A Universal Sundial Made for Sultan Mehmet II, in the Context of Astronomical Instrumentation in late-15th Century Istanbul

In this study we present a previously-unknown astronomical and mathematical instrument. This is a sundial for all latitudes made in 1477 and dedicated to the Ottoman Sultan Mehmet II who is known for his interest in astronomy. It is a unique example of a type of instrument previously known only from Arabic astronomical texts some two centuries earlier. This sundial, which enables the user to measure time from the solar altitude throughout the year, is conceived for all inhabited latitudes (as in classical geography, this would be from the equator to about 45°). By necessity, therefore, it is based on an approximate but practical formula for timekeeping. In Islamic civilization sundials have a history of over 1,000 years, but this has yet to be documented on the basis of surviving sundials and texts – of these, universal sundials form a small but significant part. The immediate source of the design for this particular universal sundial can be identified as an Egyptian treatise on astronomical instruments from the late 13th century; however, the device itself was much earlier, maybe originating as far back as Baghdad ca. 900. The formula was known already to the earliest Muslim astronomer al-Fazārī in Baghdad ca. 750. This sundial is a mathematical device as well an astronomical one, in the sense that it was not really intended to be used as a practical time-telling device. It is so small that it would be difficult to measure time with it under any circumstances; the same is true of many hand-held astronomical instruments. It may be «universal» but one could hardly travel about with it. Nevertheless, at some time it was brought from Istanbul to Bucharest, either whilst the latter was still under Ottoman control or even thereafter. In fact, it is simply an intriguing object embodying mathematical quantities associated with an elegant astronomical formula for timekeeping, and a testimony to part of the essence of the transmission of ideas from Antiquity to the early Muslim world, thence to Mamluk Egypt and on to the early Ottoman world. And similar devices appear in Renaissance Europe. This «new» sundial reveals how little we knew previously about the astronomical interests of Mehmet II and their level of sophistication; it invites a new look at the relevant sources. This study deals with materials not yet incorporated into the current history of Ottoman astronomy. It casts light on astronomy and dialling in 15th-century Istanbul and it adds substantially to our knowledge of Mehmet II’s interest in astronomy.