L-Shaped Sundials

L-shaped sundials (also called shadow clocks or shadow sticks) are first attested in the New Kingdom. They are the rarest type of Egyptian sundial and are possibly the earliest of the formal types that have survived. However, they are also the only type of sundial for which an explanatory text has been found (SL Osireion) and one of two types of sundial (with sloping sundials) that are used as hieroglyphs.

Objects

In this database, we have included information about two surviving instruments:

SL Berlin ÄM 19743: L-shaped Sundial ÄM 19743 in Berlin 

SL Berlin ÄM 19744: L-shaped Sundial ÄM 19744 in Berlin

two partial instruments:

SL Berlin ÄM 14573: L-shaped Sundial Gnomon ÄM 14573 in Berlin

SL Louvre N 781: L-shaped Sundial Gnomon N 781 in Paris

and two pieces of documentary evidence:

SL BM EA10673: L-shaped Sundial on Papyrus EA10673 in London from Tanis

SL Osireion: L-shaped Sundial Text in the Osireion

Characteristics

The characteristics of L-shaped sundials are:

  1. L-shape - a short vertical gnomon attached to a horizontal plattern, scale, ruler, or base.
  2. Fixing for a plumb bob: usually the top face and/or sides of the gnomon display small holes which could be plumb bob attachment points
  3. Reference line for plumb bob: a vertical line on one or both sides of the gnomon, used to align the plumb bob thread in order to keep the instrument level.
  4. For timekeeping, only the length (and not the direction) of the shadow is considered.
  5. Flat (linear) shadow-casting edge: more familiar types of sundial use a pointer or even a hole.
  6. Flat shadow-catching surface:the top surface of the horizontal plattern, scale, ruler, or base is flat and parallel to the bottom.
  7. Hour marks: circles, holes, or perhaps lines marking lengths long the scale from the gnomon outwards. The marks get further apart the further away from the gnomon. The text SL Osireion specifies that the length ratio between these marks should be 3:6:9:12. Surviving examples continue this to a fifth mark 15 units from the fourth.

Note that characteristics 1-3 are identical to those of a merkhet level, a type of surveying or building instrument. The relationship between these instruments is extremely close. Speculatively, one could imagine that the L-shaped sundial represents an extension in utility of the merkhet. Or one could speculate that all merkhets could have been intended to be sundials rather than levels. The existence of SL Osireion and, to a lesser extent SL BM EA10673 (a papyrus from the Roman era), show that the opposite perspective (that all L-shaped sundials are merely levels, not timekeeping devices) is not true.

Characteristics 2-5 are shared with sloping sundials. Hieroglyphic representations of sundials suggest that sloping sundials emerged from a set of variants of L-shaped sundials (although no examples of the others survived as objects). The benefit of a sloping sundial is that the instrument can be much more compact.

"Crossbar" sundials

Some researchers (starting with Ludwig Borchardt in 1910) believe that this type of sundial is missing an entire piece, a crossbar similar to a cubit rod, that in fact formed the shadow-casting edge. Reconstructions of L-shaped sundials in museums often include this addition. Symons (the main author of this database) does not.

For more information about the crossbar argument, see:

Borchardt, L., 1910. Altägyptische Sonnenuhren. Zeitschrift für Ägyptische Sprache und Altertumskunde, 48, pp.9–17.
Borchardt, L., 1920. Die Altägyptische Zeitmessung. Berlin: W. de Gruyter.
 
Bruins, E.M., 1965. The Egyptian shadow clock. Janus, 52, pp.127–137.
Symons, S.L., 2002. Egyptian shadow clocks. In: M. Dorikens, ed., Proceedings of the XXth International Congress of History of Science Vol XVI. Turnhout: Brepols, pp.13–20.