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

SUNPOS

SUNPOS

## Purpose

To compute the RA and Dec of the Sun at a given date.

## Calling Sequence

SUNPOS, jd, ra, dec, [elong, obliquity, /RADIAN ]

## Inputs

jd - The Julian date of the day (and time), scalar or vector
usually double precision

## Outputs

ra - The right ascension of the sun at that date in DEGREES
double precision, same number of elements as jd
dec - The declination of the sun at that date in DEGREES

## Optional Outputs

elong - Ecliptic longitude of the sun at that date in DEGREES.
obliquity - the obliquity of the ecliptic, in DEGREES

## Optional Input Keyword

/RADIAN - If this keyword is set and non-zero, then all output variables
are given in Radians rather than Degrees

## Notes

Patrick Wallace (Rutherford Appleton Laboratory, UK) has tested the
accuracy of a C adaptation of the sunpos.pro code and found the
following results. From 1900-2100 SUNPOS gave 7.3 arcsec maximum
error, 2.6 arcsec RMS. Over the shorter interval 1950-2050 the figures
were 6.4 arcsec max, 2.2 arcsec RMS.
The returned RA and Dec are in the given date's equinox.
Procedure was extensively revised in May 1996, and the new calling
sequence is incompatible with the old one.

## Method

Uses a truncated version of Newcomb's Sun. Adapted from the IDL
routine SUN_POS by CD Pike, which was adapted from a FORTRAN routine
by B. Emerson (RGO).

## Example

(1) Find the apparent RA and Dec of the Sun on May 1, 1982

IDL> jdcnv, 1982, 5, 1,0 ,jd ;Find Julian date jd = 2445090.5
IDL> sunpos, jd, ra, dec
02 31 32.61 +14 54 34.9
The Astronomical Almanac gives 02 31 32.58 +14 54 34.9 so the error
in SUNPOS for this case is < 0.5".
(2) Find the apparent RA and Dec of the Sun for every day in 1997
IDL> jdcnv, 1997,1,1,0, jd ;Julian date on Jan 1, 1997
IDL> sunpos, jd+ dindgen(365), ra, dec ;RA and Dec for each day

## Modification History

Written by Michael R. Greason, STX, 28 October 1988.
Accept vector arguments, W. Landsman April,1989
Eliminated negative right ascensions. MRG, Hughes STX, 6 May 1992.
Rewritten using the 1993 Almanac. Keywords added. MRG, HSTX,
10 February 1994.
Major rewrite, improved accuracy, always return values in degrees
W. Landsman May, 1996