On Mon, 22 Oct 2001, Chris Lusby Taylor wrote: > 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 best rationale I can see for the curved dialplate is to keep the shadow surface at a constant distance form the image-forming holes in the ring. With the pivoting dialplate, you also get the essence of a curve in the East-West plane, as well. By manually adjusting the plate so the perpendicular post is in the Sun plane, you have a "slice" of a hemispherical dial. > 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. I think this is the same answer? > Chris Lusby Taylor > Newbury, England > 51.4N, 1.3W Dave Bell 37.3N 122W
