In the thread 'Re: Making a Dial in The Gambia.', "Frans W. Maes" wrote:
> <snip> > Have you considered an "inverted" armillary sphere? Examples can > be found at my page "Pole-style - equatorial" in the bottom row of > thumbnails. > > Please keep us informed about the results! > > Regards, Frans > > ===================================== > Frans W. Maes > Peize, The Netherlands > 53.1 N, 6.5 E > www.biol.rug.nl/maes/sundials/ > ===================================== Can anyone help me, please, with understanding a sundial in Bremen shown in Frans's 'inverted' armillary sphere section. (See < http://www.hs-bremen.de/planetarium/astroinfo/sonnenuhren/digitalk.htm >). Its equatorial ring is pierced with many small holes which cast a pattern of lines and numbers on the dial. The ring appears to be semicircular and the dial appears to be near its centre. I don't understand why the dial is curved or how it can correct for the EoT at any time of day. The dial seems to be curved so that the top and bottom of the analemma are further from the front of the ring than the middle is. It isn't curved in the other plane. I cannot see why a curved dial should work. It seems to me that for the dial to indicate the correct apparent time regardless of the sun's declination, it should be on or parallel to the axis of the ring. Straight, in any event. The biggest contradiction I cannot resolve in my mind relates to the analemma. It is this: for the dial to indicate the correct date all day long the distance from any part of the ring to the dial should be constant. So, the ring should be circular and the dial should be on its axis. But, for the plane analemma to be correct all day long, the dial would have to be on the circumference of the ring. Clearly, it is not. So it seems to me that, if the analemma is correct at noon, it will be too narrow at other times. For instance, if it indicates an EoT of 10 minutes at noon, it will show 7 minutes at 9am/3pm and just 5 minutes at 8am/4pm. Any thoughts, please? Chris Lusby Taylor Newbury, England 51.4N, 1.3W
