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

Description:
This function 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.
Synopsis:
double astAxAngle( AstFrame this, const double a[], const double b[], int axis )
Parameters:
this
Pointer to the Frame.
a
An array of double, with one element for each Frame axis (Naxes attribute) containing the coordinates of the first point.
b
An array of double, with one element for each Frame axis (Naxes attribute) containing the coordinates of the second point.
axis
The number of the Frame axis from which the angle is to be measured (axis numbering starts at 1 for the first axis).
Returned Value:
astAxAngle
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 function is the path of shortest distance between two points, as defined by the astDistance 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: astAxDistance - Find the distance between two axis values
Up: AST Function Descriptions
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AST A Library for Handling World Coordinate Systems in Astronomy