Semicircular sundials are a type of sundial found in ancient Egypt and many other cultures. Their characteristic shape is using symmetrical, indicating that the sundial was designed to be used with the noon line, gnomon (always now missing), and noon shadow forming a triangle in the North-South plane.
In this database, we have included information about eleven semicircular sundials. Six are portable:
Two are located on rock faces:
One formed part of a free-standing permanent sundial:
and two are indeterminate (they could orignally have been permanently located, but then fell or were removed from that location):
The characteristics of semicircular sundials are:
- Shape: semicircular, with the long flat edge being horizontal at the top of the dial.
- Radial hour lines: issuing from the centre point of the semicircle, midway along the top edge. Usually there are an odd number of lines (11 or 13) with the outer lines forming the long flat top edge and the middle line at right angles to them, forming the noon mark.
- For timekeeping, primarily the direction (rather than the length) of the shadow is considered.
- Gnomon hole: at or very near the convergence of the hour lines, a small circular hole where the gnomon (presumably of metal, wood, or ivory) would be attached. No dials with intact gnomons have been found, so their shape and length is not certain.
- Flat shadow-catching surface: the semicircular surface is flat, not curved.
If the instrument is portable, holes for attaching or suspending the instrument may be visible. If the instrument was designed to be stationary, the flat shadow-catching surface is vertical or near-vertical.