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
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