In the Hemicyclium discussion, the OP mentioned having 6-inch copper tubing. So, though it was a bit off-topic, I suggested that the tubing could be used for an additional, quicker, project, to make a south windowsill sundial--a Circumference-Aprerture Cylindrical Equatorial Dial.
But, when I said that the axial dimension of the cylinder has to be at least 0.4335 times the diameter, I neglected the fact that there are south declinations as well as north declinations. (...funny, because we're in south declination now) So, with the circumference aperture in the middle of the cylinder, the cylinder's axial dimension has to be at least twice 0.4335, which is about 0.867 times the diameter. But my suggestion for marking points of the declination-lines for each hour was correct: At any hour-line, the axial displacement of a declination-line from the equinox-line is equal to the tangent of the declination times the direct distance between the circumference aperture and the intersection of that hour-line with the equinox-line That amounts to: (Tan dec)(R*2Sin(h) ). ...where h is the number hours from 12 noon.where R is the cylinderr's radius. Obviously more neatly written: (Tan dec)(DSin(h) ). ...where D is the diameter of the cylinder. ------------------------- But a cone would be better than a cylinder, because it opens toward the north, the direction from which it would be observed--making it readable from a wider-range of directions, and making the inside surface more readable in generral. The use of a cone just slightly more complicates the declination-lines, but that would take this post even more off-topic. -------------------------- I mentioned that I'd read of a drinking-cup with a hole in it being used as a cylindrical sundial. Of course if it were a Cylindrical Equatorial, orienting it just by estimation wouldn't give very accurate results. (A Cylindrical Equatorial is supposed to be a *mounted* dial, not a portable dial). But actually, maybe they were talking about a Cylindrical *Altitude* Dial. But, though that avoids the direction-estimation, the drinking-cup would need a way of hanging it in the right orientation, and so it wouldn't be much like an ordinary drinking-cup. ...and the line-marking would be complicated by the non-cylindrical shape of the cup. Michael Ossipoff 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. It appears to me the > only issue is it needs to be tilted so that the gnomon aligns with the > Earth’s rotation axis; thus the half-bowl faces south and the gnomon points > south, but the end of the gnomon that attaches to the bowl points north. > Am I missing anything? I am also looking to use an analemma-shaped gnomon > to cast the shadow on the bowl, and at least month lines for the solar > elevation. The bowl will also have a rod and bracket on the bottom to > allow it to be rotated for daylight-savings time and for local longitude > corrections. > > > > Thanks in advance -- Brad > > --------------------------------------------------- > https://lists.uni-koeln.de/mailman/listinfo/sundial > > >
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