On bifilar polar sundial

[Anselmo Pérez Serrada]

 I have been playing a bit with the equations for a bifilar dial trying to
reproduce the bifilar polar dial that I saw in your web. There you say
that the transversal gnomon is a piece of hyperbola, but I have found
that it is really a piece of ellipse (there are more solutions, but none
is an hyperbolic arc). I suppose my calculations are wrong  but I can't
find the mistake in them. Can you please provide me more
information about this topic? Do you know the exact equation for this
curve?

Hartelijke bedankt,

Anselmo P. Serrada




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