ʿAbd al-Raḥman al-Ṣūfī (903-986 AD) describes the construction and use of a water clock in two of his treatises on the astrolabe. The most elaborated version is the one preserved in MS Paris, BNF, Arabic 5098 (ff. 220r-223v), in chapters 73-80 of book XV. According to the description, the water clock is a large, concave, cone-shaped or hemispheric bowl, with a small hole at its lower part, which slowly allows water to enter inside it, when the water clock is placed on water. An astrolabe is used for the graduation of the water clock. It can be graduated in equal (equinoctial), or unequal (seasonal) hours; there are two explanatory diagrams on ff. 221r and 223v. The graduation starts at the sunrise; the water clock is placed on the water, while the time is measured with the astrolabe; every one hour, marks are put on the water clock at the level of the water, which will later be connected to produce a circle for this hour. This procedure may last two or three days, until the entire height of the water clock will be graduated.
There are instructions how to set up the water clock for the sunrise, the sunset, or any hour after them, also for determining the time that a birth occurs in day- or nighttime, when the day is cloudy and the astrolabe cannot be used.

Studies on Ottoman timekeeping traditions have highlighted the use of a seasonal timekeeping system, where instruments like the quadrant and sundial measured daylight hours. However, the methods for determining the time between sunset and sunrise and the instruments employed remain underexplored.
Our preliminary literature review has identified several sources that motivate us to address this gap: Ottoman navigator Seydi Ali b. Hüseyin’s Kitabü’l Muhit fi İlm el-Eflak ve’l-Ebhur (1562) explains nighttime timekeeping for sailors, linking seasonal hours with wind intensity variations. Similarly, Ali ibn Ahmed al Sharfi’s Mediterranean Sea Atlas (1571) incorporates North African traditions, detailing how lunar phases and positions determined nighttime hours, supported by arithmetical calculations. Evliya Çelebi’s Seyahatnâme (1682) mentions fifteen sandglass workshops in seventeenth-century Istanbul, underscoring sandglasses’ role in navigation and daily life. An anonymous 17th-century Ottoman notebook further illustrates diverse methods, describing water clocks like the şâzervân, which operated when celestial observations were impossible.
In conclusion, this paper delves into Ottoman society's practical needs for nighttime timekeeping. It explores how instruments and methods were adapted to meet the specific demands of groups such as sailors, muwaqqites, and urban dwellers. We reveal the intersection of timekeeping, social needs, and technological adaptability in Ottoman society by addressing questions like how nighttime hours were measured during obscured celestial phenomena and how tools differed across social groups.
Keywords: timekeeping, night, sand-glass, water-clock, Ottoman society

The measurement of time or duration of an event in the night is achieved in various ways, the most popular being the water clock. The functioning of water clocks in India is different from the European water clocks. The bowl with a hole is immersed in water and the time needed for it to sink is calibrated. The variants of this method and the descriptions in the classical texts for the preparation of the bowl will be described. There is a couple of interesting aspects – the first is its usage even in modern times as a ritual. The second one is a masonry construction like a pool resembling fountains in a modern par. These are now abandoned but are known for measuring longer intervals of time during medieval periods. Another unique way has been the composition of specific Sanskrit verses and its recitation to measure time, a very practical tool in total darkness. Yet another method utilises a handy instrument using the two stars of Ursa Minor. Ursa Major is not circumpolar for Indian latitudes.