I agree. I have a polar dial with an equinox line and I see this every equinox. It's not that subtle. You can see it easily over just a few hours.
-----Original Message----- From: sundial-boun...@uni-koeln.de [mailto:sundial-boun...@uni-koeln.de] On Behalf Of Frank King Sent: Wednesday, September 23, 2009 1:39 PM To: Roger Bailey Cc: sundial@uni-koeln.de Subject: Re: Equinox and Equatorial Rings Dear Roger, It is good of you to remind us of the bronze ring set in the Square Hall of Alexandria. You add... > You can repeat this experiment on any > correctly aligned equatorial sundial > or armillary sphere. You can in fact undertake the essentials of the experiment using ANY sundial equipped with a nodus and an equinoctial line. When I was setting up the Pembroke Sundial in 1997, I spent a lot of time tracing the path of the shadow of the nodus across the dial and comparing the observed results with my calculations. For each day's observations I assumed the declination at noon held good all day. Observed results and calculated results matched pretty well during June and July and I became complacent. At the autumnal solstice, observed results and calculated results didn't agree so well. I was puzzled by this at first. Then I appreciated the obvious. The declination is changing so rapidly at the equinoxes (about 1 minute an hour) that you can no longer assume the declination at noon holds good all day. Contrary to what simple sundial guides tell you, the shadow of a point cast onto a plane DOESN'T follow a straight line on the day of an equinox. It actually follows an S shape, albeit a very narrow S. Have a good winter. Frank --------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial
--------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial
