FK4 system).

The default (RA,DEC) system is FK5 based on epoch J2000.0, but FK4 based on B1950.0 is available via the /FK4 keyword. Use BPRECESS and JPRECESS to convert between FK4 and FK5 systems

References

Taff, L. G., 1983, "Computational Spherical Astronomy", Krieger Publ.

See Also

premat ten

Examples

#  The Pole Star has J2000.0 coordinates (2h, 31m, 46.3s, 
#  89d 15' 50.6"); compute its coordinates at J1985.0
#  Result: 2h 16m 22.73s, 89d 11' 47.3"

precess(ten(2,31,46.3)*15, ten(89,15,50.6), 2000, 1985)


# Precess the B1950 coordinates of Eps Ind (RA = 21h 59m,33.053s,
# DEC = (-56d, 59', 33.053") to equinox B1975.

ra = ten(21, 59, 33.053)*15
dec = ten(-56, 59, 33.053)
precess(ra, dec , 1950, 1975, fk4=TRUE)