Theory and Practice of Ottoman Sundials

The sundials are composed of a stick giving a shadow perpendicular to a table or wall plane. On a sunny day the shadow of the stick-end on a plane follow a hyperbolic curve. The sundials are generally classified as being spherical, horizontal or vertical. Most of the Greek and Roman sundials are spherical and few of them cylindrical or conical. The most of the sundials were also in use in early Islamic time. But, in contrast the sundials construct in Islamic period are generally horizontal or vertical dials.

On Islamic horizontal sundials other then the gnomon perpendicularly placed to the horizon, they are generally a second stick or stretched wire (polos) oriented parallel to the earth axis and making an angle of which is equal to the latitude j of the lieu. Since during one day the sun makes a tour around the earth axis it also makes a tour around the polos sweeping each hour an angle of 360°/24 = 15º and each degree correspond to 4 minutes. On a plane perpendicular to the earth axis the shadow of the polos give the hour angles directly. But since on horizontal sundials the dial is not perpendicular on the earth axis the hour angles have to be projected to the dial plane making an angle of j latitude degrees with the earth axis.

In this paper, the time concept used by the Ottomans is briefly summarised. Secondly the theory based on this time concept applied to the Ottoman gnomonic. And at the end some horizontal and vertical Ottoman sundials are investigated in detail. The use of this dials on day life praying and legal (Sheri) time of Ottomans sundials are discussed.

This presentation is based on work co-authored by Burak Barutçu.