if the observer is imagined to be at a finite distance from the outside of the sphere,
as in this diagram.
If the observer is imagined to be infinitely far away,
Return to "Spherical Astronomy".
the diagram should be drawn
with the z-axis emerging from the sphere a little way "in front of" the edge,
thus:
In my opinion, this version - although equally correct -
is slightly more
difficult to understand.
So throughout the rest of these pages, I am
using diagrams of the first type.
My thanks to Dr Friedrich Firneis of the Austrian Academy of
Sciences,
to Ryu Izawa of the University of Colorado, USA,
and to engineering consultant David Bosher, who provided the diagram above
for helpful discussions on this point.