Brad--
> > If the inside surface is marked with the lines analogous to lines of > longitude on a globe spaced 15 degrees apart, radiating from the “pole” of > the hemicyclium, and the entire device is tilted to align with the earth’s > axis, would it then read out in Local True Solar Time? > Yes, the hour-lines would be exactly like the 15-degree meridian longitude-lines of a globe. (plus whatever intermediate fractional-hour lines). It seems to me that, in the photos and drawings I've seen, both the Hemicyclia and the Hemispheria had a horizontal flat top edge-surface. With that bowl-edge horizontal, and with the stick-tip nodus at the same level as that bowl-edge, the dial wIould tell time whenever the Sun is above the horizon. (So do the Horizontal Dial, the Band-Equatorial, and lots of others) It also seems to me that, in the photos and drawings that I've seen, the stick for the stick-end nodus was horizontal, sticking out so that its tip, the nodus, is at the center of the bowl. If you're sure you want the bigger task of a Hemicyclium instead of a Band-Equatorial, then I'd use the traditional horizontal stick, with its end (nodus) at the bowl's center. ...instead of using a polar-parallel gnomon. Easier, for one thing, and more realistic, for a Hemicyclium or Hemispherium. Though the system of hour-lines should have its poles at the ends of an imaginary line parallel with the Earth's axis (north-south, tipped up at the north-end by an amount equal to your north-latitude), I'd just leave the top-cut, bowl-edge horizontal, like the pictures that i've seen. Anyway, if the surface that includes the bowl edge were tipped from the horizontal, wouldn't that interfere with ensuring that the dial will tell time whenever the Sun is above the horizon? I'd leave the bowl-top edge horizontal. I think that's how Hemicyclia and Hemispheria always were. I'd have declination-lines in the bowl, in addition to the hour-lines. Of course the declination-lines would be exactly like the parallels on a globe. People often mark declination-lines with the date. I'd have the lines marked with declination-degrees as well. In older centuries, they marked declination-lines with zodiac signs instead of month-names. That made sense really, because the zodiac signs coincide with exact solar ecliptic longitudes, corresponding to exact declinations (if you disregard the small change-rate of the obliquity). But of course nowadays the months are much more famiiar. But I'd mark the declination-lines in degrees too. That is my primary sticking point. I’d prefer that than the ancient Temporary hours. It would seem it would be mathematically similar to a section of an armillary sphere. Yes, just maybe a more challenging construction than an Armillary Band-Equatorial. The globe-meridian-like hour-lines would be more work than the straight hour lines on a Band-Equatorial. But, with your metal-working experience, you probably *want* something more challenging. > > > With a proper adjustable mount, I can adjust for the longitude correction > (I am currently about 4 degrees away from my nearest meridian) and DST > twice a year as well. > Ok, but that seems unnecessary extra work, since the longitude-correction could be added to EoT in your conversion-table plaque displayed near the dial. Anyway, it seems more aesthetic for a dial to give Local True Solar Time. Interesting project. I once considered proposing a project of a brass band-equatorial mounted in a concrete structure resembling a Hemicyclium. The copper bowl will have a cool ancient look when it weathers. Michael Ossipoff > > *From:* Michael Ossipoff [mailto:email9648...@gmail.com] > *Sent:* Tuesday, October 17, 2017 8:44 PM > *To:* Brad Thayer <wissenschaft...@verizon.net> > *Cc:* sundial list <sundial@uni-koeln.de> > *Subject:* Re: Hemicyclium correction > > > > > > > > On Mon, Oct 16, 2017 at 8:48 AM, Brad Thayer <wissenschaft...