- Heliocentric Radial - A 3-D orthonormal coordinate system that is primarily intended to specify
with two dimensions a point on the solar disk. The Z axis points toward the observer. The Y
axis lies in the plane defined by the solar spin vector and the Z axis, positive northward. The X
axis is perpendicular to the Y and Z axes, positive toward solar west. Standard representation
for this system is via the point's distance rho from the Z axis [Rho = SQRT(x**2 + y**2)] and
its phase angle psi measured counterclockwise from the +Y axis [psi = arctan (-y/x)].