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XYZ

XYZ

## Purpose

Calculate geocentric X,Y, and Z and velocity coordinates of the Sun

## Explanation

Calculates geocentric X,Y, and Z vectors and velocity coordinates
(dx, dy and dz) of the Sun. (The positive X axis is directed towards
the equinox, the y-axis, towards the point on the equator at right
ascension 6h, and the z axis toward the north pole of the equator).
Typical position accuracy is <1e-4 AU (15000 km).

## Calling Sequence

XYZ, date, x, y, z, [ xvel, yvel, zvel, EQUINOX = ]

## Input

date: reduced julian date (=JD - 2400000), scalar or vector

## Output

x,y,z: scalars or vectors giving heliocentric rectangular coordinates
(in A.U) for each date supplied. Note that sqrt(x^2 + y^2
+ z^2) gives the Earth-Sun distance for the given date.
xvel, yvel, zvel: velocity vectors corresponding to X, Y and Z.

## Optional Keyword Input

EQUINOX: equinox of output. Default is 1950.

## Example

What were the rectangular coordinates and velocities of the Sun on
Jan 22, 1999 0h UT (= JD 2451200.5) in J2000 coords? NOTE:
Astronomical Almanac (AA) is in TDT, so add 64 seconds to
UT to convert.
IDL> xyz,51200.5+64.d/86400.d,x,y,z,xv,yv,zv,equinox = 2000
Compare to Astronomical Almanac (1999 page C20)
X (AU) Y (AU) Z (AU)
XYZ: 0.51456871 -0.76963263 -0.33376880
AA: 0.51453130 -0.7697110 -0.3337152
abs(err): 0.00003739 0.00007839 0.00005360
abs(err)
(km): 5609 11759 8040
NOTE: Velocities in AA are for Earth/Moon barycenter
(a very minor offset) see AA 1999 page E3
X VEL (AU/DAY) YVEL (AU/DAY) Z VEL (AU/DAY)
XYZ: -0.014947268 -0.0083148382 -0.0036068577
AA: -0.01494574 -0.00831185 -0.00360365
abs(err): 0.000001583 0.0000029886 0.0000032077
abs(err)
(km/sec): 0.00265 0.00519 0.00557

## Procedure Calls

PRECESS_XYZ

## Revision History

Original algorithm from Almanac for Computers, Doggett et al. USNO 1978
Adapted from the book Astronomical Photometry by A. Henden
Written W. Landsman STX June 1989
Correct error in X coefficient W. Landsman HSTX January 1995
Added velocities, more terms to positions and EQUINOX keyword,