This is a type of non-planar portable clock which is constructed using a thin ring which is it's own gnomon. A single hole in the ring allows sunlight to fall on the interior surface to read the time.
To use it correctly, the ring should be suspended and rotated about the vertical toward the sun until a ray of sunlight falls on the correct date line. This way the vertical plane that forms the ring should be aligned to the sun.
The parameters that define this clock type are as follows:
WIDTH OF THE RING
The ring width must be sized to accommodate an easily read calendar.
RING RADIUS
This is the cylinder radius the ring is constructed from. To be reasonably accurate, the radius should be sized to accommodate an easily readable calendar without being too large to be portable as well as too large to be easily aligned
ANGLE OF THE HOLE WITH THE VERTICAL
This is the angle between the hole and the vertical. It is measured from the center of the ring and from the vertical. This angle must equal the local
latitude.
