Sundials have been around for several thousand years. Although they have lost their practical value as time pieces, they still seem surprisingly popular. (Just go and google for "sundials"!)
I think the reason for the interest in these after all rather simple "obsolete" technical contraptions is that in reality they represent an intriguing combination of
HISTORY, ASTRONOMY, GEOMETRY, MATHEMATICS and last not least ART.
Most of them require an exact knowledge of the direction of geographic North for their installation.
While that constitutes in general no major problem for stationary sundials, it is rather inconvenient for portable ones. Therefore the design of the latter normally allows them to be used without exactly knowing where North is.
Below is an example of a simple portable specimen I found here:
Pict.1 Embroidery hoop sundial
This one is not difficult to make, the parts are readily available and cheap, and with some care it can even indicate the time. :-)
I decided to make a copy .
The bad news is that my product failed miserably because during those days the rubber washer (located at the center of Pict1), whose shadow is supposed to indicate the time, lay in the shadow of the frame and therefore the shadow of the washer vanished just when you needed it most.
In other words I found out the hard way that simple equatorial ring sundials do not work near the equinoxes.
And it was March 19! Murphy's law...
So, a way around this problem had to be found. Here is a picture of the result:
As can be seen the little rubber washer has been replaced by a music cassette box with (cleaned) CDs attached to its largest sides. The CDs can be rotated around the center screw to adjust for the declination of the sun.
Since the holes of the CDs are much to big I filled each with a stainless steel washer with its external diameter close to the diameter of the hole in the CD. Each CD is held in place by a 10-32 screw and a coin with a hole drilled into its center.
Each CD carries two IC sockets. The upper socket has two holes. The holes of the lower sockets have been filled with diminutive white cardboard disks.
Reading the Time
1)Get the declination of the sun and turn the CDs around their center screws so that the thin lines coincides with that angle.
2)The long steel rod must be at an angle with the horizontal plane equal to the latitude of your location and have the round steel grip (at the right edge near the top of the picture) point approximately north. The outer ring must be vertical. (you can suspend it with a piece of string for that purpose or mount it in the correct fixed position as shown in the picture)
3)Rotate hoop and cassette so that the sunlight falling through the hole(s) in the upper IC socket hits the corresponding white spot(s) in the lower one. Since there are altogether four holes at distances that are larger than the hoop width, there will be always some that do not lie in the shadow of the hoops.
The red pointer sticking out from the center of the small side of the cassette box at the lower right of Pict.2 points to the sun time on the inner hoop. Do not forget to correct the time indicated by the sundial in accordance with the Equation of Time.
Evidently things have become a little bit more complicated. But now the embroidery hoop sundial works also during the equinoxes.
The idea for this type of mechanism came from a close look at Robert Felix' Sunmaster :
Pict.2 - The final product
A careful look reveals that the KARFEHS* and Sunmaster design are based on the same principle.
*Karen and Rainer´s Funny Embroidery Hoop Sundial
SUPER MODERN CHEAPO
(MORE OR LESS) UNIVERSAL ARMILLARY or RING SUNDIAL
Note: The hour marks on the inner ring are spaced at 15 degrees intervals with the noon mark placed at the lowest point.