Sun clocks are clocks which use the movements of the sun to tell time. These clocks are somewhat limited in scope, as they do not work at night, but they are the earliest form of clock in the world, and they laid the groundwork for the invention of later timekeeping devices. People continue to use sun clocks in a variety of settings, both as novelties and as actual working clocks which help communities keep track of time.
The earliest sun clocks divided the day into two parts: morning and afternoon. These clocks were devised by establishing a large pillar or obelisk and tracking the movement of the shadow cast by this object as the sun moved across the day. The middle of the day occurred when the shadow was shortest, indicating that the sun was directly overhead. Numerous Middle Eastern cultures used this basic design, as did the Egyptians.
Eventually, the two-part day was deemed too simplistic, and societies started adding markers to create more segments in the day, creating sun clocks which would be recognizable to modern users, even if the length of the hours might be slightly different than we are accustomed to. Several cultures also devised versions which could change to adapt to the seasons, recognizing that the sun changes position in the sky over the course of the year, and some sun clocks marked out major events like solstices and equinoxes as well as the hours.
The sun dial is a classic example of a sun clock, although numerous variations on the design were developed by ancient cultures. Some communities also developed human sun clocks, in which a person stood on a clearly marked date bar and checked the time by seeing where his or her shadow fell. Human sun clocks are sometimes installed on the grounds of science museums and educational centers to get people interested in the history of timekeeping and sun clocks.
In order for sun clocks to work right, they must be positioned properly. Someone installing a sun dial at a northern latitude, for example, needs to angle the device so that it will reflect time accurately when one considers the position of the sun in relation to the clock. The same position used in Kansas City, Missouri would not work in Cape Town, South Africa, because the position of the sun in the sky is different. One way to do this is to orient a sun clock so that the shadow falls on noon when the sun is directly overhead, although there are also scientific calculations which can be used to position a sun clock.