Principle and Use of Ottoman Sundials
By Atilla Bir*
1. The definition of the day, the hour and the prayer times 
In the Islamic world the new day begins with the sunset. When the sun is lost in the horizon it is 12 or 0 hours. The interval until the next sunset is divided into 2 x 12 hours. As defined in the "ezanic hour", the start of the day is changing but the duration of the hour-intervals remains the same throughout the day (7).
Besides this, another concept of the hour remaining from the Hellenistic age, the "unequal hour" is also used. In this concept of the hour, the periods of night and daylight are separately divided into 12 equal parts in themselves. As the length of the day remains constant by definition, the lengths of the night and daylight hours change according to the season.
- The prayer times are defined as stated below (Fig. 1) (3):
- 1.1. The Evening Prayer: Is called at the moment of sunset as observed from a point with an elevation of 625 metres. In this situation it is 12 hours according to the ezanic hour and the new day begins.
- 1.2. The Night Prayer: Is called at the moment when the sun is 17° below the horizon. Subjectively, this is defined as the moment in which two objects in black and white standing together can not be differentiated.
- 1.3. The Morning Prayer: Since the Morning Prayer must have ended at the moment of sunrise, its beginning is arranged accordingly.
- 1.4. The Midday Prayer: Is called at the moment when the shadow of a stick placed perpendicular to the horizon begins to get longer (Fig. 2).
- 1.5. The Afternoon Prayer: Is called between the moments named "Asr-i evvel" and "Asr-i sani" (Fig. 2).
The "Asr-i evvel": Is defined as the moment at which the length of the shadow of the stick is equal to the length of the shortest shadow of the stick at midday of the same day plus the length of the stick itself.
The "Asr-i sani": Is defined as the moment at which the length of the shadow of the stick is equal to the length of the shortest shadow at midday of the same day plus twice the length of the stick itself.
Figure 1: The determination of the prayer times according to the positions of the sun.
Besides these definitions, during the month of Ramadan, fasting begins at down (Imsak): the moment at which the sun is 19° beneath the horizon and there is still 1 hour and 16 minutes to the sunrise (1º 4 minutes). This time is also subjectively defined as the moment at which one can begin to differentiate between two objects in black and white which are standing together. Fasting continues until the sunset. The Bayram prayer (iyd) is conducted in mosques when the sun is 5° above the horizon or 20 minutes after sunrise.
Since the exact determination of the time is only possible for the midday and afternoon prayers, a tolerance limit of 10 minutes is recognised for the times of the other prayers and the time of commencing the fast. Accordingly the time of the Morning Prayer and the commencement of the fasting can begin 10 minutes earlier, the evening and night prayers alongside the ending of the fast may be 10 minutes later.
Figure 2: The determination of the "asr-i awwal" and asr-i sani" time according to the shadow of the stick.
The "Eid al-Fitr" begins with the first observation of the crescent at the end of the month of Ramadan which lasts 29 days. The "Eid al-Adha" is celebrated 68 days after the former bayram. Thus the Eid al-fitr is celebrated in the first day of Shawwal, whereas the Eid al-Adha is celebrated in the 10th day of zilhicce, which is 2 months and 10 days later.
Although it is known that sundials were used extensively for the establishment of time in the Islamic world, in our day the original examples of sundials can rarely be seen except the ones embodied into the walls of the mosques (2, 3, 4). The sundials which had lost their importance with the extensive use of mechanical clocks after the 17th century have quickly vanished due to the wear and tear of the external effects.
2.1. Horizontal Sundials
Picture 1: The sundial near the library of Ahmet III in the 3rd courtyard of the Topkapi Palace.(2)
The sundial near the Library of Ahmet III in the 3rd courtyard of the Topkapi Palace can be given as an example of a horizontal sundial which has been protected due to its location (Picture 1). One has to climb four steps on a marble stairway in order to take a glance at the dial settled above an elevated base. Under the scale at the eastern side of the dial is seen the inscription "Ameli Suleyman Katib-i evvel" meaning that it has been built by Suleyman, the first secretary of the Treasury. At the western side the inscription "This sundial has been established during the reign of Fatih Sultan Mehmet, let the rule of Sultan Selim, the son of Sultan Mustafa, extend to the world to come, it has been renovated by Seyyid Abdullah, his guard, year 1208,-the month of Shaban (February/April 1794)" can be read (Picture 2).
Figure 3: The drawings of the horizontal sundial near the Library of Ahmet III in the Topkapi Palace.
According to these registrations this sundial has been drawn by Suleyman Bey, the secretary of the Treasury in the era of Sultan Mehmet the Conqueror (1453-1481 ) and has been rest
by: Atilla Bir