The basic concept of a sundial is a familiar one in many civilisations: the change in length and/or direction of the shadow cast by sunlight can mark the passage of time. However, the simplicity of the idea masks a great deal of nuance and complexity. From the archaeological perspective, for example, since any object can cast a shadow, which objects should we classify as sundials? From the technical perspective, as a second example, which measurements of which time periods at which location and with what accuracy were intended? In ancient Egypt, we are fortunate enough to have a few examples of texts which deal with sundials, some hieroglyphs which depict sundials, and several objects which match these descriptions. Indeed, we have enough objects to make a rough classification into five distinct "types" of sundial:
L-shaped sundials (also known as L-shaped shadow clocks or shadow sticks)
Sloping sundials (also known as sloping shadow clocks)
Semicircular sundials (vertical planar sundials)
Concave sundials (spherical and conical sundials and variants)
Miscellaneous sundials (sundial candidates, sundial-related objects, etc.)
Of these, the first two types (L-shaped and sloping) are unique to ancient Egyptian culture. The semicircular type is not unique to Egypt, but seems likely to originate or to have been independently invented there. Concave sundials are common throughout the Greco-Roman world - we list here only those that were found in culturally Egyptian locations.