astAngle

Calculate the angle subtended by two points at a third point

Description:

This function finds the angle at point B between the line joining points A and B, and the line joining points C and B. These lines will in fact be geodesic curves appropriate to the Frame in use. For instance, in SkyFrame, they will be great circles.

Synopsis

double astAngle( AstFrame $\ast$this, const double a[], const double b[], const double c[] )

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.
c
An array of double, with one element for each Frame axis (Naxes attribute) containing the coordinates of the third point.

Returned Value

astAngle
The angle in radians, from the line AB to the line CB. If the Frame is 2-dimensional, it will be in the range $$\setminus$pm $\setminus$pi$, and positive rotation is in the same sense as rotation from the positive direction of axis 2 to the positive direction of axis 1. If the Frame has more than 2 axes, a positive value will always be returned in the range zero to $$\setminus$pi$.

Notes:

• A value of AST__BAD will also be returned if points A and B are co-incident, or if points B and C are co-incident.

• A value of AST__BAD will also be returned if this function is invoked with the AST error status set, or if it should fail for any reason.