Dear Sasson, Your sundial looks pretty, but it does not seem cylindrical to me in the photo.
As for your questions concerning the preference of the Greeks and Romans to concave spherical sundials, there are several answers: 1. The bowl mirrored the spherical shape of the heavens, and this was satisfying from a cosmological and philosophical standpoint. 2. Finding time to the hour was accurate enough for public lives, and indeed, many thought it was more of a bother to divide the day into twelve parts. Other cultural divisions of the day were also in use, and had names like, the time of the cock crowing. 3. It is a misconception to think that the Greeks and Romans strongly preferred concave sundials. Perhaps the most famous of Greek sundials is the Tower of the Winds in Athens. It has eight vertical sundials with pin gnomons. The most famous Roman sundial was likely the large horizontal azimuth dial in Rome which used an Egyptian obelisk for a gnomon. Romans also used altitude dials, including the pillar dial. Cheers, Sara Sara J. Schechner, Ph.D. David P. Wheatland Curator of the Collection of Historical Scientific Instruments Department of the History of Science, Harvard University Science Center 251c, 1 Oxford Street, Cambridge, MA 02138 Tel: 617-496-9542 | Fax: 617-496-5932 | sche...@fas.harvard.edu http://www.fas.harvard.edu/~hsdept/chsi.html From: sundial [mailto:sundial-boun...@uni-koeln.de] On Behalf Of Sasson Kaufman Sent: Tuesday, October 22, 2013 6:40 AM To: sundial@uni-koeln.de Subject: Shape of Greek and Roman sundials Hi. Greek and Roman sundials measured temporal time. They were generally spherical or conical, and, if I understand correctly, their surface was designed to provide a mirror image of the sun's paths in the sky throughout the different seasons. These paths, whether short or long, were then divided into twelve ostensibly equal parts. Yet this seems rather odd, since the sun's orbit remains at the same tilt (depending on altitude) throughout the year, only going back and forth, thus creating the shape of a tilted tunnel or cylinder. If the objective of the dial was to divide the day into equal hours, wouldn't it have been more appropriate for the surface of the dial to be that of the inner part of a cylinder? Shaping the surface as a sphere or cone would seemingly distort the shape of the sun's path projected on the surface and complicate the calculations for line plotting. This leads me to suspect that the ancient dials were of limited accuracy. With this question in mind, I recently created a portable dial (see the attached image) with a cylindrical surface tilted according to latitude, and so far, to the best of my reckoning, it's precise to about a minute. The dial was created using TinkerCad, and printed in 3d. Plotting the lines for my dial was fairly simple using paper. I created a 2d insert equal in shape to that of the cylinder surface, divided it into 12 equal-width parts, and glued it to the dial. I haven't found though the formula to do this in 3d. My questions- - Why did the Greeks and Romans prefer spheres and cones over cylinders which presumably would have been more accurate and simpler to plot the lines on? - Would anyone here have an idea how to plot the lines for my dial using a CAD program? Thanks in advance, Sasson Kaufman
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