How Did Ancient Civilizations Record Time?
After the Sumerians, the Egyptians were the next to formally divide their day into parts something like hours. Slender, tapering, four-sided monuments (Obelisks) 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 shortest or longest of the year.
Another Egyptian shadow clock or sundial, possibly the first portable time piece, came into use around 1500 B.C. to measure the passage of ‘hours’. This device divided a sunlit day into ten 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’.
Other ancient timekeeping devices include the candle clock, used in ancient China, ancient Japan, England and Mesopotamia; the time stick, widely used in India and Tibet, as well as some parts of Europe; and the hourglass, which functioned similarly to a water clock. The sundial, another early clock, relies on shadows to provide a good estimate of the hour on a sunny day.
It is not so useful in cloudy weather or at night and requires recalibration as the seasons change (if the gnomon was not aligned with the Earth’s axis).
The earliest known clock with a water-powered escapement mechanism, which transferred rotational energy into intermittent motions, dates back to 3rd century BC in ancient Greece; Chinese engineers later invented clocks incorporating mercury-powered escapement mechanisms in the 10th century, followed by Arabic engineers inventing water clocks driven by gears and weights in the 11th century.
The first mechanical clocks, employing the verge escapement mechanism with a foliot or balance wheel timekeeper, were invented in Europe at around the start of the 14th century, and became the standard timekeeping device until the pendulum clock was invented in 1656.
The invention of the mainspring in the early 15th century allowed portable clocks to be built, evolving into the first pocket watches by the 17th century, but these were not very accurate until the balance spring was added to the balance wheel in the mid 17th century.
The pendulum clock remained the most accurate timekeeper until the 1930s, when quartz oscillators were invented, followed by atomic clocks after World War 2. Although initially limited to laboratories, the development of microelectronics in the 1960s made quartz clocks both compact and cheap to produce, and by the 1980s they became the world’s dominant timekeeping technology in both clocks and wristwatches.
Atomic clocks are far more accurate than any previous timekeeping device, and are used to calibrate other clocks and to calculate the International Atomic Time; a standardized civil system, Coordinated Universal Time, is based on atomic time.