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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