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## AST_AXANGLE - Returns the angle from an axis, to a line through two points

Description:
This routine finds the angle, as seen from point A, between the positive direction of a specified axis, and the geodesic curve joining point A to point B.
Invocation:
RESULT = AST_AXANGLE( THIS, A, B, AXIS, STATUS )
Arguments:
THIS = INTEGER (Given)
Pointer to the Frame.
A( ) = DOUBLE PRECISION (Given)
An array with one element for each Frame axis (Naxes attribute) containing the coordinates of the first point.
B( ) = DOUBLE PRECISION (Given)
An array with one element for each Frame axis (Naxes attribute) containing the coordinates of the second point.
AXIS = INTEGER (Given)
The number of the Frame axis from which the angle is to be measured (axis numbering starts at 1 for the first axis).
STATUS = INTEGER (Given and Returned)
The global status.
Returned Value:
AST_AXANGLE = DOUBLE PRECISION
The angle in radians, from the positive direction of the specified axis, to the line AB. If the Frame is 2-dimensional, it will be in the range [-PI/2,PI/2], and positive rotation is in the same sense as rotation from the positive direction of axis 2 to the positive direction of axis 1. If the Frame has more than 2 axes, a positive value will always be returned in the range zero to PI.
Notes:
• The geodesic curve used by this routine is the path of shortest distance between two points, as defined by the AST_DISTANCE function.

• This function will return "bad" coordinate values (AST__BAD) if any of the input coordinates has this value, or if the require position angle is undefined.

Next: AST_AXDISTANCE - Find the distance between two axis values
Up: AST Routine Descriptions
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AST A Library for Handling World Coordinate Systems in Astronomy
Starlink User Note 210
R.F. Warren-Smith & D.S. Berry
7th December 2012