Sunday, September 28, 2014

Archeoastronomy: Sundials

As societies became more complex, people had to organize their timekeeping into smaller increments, especially during the daylight hours. Whether they had to establish a meeting time for a council session or a starting time for a religious ritual, people needed to devise a way to coordinate their time. Beyond “morning”, “noon,” and “evening,” they needed a finer division of time such as “hours.”

 Sundials were invented to solve this problem. Because they measure small intervals of time using cosmic activity, sundials were considered to be one of the earliest scientific instruments. The Egyptians were the first people recorded in the West to use the sundial. Their sundial consisted of the gnomon (a crossbar) and a flat base divided into six “hours.” In the morning, the gnomon faced east, and the afternoon west. The shadow cast by the gnomon on the base determined the hour.

 Using their knowledge of geometry, the Greeks refined the sun dial of the Egyptians. They invented the hemicyclium (hemispherium), which is a block of stone with a hemisphere cut into it. The gnomon was placed on top. The half-sphere created a circular arc that determined time more accurately. The hemicyclium resembled a hollowed out bowl with a stick on top.

 Sundials used in Europe were adapted from the Greeks and the Egyptians. These instruments featured twelve “hour” days and nights. However, the Norse and Saxons based theirs on the ebb and flow of tides. They marked two high and two low tides, which were further divided into halves and quarters for a total of sixteen “hours.”

One major problem with ancient sundials is that they measured “unequal” (“temporary”) “hours.” Summer, which has longer days, produces longer “hours,” than winter. Moreover, the further north (or south) from the Equator, the location of the sundial is, the more pronounced the difference between summer and winter “hours” are.

 In the 1300s, Abu’l-Hasan (Ibn al-Shatir), an Arab Muslim astronomer and religious timekeeper, invented “equal” (“equinoctial”) “hours” based on trigonometry. He reasoned that a gnomon parallel to the Earth’s axis (i.e. polar axis) would produce “equal hours.” “Equal hours” are measured from the passing of the low meridian (12 Midnight) until the next low meridian, and then divided into twenty-four hours. Once sundials were adjusted to the latitude of where they were, they kept accurate time. People, then, used sundials to set mechanical clocks.

 Sundials are divided into two groups – Altitude and Azimuth. The Altitude sundials determine time by the sun’s altitude. These Altitude sundials were either aligned with either the sun or the cardinal directions. Azimuth sundials determine the time from the “hour angle of the sun.” (Azimuth refers to direction of the sun’s angle.) These sundials are oriented by a compass.

 Example of an Altitude sundial is the pole (pillar) dial, which is a pole with a gnomon at the top. Meanwhile Azimuth sundials are often found in gardens. These are the tilted horizontal ones with the gnomon inclined at the latitude of the garden.

 As society became more complex, instruments to tract time in finer increments were invented. Though seemingly simple, sundials are actually intricate devices for keeping time. Because of their importance, people, over the centuries, worked to improve sundials. In fact, sundials were still being used in the 19th Century to set mechanical clocks.

 Works Used:
 Ling Liew Huay and Yee, Lim Siew, “The Mathematics of Sundials,” National University of Singapore, 2001,
 Marie, Niclas, “When Time Began: The History and Science of Sundials,” TimeCenter, 2014, .

 Nordoff, Helena, “Fun in the Sun: A Sundial Tutorial,” 21 August, 2003,

 “Unit 6: Sundials,” Polaris Project, Iowa State University, 2001,
 Ward, John and Margaret Folkard, “Sundials Part 2: Definitions and Basic Types,” Royal New Zealand Institute of Horticulture, 1997,

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