Re: About the circumpolar parabola

[Anselmo P�rez Serrada]

Hello Fer and all,      Thanks for the reference. I hadn't seen Fantoni's book but I have heard that it is very good. As I told you, I heard about that topic from somebody else.   Yes, you are right in saying that the curve can be some other conical if the sundial reclines, but if it is horizontal it doesn't matter which the latitude is: the curve is a parabola (more or less open).   An special case rises when the sundial is equatorial: the evolving curve of the ita-bab lines is a circle; and if we make an stereographical projection instead of a gnomonical one, we get a nice diagram that looks like a rose window or a ball bearing, if you prefer.   Greetings,   Anselmo Perez Serrada   [41.652 N  4.723 W] ----- Original Message ----- From: fer j. de vries To: sundial@rrz.uni-koeln.de Sent: Sunday, March 17, 2002 11:30 AM Subject: Re: About the circumpolar parabola Hello Anselmo,   About the property you mention a description may be read in the Italian book "Orologi Solari" by Girolamo Fantoni, 1988. He describes the Babylonian and Italian hourlines and draw figures with the "Cerchio circumpolari visibili".   Have in mind that you only get a parabola on an horizontal sundial. For other sundials the intersection of the cone with the sundial's face will be an ellipse or hyperbola .   Best wishes, Fer.       Fer J. de Vries mailto:[EMAIL PROTECTED] http://www.iae.nl/users/ferdv/ Eindhoven, Netherlands lat.  51:30 N      long.  5:30 E