How it began
Ibn al-Shatir, who died in 1375, was the muwaqqit, or timekeeper, of the Umayyad Mosque in Damascus. In 1371 he designed and built a horizontal sundial that was installed on the mosque's northern minaret, the Minaret of the Bride. The dial was part of the mosque's timekeeping equipment and was used chiefly to determine the times of the five daily prayers. The idea of using hours of equal length throughout the year built on earlier trigonometry by al-Battani.
Before it existed
Sundials with an upright gnomon show seasonal hours, which divide daylight into twelve parts that are longer in summer and shorter in winter. Ibn al-Shatir's own dial still included scales for these unequal hours, counting hours since sunrise and hours remaining until sunset, alongside the new equal-hour markings.
How it works
The dial is engraved on a marble slab about 2 meters by 1 meter. Its gnomon is not perpendicular to the plate. It is tilted to lie parallel to the Earth's rotational axis, at about 33.5 degrees to the horizontal, the medieval value for the latitude of Damascus. The shadow of this sloping edge marks equinoctial hours that stay the same length on every day of the year. Three dials share the slab, with subdivisions down to about four minutes and curves marking prayer times.
What lasted
The instrument is described as the oldest polar-axis sundial still in existence. Ibn al-Shatir also made a much smaller sundial and a compendium called the jewel box that combined a universal sundial with a magnetic compass, used to find prayer times, the local meridian and the direction of Mecca.
Where it stands today
The original was damaged beyond repair, and a replica made in 1890 stands on the Minaret of the Bride at the Umayyad Mosque. The northernmost of the three dials has lost its vertical gnomon. The slab still shows the engraved curves for the afternoon and daybreak prayers that tied the instrument to the daily routine of the mosque.
