Usually the site decides the size, but if you can make it any size
you wish, I'd suggest a major axis of about 4 meters. Obviously
these "human-gnomon" dials aren't designed to be read to within a few
minutes of time, so half-hour or even just hour marks will suffice.
It helps if the surface is flat, smooth, and level, but that isn't
necessary. It's easier to use the dial if the user faces his/her
shadow with arms held up.
The analemma on the dial in your photo may be thought of as an
informational decoration. It gives the EoT and would help convert
the dial time to clock time, should that be important to anyone.
However, putting the analemmatic curve in the location shown invites
users to stand on the date point of the analemma, which isn't
correct. The user must stand on the minor axis of the ellipse.
Although Zonwvlak doesn't design analemmatic sundials, it will give
you the x,y coordinate data for an analemma for noon, which would be
what you want.
On the analemmatic sundials I've laid out (usually with school
children), I've established the meridian line for a "y" axis
(increasing to the north), drawn an east-west line for the "x" axis
(increasing to the east), then, with their intersection as the
origin, used x,y coordinates to locate the hour points and
declination (date) marks. If S is the size of the major axis, L the
latitude of the place, H the hours from noon (negative for morning),
and D the declination of the sun, then, for the hour points,
x = SIN(15*H)*S/2 and y = COS(15*H)*SIN(L)*S/2.
For the declination (or date) points,
x = 0 and y = TAN(D)*COS(L)*S/2.
The COMPENDIUM for September 1995 (Vol 2 No 3) has articles about
designing and constructing analemmatic sundials, including a Basic
program to generate the x,y coordinate data for the hour points and
date marks. The digital edition has the program in both text and
.exe form.
Good luck with your project,
Mac Oglesby
sorry....forgot the photo! :)
-----Original M essage-----
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of Pace
Sent: Thursday, July 15, 1999 4:36 PM
To: [email protected]
Subject: analemmatic sundial
Hello sundialists...
I am thinking about building a large analemmatic sundial, the one
where you use your own shadow to read the time.
In practice, the final result i want would be something similar to
the dial I am attaching to this mail (its only a 6kb .jpg file)
I would like to ask some questions, if anyone would be kind to
answer, i would be very grateful.
What is the minimum major axis diameter of the ellipse to be
constructed if the time has to be read clearly? I intend only to
mark hour and half hour markings on the dial.
How may I construct the analemma (as seen in the attached
photograph) ? Programs like zonwlak dont seem to design analemmatic
sundials. How large has this to be?
Finally, what would be the best way to lay out the ellipse and to
make the markings on the ground.
Looking forward to your insights..
regards
Alexei Pace
Lat: 35°50'33"N
Lon: 14°32'44"E
Marsaxlokk, MALTA
Attachment converted: Macintosh HD:analemmatic.jpg (JPEG/JVWR) (000067D9)
