- Helioprojective Cartesian = A 3-D orthonormal (left-handed) coordinate system that is
primarily intended to specify with two dimensions a point on the solar disk. The Z axis points
from the observer to the center of the solar disk. 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. Given as the distance between the observer and the center of
the solar disk, the standard representation of an (x,y) point on the solar disk is via the point's
longitude angle [arctan (x/d)] and latitude angle [arctan y/d].