Fellow Shadow Watchers,
Mac Oglesby has reminded me af an email discussion
we had on this problem some months ago. The upshot being that a dial on
a direct north or direct south slope could be treated as one displaced in
latitude BUT the gnomon must be perpendicular to the sloped surface NOT
vertical. On a steep slope a 'human gnomon' would have the difficult
task of leaning forward or backwards at the correct angle! - so a movable
'pylon' would be required.
I'd forgotten that as part of our discussion I'd produced a diagram for a
'compromise analemmatic' which can be applied to almost any surface. The
principle being that the 'calendar line' is built up like a wedge to be
truly horizontal but the ellipse is projected with strings and plumb bob
onto whatever surface is available. No doubt I've just 'reinvented a
long-forgotten wheel'.
GIF available to show the working principle in detail on request.
Tony Moss
