The History of Sun Clocks, Water Clocks and Obelisks

Sun Clocks, Water Clocks and Obelisks

It wasn’t until somewhat recently -- at least in terms of human history -- that people felt the need to know the time of day. Great civilizations in the Middle East and North Africa first initiated clock making some 5,000 to 6,000 years ago. With their attendant bureaucracies and formal religions, these cultures found a need to organize their time more efficiently.

The Elements of a Clock 

All clocks must have two basic components: They must have a regular, constant or repetitive process or action by which to mark off equal increments of time.

Early examples of such processes include the movement of the sun across the sky, candles marked in increments, oil lamps with marked reservoirs, sand glasses or "hourglasses,” and, in the Orient, small stone or metal mazes filled with incense that would burn at a certain pace.

Clocks must also have a means of keeping track of the increments of time and be able to display the result.

The history of timekeeping is the story of the search for ever more consistent actions or processes to regulate the rate of a clock.


The Egyptians were among the first to formally divide their days into parts resembling hours. Obelisks – slender, tapering, four-sided monuments – were built as early as 3500 B.C. Their moving shadows formed a kind of sundial, enabling citizens to partition the day into two parts by indicating noon. They also showed the year's longest and shortest days when the shadow at noon was the shortest or longest of the year.

Later, markers were added around the base of the monument to indicate further time subdivisions.

Other Sun Clocks 

Another Egyptian shadow clock or sundial -- possibly the first portable timepiece -- came into use around 1500 B.C. to measure the passage of "hours." This device divided a sunlit day into 10 parts, plus two "twilight hours" in the morning and evening.

When the long stem with five variably spaced marks was oriented east and west in the morning, an elevated crossbar on the east end cast a moving shadow over the marks. At noon, the device was turned in the opposite direction to measure the afternoon "hours."

The merkhet, the oldest known astronomical tool, was an Egyptian development in around 600 B.C. Two merkhets were used to establish a north-south line by lining them up with the Pole Star. They could then be used to mark off nighttime hours by determining when certain other stars crossed the meridian.

In the quest for more year-round accuracy, sundials evolved from flat horizontal or vertical plates to forms that were more elaborate. One version was the hemispherical dial, a bowl-shaped depression cut into a block of stone that carried a central vertical gnomon or pointer and scribed with sets of hour lines. The hemicycle, said to have been invented around 300 B.C., removed the useless half of the hemisphere to give an appearance of a half-bowl cut into the edge of a squared block. By 30 B.C., Vitruvius could describe 13 different sundial styles in use in Greece, Asia Minor and Italy.

Water Clocks 

Water clocks were among the earliest timekeepers that did not depend on the observation of celestial bodies.

One of the oldest was found in the tomb of Amenhotep I who was buried around 1500 B.C. Later named clepsydras or "water thieves” by the Greeks who began using them around 325 B.C., these were stone vessels with sloping sides that allowed water to drip at a nearly constant rate from a small hole near the bottom. 

Other clepsydras were cylindrical or bowl-shaped containers designed to slowly fill with water coming in at a constant rate. Markings on the inside surfaces measured the passage of "hours" as the water level reached them. These clocks were used to determine hours at night, but they may have been used in daylight as well. Another version consisted of a metal bowl with a hole in the bottom. The bowl would fill and sink in a certain time when placed in a container of water. These are still in use in North Africa in the 21st century.


More elaborate and impressive mechanized water clocks were developed between 100 B.C. and 500 A.D. by Greek and Roman horologists and astronomers. The added complexity was aimed at making the flow more constant by regulating the pressure of the water and at providing fancier displays of the passage of time. Some water clocks rang bells and gongs. Others opened doors and windows to show little figures of people or moved pointers, dials and astrological models of the universe.

The rate of flow of water is very difficult to control accurately, so a clock based on that flow could never achieve excellent accuracy. People were naturally led to other approaches.

Mechanized Clocks 

A Greek astronomer, Andronikos, supervised the construction of the Tower of the Winds in Athens in the first century B.C. This octagonal structure showed both sundials and mechanical hour indicators. It featured a 24-hour mechanized clepsydra and indicators for the eight winds from which the tower got its name. It displayed the seasons of the year and astrological dates and periods. The Romans also developed mechanized clepsydras, but their complexity accomplished little improvement over simpler methods for determining the passage of time.

In the Far East, mechanized astronomical/astrological clock making developed from 200 to 1300 A.D. Third-century Chinese clepsydras drove various mechanisms that illustrated astronomical phenomena.

One of the most elaborate clock towers was built by Su Sung and his associates in 1088 A.D. Su Sung's mechanism incorporated a water-driven escapement invented around 725 A.D. The Su Sung clock tower, over 30 feet tall, possessed a bronze power-driven armillary sphere for observations, an automatically-rotating celestial globe, and five front panels with doors that permitted the viewing of changing manikins which rang bells or gongs. It held tablets indicating the hour or other special times of the day.

Information and illustrations provided by the National Institute of Standards and Technology and the U.S. Department of Commerce.