This projection puts the viewer somewhere in space, looking back at the Earth. It has the appearance of a globe and only one hemisphere can be seen at one time. It is an ancient projection, in use by the Greeks and Egyptians thousands of years ago.
It is not equal-area or conformal. Meridians and parallels can be straight lines, circles or ellipses with large distortion around the edges.
Two variations of this projection are handled. In the first variation, the orientation of the positive Y axis is defined by a latitude and longitude of a point on the positive Y axis. In the second form, the azimuth of the Y axis is defined by an actual angle, east of north, of the Y axis. The most common case is a coordinate system where the Y axis is true north.