At 07:49 AM 4/15/99 -0400, you wrote:
>Dear Friends,
>
>Fernando Cabral visited the unusual sundial at Randolph Air Force Base in
>San Antonio Texas recently, and returned with some new photographs,
>including one showing the new broken part.
Dear Bob and All
I think the newly found part is one half of an Equatorial Circle. It
was
probably supported by an Horizon Circle and by a missing piece of the
Meridian Circle. The Horizon Circle may have had additional support by
braces from the two rusty machine screws shown in the photo entitled "This
is the EOT."
Inside the above circles, all of the same inside diameter, the was
probably the "HOUR ANGLE ARC" referred to in the directions, and pivoted on
the polar axis just inside the Meridian Circle. The position of the HOUR
ANGLE ARC could be read from the hour calibrations on the inside edge of
the Equatorial Circle (newly found part).
Inside all of this would be the"GLOBE" referred to in the directions and
nicely portrayed in your drawing of a "conjectural restoration." This
GLOBE could have simply had geographical or celestial coordinates on it as
you have shown, or more likely was a terrestrial globe. A third, but less
likely possibility was that it had a horizon system of coordinates. This
is suggested by the statement that it "SOLVES ASTRONOMICAL TRIANGLE ETC."
No one of these globes need be transparent. The GLOBE is probably fixed so
that it usually does not rotate. (Note the rusty holes in the Meridian and
the Longitude circles in your third photo. A rod through them would fix
the Longitude Circle with 100 W just under the missing portion of the
Meridian Circle. This is close to the longitude of San Antonio. Also note
that the polar axis in inclined from the horizontal at an angle close to 30
degrees, near the latitude of San Antonio. I think San Antonio (R.A.F.B.)
was meant to be fixed at the top of the oriented globe)
An important feature of the HOUR ANGLE ARC was that it was probably made
of two parallel flat circular plates or rings with a small space between
them that the sun could shine through. Mounted on this HOUR ANGLE ARC
should be a SLIDE (a short, sheet metal sleeve that surrounds the ARC with
holes on the inner and outer surfaces that let the sun shine through
between the plates. The SLIDE moves up and down along the ARC between 23
1/2 degrees north and south of the equator. This part of the ARC in
calibrated with dates corresponding to the declination of the Sun.
Now the directions can be interpreted:
A. SUN'S PATH AND TIME OF SUNRISE AND SUNSET
1. MOVE THE SLIDE TO DESIRED DATE.
2. MOVE HOUR ANGLE ARC UNTIL THE HORIZON IS ALIGNED WITH THE HOLE. The
outer hole of the SLIDE meets the Horizon Circle at the time and direction
of SUNRISE.
3. READ THE TIME OPPOSITE THE POINTER. Read the time on the Equatorial
Circle.
4. MOVE HOUR ANGLE FROM SUNRISE AND SUNSET--THIS REPRESENTS SUN'S PATH
FOR THAT PARTICULAR DATE. Follow the movement of the inner hole on the
SLIDE across the GLOBE. If the GLOBE is an oriented terrestrial globe the
hole traces the path of the overhead Sun over the Earth. If the GLOBE is
an Horizon Coordinate System the hole traces the path of the Sun across the
sky.
B. SUN TIME
1. SET DATE IN SLIDE
2. MOVE HOUR ANGLE UNTIL A CIRCULAR SUN SPOT IS FORMED ON THE GLOBE.
This would be a spot just under the inner hole of the slider. The Sun is
then directly over the slider and the HOUR ANGLE ARC is aligned with the
hour angle of the Sun. This shows the spot on the Earth when the Sun is
overhead at the moment.
3. READ THE TIME OPPOSITE POINTER. This would be read on the inner
scale
of the Equatorial Circle.
There, I hope you have been able to follow this. A picture or two would
have done this more briefly and clearer, but I have not yet learned how to
convert my drawings into files you can read. I have in the distant past
made a device I used in teaching very much along the lines of what I have
described. It was made of wood and Hula-hoop rings. An oriented
terrestrial globe in the sunlight shows many other interesting things.
Among them it shows the portion of the Earth in sunlight an any one time.
Thanks for listening.
Bill
