Every clock in history answers the same question — how do you make something repeat at a perfectly steady rate? — and each era found a better oscillator. Here is the whole arc, from dripping water to vibrating atoms.
Water clocks (~1500 BC)
The Egyptian clepsydra was a bowl with a small hole: water dripped out at a roughly constant rate, and level marks inside gave the hour. Unlike a sundial it worked at night and indoors — which is why Greek courts used water clocks to time speeches. The flaw: water flow changes with temperature and pressure, so accuracy plateaued around minutes per day.
Candle clocks, incense clocks, hourglasses
- Candle clocks (described in China by ~520 AD, famously used by Alfred the Great ~880 AD): a candle with marked rings burns down the hours.
- Incense clocks in China and Japan burned along a calibrated trail — some changed scent to "chime" the hour.
- The hourglass appeared in Europe by the 14th century — sealed, ship-friendly (unaffected by rocking), and standard for navigation watches well into the 1700s.
The mechanical escapement (~1300 AD)
The breakthrough of medieval Europe: the verge escapement, a mechanism that lets a weight-driven gear train advance in small equal ticks. Tower clocks spread across European cities in the 1300s. Accuracy was still poor — ±15 minutes a day — but for the first time, time came from a machine, not the sky.
The pendulum (1656)
Galileo noticed a swinging lamp keeps near-constant period; Christiaan Huygens built the first pendulum clock in 1656. Accuracy jumped a hundredfold, to ~15 seconds a day. In 1761 John Harrison's H4 marine chronometer solved the longitude problem — losing only seconds over a trans-Atlantic voyage — and navigation was transformed.
Quartz (1927)
A quartz crystal vibrates at an extremely stable frequency when driven electrically (the piezoelectric effect). The first quartz clock (Bell Labs, 1927) was better than any pendulum; by the 1970s quartz wristwatches made near-perfect time cheap. Typical drift: about half a second a day for a watch, far better for lab units.
Atomic time (1955)
Atoms are nature's perfect oscillators. The first practical caesium atomic clock (Louis Essen, NPL England, 1955) was so stable that in 1967 the SI second was redefined: 9,192,631,770 oscillations of the caesium-133 atom. Modern caesium fountains are accurate to ~1 second in 100 million years; experimental optical-lattice clocks push past 1 second in the age of the universe.
Atomic time is why we have leap seconds — Earth's rotation can no longer keep up with our own clocks.
The arc in one line
Each oscillator beat the last by orders of magnitude: water flow → escapement tick (minutes/day) → pendulum swing (seconds/day) → quartz vibration (fractions of a second/day) → atomic resonance (nanoseconds/day).
Quick answers
What was the first clock? Shadow/sun clocks (~1500 BC Egypt) are the oldest known time-telling devices; water clocks of the same era were the first that worked without the Sun.
Who invented the mechanical clock? No single inventor is documented — verge-escapement tower clocks appeared in late-13th/early-14th-century Europe.
What defines a second today? Since 1967: 9,192,631,770 cycles of microwave radiation from the caesium-133 atom's hyperfine transition.