The Function of Sundials in Ancient Civilizations

The sundial is the oldest known device used for the measurement of time. The history of sundials dates back to approximately 3,500 years ago. The function of a sundial depends on the position of the Sun in the sky at a given time. It´s based on the fact that the shadow of an object will move sideways as the Sun travels from east to west. Before the invention of clocks and watches, sundials had been used by diverse civilizations, to tell time. The purpose of historical sundials for early civilizations might have been to maintain a record of time throughout the day.

The earliest and simplest type of sundial is the shadow stick (gnomon). The time of day is determined by the position of the shadow cast on the ground.  There are many types of sundials. Some sundials use the edge of a shadow to tell the time. Others use a line of light. On a shadow casting sundial, the sundial´s gnomon may be a thin rod or any other object with a sharp end. Sundials utilize distinct types of gnomon; a gnomon may be rigid or movable according to the season. The surfaces of sundials may also be plane, spherical, cylindrical, or conical. The use of many sundials requires knowing the local latitude and a precise vertical orientation.

The Egyptians were the first civilization known to have used sundials. Their sundials consisted in a t-shaped cross rod with a vertical stick, which was marked with five lines, representing five hours. In the morning, the stick would be placed facing east to measure the hours, and later, the stick would be placed facing west to measure the following 5 hours. Later, the Egyptians built obelisks at the entrance of their temples. Obelisks are tall four sided structures that end with a pyramid-like top. The Egyptians built obelisks, utilizing the shadow cast of a sundial to calculate time. Obelisks also helped them calculate the longest and shortest day of the year (solstices). For the Egyptians, Obelisks represented the flesh of Gods. Overtime, the Egyptians built smaller versions of the obelisks to use them as portable sundials.

The Greeks created a sundial known as the “pelekinon” which consisted on placing a vertical rod on a horizontal half spherical face. The sundial was marked to help them more accurately predict time in the course of a year. Based on this idea, they later invented the hemicycle which consisted of a cubical block of wood or stone that is cut in half and a rod attached on one end. The hemicycle varied in length depending on the season of the year. The hemicycle was later divided into 12 equal quadrants, indicating the length of each day. The Tower of Winds in Athens used eight sundials, each, facing a cardinal point.

In china, sundials date back to (1368-1644) in the era of the Ming Dynasty. The Rigou sundial, as it was known in China, comprise a base made of stone, which is aligned with the equatorial plane, and a gnomon that is placed perpendicular to the base. As in any culture, time keeping was an important function in ancient China. In China, the emperor was considered the basis of the norms of time. Two examples of Rigou sundials can be seen: one at the Beijing Ancient Observatory, and another at the Hall of Supreme Harmony.

There are three different types of sundials: the horizontal, equatorial, and vertical sundials. The base of the horizontal type is set horizontally, and the gnomon is slanted according to the Earth´s axis.  The base of the equatorial sundial is aligned with the equatorial plane, while the gnomon is positioned perpendicular to the base. The base of a vertical sundial, which is the most common, is placed vertically, while the gnomon is placed in line with the Earth´s axis. To tell the time accurately, vertical sundials must be designed for specific Earth latitudes and longitudes. The Sun´s rotation is also taken into account when designing and finding a location for a sundial.