Correction: I'd said:
(Tan dec)(R*2Sin(h) ). ...where h is the number hours from 12 noon.where R is the cylinder's radius. Here's the correction: Instead of "hours from 12 noon", It should say: "...where h is 15 degrees times the number of hours from 6 a.m., during the a.m. hours, or the number of hours from 6 p.m., during the p.m. hours." ...which could also be said as: " 15 degrees times (6 minus the number of hours from 12 noon)". ...for the hours from 6 a.m. to 6 p.m. Michael Ossipoff On Mon, Oct 23, 2017 at 7:31 PM, Michael Ossipoff <email9648...@gmail.com> wrote: > > 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 >> >> >> >
--------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial
