Welcome to the L3 Harris Geospatial documentation center. Here you will find reference guides and help documents.
﻿
>  Docs Center  >  Libraries  >  ASTROLIB  >  PRECESS

PRECESS

PRECESS

## Purpose

Precess coordinates from EQUINOX1 to EQUINOX2.

## Explanation

For interactive display, one can use the procedure ASTRO which calls
PRECESS or use the /PRINT keyword. 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

## Calling Sequence

PRECESS, ra, dec, [ equinox1, equinox2, /PRINT, /FK4, /RADIAN ]
INPUT - OUTPUT:
RA - Input right ascension (scalar or vector) in DEGREES, unless the
/RADIAN keyword is set
DEC - Input declination in DEGREES (scalar or vector), unless the
/RADIAN keyword is set

The input RA and DEC are modified by PRECESS to give the
values after precession.

## Optional Inputs

EQUINOX1 - Original equinox of coordinates, numeric scalar. If
omitted, then PRECESS will query for EQUINOX1 and EQUINOX2.
EQUINOX2 - Equinox of precessed coordinates.

## Optional Input Keywords

/PRINT - If this keyword is set and non-zero, then the precessed
coordinates are displayed at the terminal. Cannot be used
with the /RADIAN keyword
/FK4 - If this keyword is set and non-zero, the FK4 (B1950.0) system
will be used otherwise FK5 (J2000.0) will be used instead.
/RADIAN - If this keyword is set and non-zero, then the input and
output RA and DEC vectors are in radians rather than degrees

## Restrictions

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).

## Examples

(1) The Pole Star has J2000.0 coordinates (2h, 31m, 46.3s,
89d 15' 50.6"); compute its coordinates at J1985.0
IDL> precess, ten(2,31,46.3)*15, ten(89,15,50.6), 2000, 1985, /PRINT
====> 2h 16m 22.73s, 89d 11' 47.3"
(2) Precess the B1950 coordinates of Eps Ind (RA = 21h 59m,33.053s,
DEC = (-56d, 59', 33.053") to equinox B1975.
IDL> ra = ten(21, 59, 33.053)*15
IDL> dec = ten(-56, 59, 33.053)
IDL> precess, ra, dec ,1950, 1975, /fk4

## Procedure

Algorithm from Computational Spherical Astronomy by Taff (1983),
p. 24. (FK4). FK5 constants from "Astronomical Almanac Explanatory
Supplement 1992, page 104 Table 3.211.1.
PROCEDURE CALLED:
Function PREMAT - computes precession matrix

## Revision History

Written, Wayne Landsman, STI Corporation August 1986
Correct negative output RA values February 1989
Added /PRINT keyword W. Landsman November, 1991
Provided FK5 (J2000.0) I. Freedman January 1994
Precession Matrix computation now in PREMAT W. Landsman June 1994
Added /RADIAN keyword W. Landsman June 1997
Converted to IDL V5.0 W. Landsman September 1997
Correct negative output RA values when /RADIAN used March 1999
Work for arrays, not just vectors W. Landsman September 2003

© 2019 Harris Geospatial Solutions, Inc. |  Legal
My Account    |    Store    |    Contact Us