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POSANG

POSANG

## Purpose

Computes rigorous position angle of source 2 relative to source 1

## Explanation

Computes the rigorous position angle of source 2 (with given RA, Dec)
using source 1 (with given RA, Dec) as the center.

## Calling Sequence

POSANG, U, RA1, DC1, RA2, DC2, ANGLE

## Inputs

U -- Describes units of inputs and output:
1: RAx in decimal hours, DCx in decimal
degrees, ANGLE in degrees
RA1 -- Right ascension of point 1
DC1 -- Declination of point 1
RA2 -- Right ascension of point 2
DC2 -- Declination of point 2

## Outputs

ANGLE-- Angle of the great circle containing [ra2, dc2] from
the meridian containing [ra1, dc1], in the sense north
through east rotating about [ra1, dc1]. See U above
for units.

## Procedure

The "four-parts formula" from spherical trig (p. 12 of Smart's
Spherical Astronomy or p. 12 of Green' Spherical Astronomy).

## Example

For the star 56 Per, the Hipparcos catalog gives a position of
RA = 66.15593384, Dec = 33.94988843 for component A, and
RA = 66.15646079, Dec = 33.96100069 for component B. What is the
position angle of B relative to A?
IDL> RA1 = 66.15593384/15.d & DC1 = 33.95988843
IDL> RA2 = 66.15646079/15.d & DC2 = 33.96100069
IDL> posang,1,ra1,dc1,ra2,dc2, ang
will give the answer of ang = 21.4 degrees

## Notes

(1) If RA1,DC1 are scalars, and RA2,DC2 are vectors, then ANGLE is a
vector giving the position angle between each element of RA2,DC2 and
RA1,DC1. Similarly, if RA1,DC1 are vectors, and RA2, DC2 are scalars,
then DIS is a vector giving the position angle of each element of RA1,
DC1 and RA2, DC2. If both RA1,DC1 and RA2,DC2 are vectors then ANGLE
is a vector giving the position angle between each element of RA1,DC1
and the corresponding element of RA2,DC2. If then vectors are not the
same length, then excess elements of the longer one will be ignored.
(2) Note that POSANG is not commutative -- the position angle between
A and B is theta, then the position angle between B and A is 180+theta

## Procedure Calls

ISARRAY()

## History

Modified from GCIRC, R. S. Hill, RSTX, 1 Apr. 1998
Use V6.0 notation W.L. Mar 2011

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