@verizon.net> > wrote: > > I am looking to make a hemicyclium-type sundial (half-hemisphere) in a > metal working class. What little I can find on them says they are > inaccurate, without being very clear on the problem. > > > > But the way, I've seen it spelled "Hemicycleum" as well. I don't know > which is correct, but "Hemicycleum" looks better, it seems to me. But I'll > use your spelling. It's probably better-accpeted. > > > > Whoever said that was mistaken. Hemicyclia are as in principle as accurate > as any sundial can be. > > > > In ancient times, Hemicyclia were devised, instead of our modern > polar-gnomon garden sundial, or our equatorial sundials, because in those > days, they weren't using our Equal-Hours for civil time (Astronomers used > it though). They were using "Temporary Hours), that divided each day into > 12 equal hours, for their civil time. > > > > To achieve that, they used a stick-tip nodus or bead-nodus. to cast a > point-shadow on the hemicyclium surface. As the Sun's declination changes > seasonally, of course the tip-nodus's path on the hemicyclium changes too. > So the hour-lines were curves, drawn so that, for any particular solar > declination, those marks divide the day (sunrise to sunset) into 12 equal > parts. > > > > As you can imagine, that makes the dial more complicated than modern ones. > "Early" doesn't always mean "simpler". > > > > ...and so there was much opportunity for error in the calclulations and > drafting,for those curved hour-lines. > > > > So maybe some hemicyclia *were* inaccurately drafted. For one thing, of > course the exact time of day wasn't as crucial in those days, and of course > the more precisely made, and prestigiously made, a sundial was, the more it > was likely to cost. > > > > But there's no reason why your hemicyclium should be made for Temporary > Hours. Make it for our modern Equal-Hours, also referred to as Local True > Solar Time (LTST). > > > > > > It appears to me the only issue is it needs to be tilted so that the > gnomon aligns with the Earth’s rotation axis; > > > > The equinox circle on the Hemicyclium, the line that the tip-nodus follows > on the day of the equinox, should be a circle parallel to the celestial > equator. > > > > And yes, if you're going to use a polar gnomon instead of a tip-nodus, > then that polar gnomon should be parallel to the Earth's axis, pointed at > the north celestial pole. ...and should go through the center of the sphere > from which the Hemicyclium is cut. > > > > But, if you're going to do that, then why make it a Hemicyclium? Why not > just a Band-Equatorial dial? You could mount a cylindrical brass band, > parallel to the equator, with a polar-parallel rod-gnomon mounted to go > through the central axis of that band. > > > > You could mount that brass Band-Equatorial on a mount of whatever kind. > For extra durability, you could mount it inside a concrete structure > resembling a Hemicyclium. The durability of a Hemicyclium, with the > simplicity and modernity of a Band-Equatorial. > > > > So then the hour-lines would just be polar-parallel lines equidistantly > drawn along the inside of the band, dividing the lower half of the band > into 12 equal parts. Of course you'd want half-hour lines too. Maybe, if > you want it to be fancier and more accurate, you could divide each hour, > instead, into 3, 4, or even 6 parts. ...depending on how much precision > and work you want. > > > > But that's a lot of work. > > > > There's a good reason why the garden-style Horizontal-Dial is the most > popular design for a stationary dial-- > > > > 1. It's easy to construct. > > > > 2. It tells the time whenever the sun is up (a Hemicyclium can do that > too). > > > > 3. Unlike a Hemicyclium, a Horizontal-Dial is readable from all > directions, provided that you're close enough to look at it from above. > > > > I'd say, forget about the analemic-edge gnomon. For one thing, that of > course hugely complicates the design and construction. For another thing, > if someone wants clock time, they can look at a clock or watch. A sundial > should give sundial time, Local True Solar Time. > > > > You can have a nearby plaque-chart that tells the corrections, what to add > to the sundial's time, to give the Standard-Time for your time-zone at > various times of year.. ...and a reminder to add an hour for > DaylightSavingTime, between the specified dates. > > > > I'd make the sundial for Local True Solar Time, equal-hours, instead of > for Temporary Hours, because, not only is that easier, but it's also the > basis Standard-Time (when adjusted for Equation-of-Time and for your > longitude). > > > > I'd suggest changing your project to a garden-style Horizontal-Dial. > > > > Michael Ossipoff > > > > >
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