Water clock

A water clock, also called a clepsydra, is one of the oldest ways people measured time. It works by letting water flow slowly into or out of a container. By watching how much water had moved, people could tell how much time had passed.

Water clocks have been used for thousands of years. The simplest ones were found in places like Babylon, Egypt, and Persia as far back as the 16th century BC. Other countries, such as India and China, also used them. Important thinkers from ancient Greece and ancient Rome wrote about how these clocks were made and used.

Designs

A water clock measures time by letting water flow. There are two main types: outflow and inflow. In an outflow water clock, a container full of water drains out slowly. Marks on the container show how much time has passed as the water level goes down. In an inflow water clock, the container starts empty and fills with water. Marks on the container show how much time has passed as it fills up.

The Greeks and Romans made water clocks better by adding parts like gears. These changes made the clocks more accurate and were sometimes used to move statues and other displays. Similar improvements were made in places like Byzantium, Syria, and Mesopotamia. In Europe, these ideas helped create better timepieces. Around the same time, people in China also made advanced water clocks and shared their ideas with Korea and Japan.

Early water clocks were checked against the sun using a sundial. Though not as accurate as clocks today, water clocks were used for thousands of years until they were replaced by more precise mechanical clocks in Europe around 1300.

Regional development

Egypt

The oldest known water clock comes from around 1417–1379 BC in Egypt, during the time of the pharaoh Amenhotep III. It was used in a temple area at Karnak. These early water clocks were stone containers with sloping sides that let water drip out slowly. They had twelve columns inside with markings to track the passing of time, helping priests know when to perform temple rites at night.

Babylon

In Babylon, water clocks were cylindrical and used for tracking time in astronomical studies from about 2000 to 1600 BC. Evidence comes from writings on clay tablets. These clocks measured time by the weight of water flowing out. They did not have moving hands but instead used the amount of water to show the time. The length of time changed with the seasons, so different amounts of water were needed at different times of the year.

India

Water clocks may have been used in ancient India as early as the second millennium BC. Some pottery from the Indus Valley site of Mohenjo-daro might have been used this way. Later, detailed descriptions of water clocks appear in Indian texts, describing devices that measured time in units of about 24 minutes. These clocks were used at ancient universities to mark time intervals.

China

Water clocks were very important in ancient China for studying astronomy and astrology. The oldest written record of water clocks in China dates to the 6th century BC. Over time, these clocks evolved from simple outflow types to more complex inflow designs with floating indicators. Chinese astronomers noted how temperature and humidity affected the clocks. Later inventions used mercury instead of water to avoid freezing problems.

Persia

In ancient Persia, water clocks were used from around 500 BC, especially in desert areas. They helped determine important religious days and were crucial for fairly sharing water for farming. These clocks were simple pots with small holes, and managers would count how many times the water filled the pot to track time. The Zibad water clock was used until 1965 before being replaced by modern timepieces.

Greco-Roman world

The term "water thief" comes from ancient Greek. Greeks improved water clocks by solving flow problems and adding features like dials and pointers. Romans used simple outflow clocks in courts to time speeches. Some water clocks even rang bells or showed moving figures to indicate the time. The Tower of the Winds in Athens combined a water clock with sundials and a wind indicator.

Medieval Islamic world

In the medieval Islamic world, water clocks continued to develop with roots from earlier Egyptian and Byzantine designs. The engineer Al-Jazari created advanced water clocks in 1206, including one shaped like an elephant that showed the passage of time. These clocks had to adjust daily for changing day lengths and included complex mechanisms to display time and astronomical information.

Korea

In 718, Korea began using water clocks, inspired by the Tang Dynasty of China. In 1434, the engineer Chang Yŏngsil built the Borugak water clock for King Sejong. This clock could strike its own hours using wooden figures, removing the need for human operators. It was the first hydro-mechanically engineered clock to announce time automatically with both visual and audible signals.

Japan

Emperor Tenji created Japan's first water clock, called a Rokoku. These clocks were important in society and were managed by specialized doctors of water clocks.

Temperature, water viscosity, and clock accuracy

When water flows out of a very thin opening, its speed depends on how thick, or viscous, the water is. The thickness of water changes with temperature — it becomes less thick when it gets warmer.

Because of this, a water clock could run faster when it’s hot and slower when it’s cold. To keep good time, the temperature around the clock would need to stay steady. There is no proof that ancient people could control the temperature this well, so older water clocks were not very accurate. But since these clocks were often checked and reset each day using the sun, they were still useful for telling time.