The question is not as easy as it seems. Sundials used by the Romans and that
are more in number and the most common ones are spherical and concave :
hemisphaerium , hemicyclium , and those who called Glass " roof spherical
sundials ." The issue is that often , to facilitate the processing of marble ,
were adopted sundials with concave conical faces . Less used , but still famous
, are the vertical and horizotnal sundials. The Horologium of Caesar Augustus
in the Campus Martius was excavated only with regard to the meridian line , but
there is no certainty that it is treated only a meridian line or a complete
sundial . The prints show sundials strange and approximate, but the one by
Athanasius Kirche in Obeliscus Pamphilius (XVII century ) is perfect and is at
temporal hours . If it is difficult to think of a possible reading of a sundial
with hour lines so long , it is not so easy to think of an easier reading even
on the meridian line . (transtaled with Google) The best wishes, Nicola Severino
----Messaggio originale----
Da: sche...@fas.harvard.edu
Data: 22/10/2013 18.02
A: "saskauf...@gmail.com"<saskauf...@gmail.com>,
"sundial@uni-koeln.de"<sundial@uni-koeln.de>
Ogg: RE: Shape of Greek and Roman sundials
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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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