c. 1500 BCE – 1400 CE
How did ancient clocks work? Sundials, water clocks, candles and hourglasses
Before mechanical clocks, people measured time with shadows, water, fire and sand. Sundials read the sun's shadow, water clocks measured steady flow into or out of a vessel, and candles, incense and hourglasses burned or poured at a known rate. Water clocks were the most advanced, reaching geared, automated towers in China and the Islamic world.
Key facts
| Oldest surviving water clock | Karnak, Egypt, about 1400 BCE (reign of Amenhotep III) |
|---|---|
| Oldest sundials | Egyptian shadow clocks, about 1500 BCE |
| Constant-flow water clock | Ctesibius of Alexandria, 3rd century BCE |
| Astronomical clock tower | Su Song, Kaifeng, China, completed 1092, about 12 m tall |
| Elephant clock and castle clock | Ismail al-Jazari, described in 1206 |
| Earliest European picture of an hourglass | Ambrogio Lorenzetti fresco, Siena, 1338 |
Sundials have an obvious flaw: they don't work at night, indoors, or under clouds. The long search for a clock that keeps going regardless led to some of the most ingenious machines of the ancient and medieval world.
Shadows
A sundial is a stick, called a gnomon, whose shadow moves as the sun crosses the sky. Egyptian shadow clocks from about 1500 BCE are among the oldest. Greek and Roman makers learned to curve the dial and tilt the gnomon to match the latitude, which made the hours more even. Sundials stayed the reference for setting other clocks until the 1800s.
Water
A water clock (Greek klepsydra, "water thief") measures time by water dripping out of a marked vessel, or filling one. The oldest surviving example, from the temple of Karnak in Egypt, dates to about 1400 BCE. Its sloping sides were a clever fix for a basic problem: water flows faster when the vessel is full.
In Alexandria in the 3rd century BCE, Ctesibius solved that problem properly with a constant-level tank and a float that drove a pointer. His clocks could ring bells and move figures, and they used one of the earliest known feedback controls. Athenian courts used simple water clocks to limit how long each side could speak.
Water clocks reached their peak in China. In 1092 the official Su Song completed a water-driven astronomical clock tower in Kaifeng, about 12 metres tall. A water wheel advanced in steps controlled by a mechanism that works like an escapement, turning an armillary sphere, a star globe, and figures that announced the hours with bells, gongs and drums.
In 1206 the engineer Ismail al-Jazari, working in what is now Turkey, described a series of automated water clocks, including the famous elephant clock and the "castle clock," with doors, musicians and moving figures.
Fire
A candle with marks along its side shows hours as it burns. Legend credits King Alfred of England with candle clocks in the 9th century. In China and Japan, incense clocks burned along carefully laid trails of powder, sometimes with different scents for different hours, or with threads that dropped small weights onto a gong.
Sand
The hourglass appears in Europe surprisingly late. The first clear picture is in a fresco by Ambrogio Lorenzetti in Siena, painted in 1338. Hourglasses didn't freeze in winter like water clocks and didn't care about the rocking of a ship, so sailors used them to time watches on deck and to measure speed with a log line.
Why these clocks weren't enough
Flow clocks depend on temperature, pressure, purity of the water or sand, and constant attention. They were good for intervals but not for keeping the time of day across weeks. That needed a regular, self-sustaining tick, which arrived with the mechanical escapement.
Questions people ask
What was the first clock?
The sundial and the water clock both date to about 1500 BCE in Egypt. Water clocks were the first that worked at night and indoors.
How accurate were water clocks?
Good ones could keep time to within several minutes a day, but they needed constant adjustment for temperature and water level.
Who invented the hourglass?
No one knows. It appears in Europe in the 1300s, with the first clear picture in a 1338 fresco in Siena.
Sources
Last updated October 18, 2026. Found an error? Tell us.