Or the slot near one end of the adelaide is just to guide the adelaide as it slides back and forth in the internal geared mechanism. Clearly a vertical gnomon had to be present on some part of the adelaide. I think Steve is on the right track. In terms of the screws or pins blocking movement of the adelaide, I think the whole center section (curcular brass plates) pivots along with the adelaide, so that is moot.
-Bill again. On Fri, Oct 29, 2021 at 9:24 PM Bill Gottesman <billgottes...@gmail.com> wrote: > My two cents: > I have a different guess about the round dial with the indicator. My > guess is that it is a calendar of months, and that turning the indicator to > the correct month adjusts the length of the adelaide to the correct > distance for reading time on the azimuth dial. I do not know what casts > the shadow, but I will guess that Sara is correct, that the slot holds a > vertical vane. > > Being an azimuth dial, the correct declination scale must be used. I see > the circular declination lines, but they are not named. So I think the > pointer of the adelaide is set to the correct circle radius (correct > length) by turning the indicator. > > -Bill > > On Fri, Oct 29, 2021 at 6:04 PM Steve Lelievre < > steve.lelievre.can...@gmail.com> wrote: > >> Thank you, Sara and Patrick for your replies to my question. >> >> I shall try to get to the Science Museum sometime to have a look at the >> dial, if that can be arranged. >> >> I’ve been trying to figure out how the cam that Sara mentioned might work. >> >> I’ve never studied the working of cams and this case isn’t obvious to me >> as there are two inputs, azimuth and declination, that must drive the >> minutes shown. >> >> If anyone can send me an explanation or drawing, it would me much >> appreciated. >> >> Steve >> >> On Mon, Oct 25, 2021 at 19:19, Schechner, Sara <sche...@fas.harvard.edu> >> wrote: >> >>> Hi Steve, >>> The photo of the azimuth dial is hard to read. I don't know what screws >>> you are talking about preventing the arm from turning. The arm is >>> backwards at the moment since the pointed end should be on the scale of >>> hour lines. I am not convinced that there is a flap on the square end of >>> the arm for a vane. The sun at most angles would not fall far along the >>> arm to reach the other end where the slot is. Rather, I suspect there was >>> a vertical gnomon in the slot at the pointed end. Its shadow could have >>> been aligned with the point so that the point was in line with the sun's >>> azimuth. As for the round dial, it almost always shows minutes and is >>> geared to the rotation of the arm. >>> >>> That's my best guess. >>> Sara >>> >>> -----Original Message----- >>> From: sundial <sundial-boun...@uni-koeln.de> On Behalf Of Steve Lelievre >>> Sent: Monday, October 25, 2021 1:22 PM >>> To: Sundial List <sundial@uni-koeln.de> >>> Subject: What's the inner scale on this photo for? >>> >>> Hi, >>> >>> Today a website called Vermont Free Press published an appallingly >>> confusing (to me) summary of types of sundials. If you can bear to look, >>> it's at https://www.vermontpressbureau.com/types-of-sundials/ >>> >>> However, there was one thing about it that piqued my interest: the photo >>> of an azimuth sundial ( >>> https://www.vermontpressbureau.com/wp-content/uploads/2021/10/Azimuthal.jpg >>> ). >>> >>> From what I can make out, there is a metal flap at the end of the >>> alidade / sighting arm (the end at top in the photo). It must get turned up >>> to make a shadow-caster. I guess the arm has to be rotated so that the >>> shadow falls along it, and time is then read from where the right-hand edge >>> of the arm crosses the net of hour and declination lines. But then, >>> wouldn't the screws seen in the upper plate block the arm from being turned >>> to the required orientation? >>> >>> Another bit I can't figure is the little circular scale just north of >>> the centre of the dial, with the pointer. Perhaps just an Equation of Time >>> scale? Or perhaps a cam connects it to the arm so that it can be used to >>> set the arm's length? (The slot in the arm suggests it can be slid in and >>> out to set the tip at the applicable declination circle, which is a nifty >>> feature.) >>> >>> Cheers, >>> >>> Steve >>> >>> --------------------------------------------------- >>> https://lists.uni-koeln.de/mailman/listinfo/sundial >>> >>> -- >> Cell +1 778 837 5771 >> --------------------------------------------------- >> https://lists.uni-koeln.de/mailman/listinfo/sundial >> >>
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