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CO_ABERRATION

## Name

CO_ABERRATION

## Purpose

Calculate changes to Ra and Dec due to the effect of annual aberration

## Explanation

as described in Meeus, Chap 23.

## Calling Sequence

co_aberration, jd, ra, dec, d_ra, d_dec, [EPS = ]

## Inputs

jd : Julian Date [scalar or vector]
ra, dec : Arrays (or scalars) of the ra and dec's in degrees
Note: if jd is a vector, then ra and dec must either be scalars, or
vectors of the same length.

## Outputs

d_ra, d_dec: the corrections to ra and dec due to aberration in
arcseconds. (These values can be added to the true RA
and dec to get the apparent position). Note that d_ra
is *not* multiplied by cos(dec), so that
apparent_ra = ra + d_ra/3600.

## Optional Input Keyword

eps : set this to the true obliquity of the ecliptic (in radians), or
it will be set for you if you don't know it (in that case, set it to
an empty variable).

## Example

Compute the change in RA and Dec of Theta Persei (RA = 2h46m,11.331s, Dec =
49d20',54.54") due to aberration on 2028 Nov 13.19 TD
IDL> jdcnv,2028,11,13,.19*24,jd ;Get Julian date
IDL> co_aberration,jd,ten(2,46,11.331)*15,ten(49,20,54.54),d_ra,d_dec
==> d_ra = 30.045" (=2.003s) d_dec = 6.697"

## Notes

These formula are from Meeus, Chapters 23. Accuracy is much better than 1
arcsecond.
The maximum deviation due to annual aberration is 20.49" and occurs when the
Earth velocity is perpendicular to the direction of the star.

## Revision History

Written, June 2002, Chris O'Dell, U. of Wisconsin
Fix error with vector input W. Landsman June 2009
June 2009 update fixed case where JD was scalar but RA,Dec were vectors, but
broke the case when both JD and RA,Dec were vectors Aug 2012 W. Landsman
Further fix when JD is 1 element vector W. Landsman

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