History of timekeeping devices

The origins of our current measurement system go back to the Sumerian civilization of approximately 2000 BC. This is known as the sexagesimal system based on the number 60. 60 seconds in a minute, 60 minutes in an hour – and possibly a calendar with 360 (60x6) days in a year (with a few more days added on). Twelve also features prominently, with roughly 12 hours of day and 12 of night, and 12 months in a year.

Early measurement devices

Sundial

Horizontal sundial in Taganrog (1833)

A large variety of measuring instruments have been adapted to measure time. The study of these devices is called horology.

A sundial uses a gnomon to cast a shadow on a set of markings which were calibrated to the hour. The position of the shadow marked the hour in local time. Pliny the Elder records that the first sundial in Rome was looted from Catania, Sicily (264 BCE), which gave the incorrect time for a century, until the markings appropriate for the latitude of Rome were used (164 BCE). Noontime was an event which could be marked by the time of the shortest shadow on a sundial. This was used in Rome to judge when a court of law was open; lawyers had to be at the court by that time. An earlier invention also using a cast shadow to determine time is an Egyptian device dating to c.1500 BCE, similar in shape to a bent T-square, which measured the passage of time from the shadow cast by its crossbar on a non-linear rule. The T was oriented eastward in the mornings. At noon, the device was turned around so that it could cast its shadow in the evening direction.

Waterclock

The most accurate timekeeping devices of the ancient world were the waterclock or clepsydra, first found in Egypt. A waterclock was found in the tomb of pharaoh Amenhotep I (1525–1504 BCE). They could be used to measure the hours even at night, but required manual timekeeping to replenish the flow of water. Plato introduced the waterclock to Greece , and invented a water-based alarm clock . One account says it depended on the nightly overflow of a vessel containing lead balls, which would float in a columnar vat. The vat would hold an increasing supply of water supplied by a cistern. Eventually the vessel would float high enough to tip over. The lead balls would then cascade onto a copper platter. The resultant clangor would then awaken his students at the Academy (378 BCE). Another account says that it used two jars and a siphon. Water empties until it reaches the siphon, which transported the water via the siphon to the other jar. Water rising in the other jar forces air through a whistle, sounding the alarm. The Greeks and Chaldeans regularly maintained timekeeping records as an essential part of their astronomical observations. In particular, Arab engineers improved on the use of waterclocks up to the Middle Ages.

Waterclocks, and later, mechanical clocks, were used to mark the events of the abbeys and monasteries of the Middle Ages. Richard of Wallingford (1292–1336), abbot of St. Alban's abbey, famously built a mechanical clock as an astronomical orrery about 1330.

Hourglass

The hourglass uses the flow of sand to measure the flow of time. Ferdinand Magellan used 18 hourglasses on each ship for his circumnavigation of the globe (1522).

Modern Devices

Mechanical clocks

A contemporary quartz watch

The most common devices in day-to-day life are the clock, for periods less than a day, and the calendar, for periods longer than a day. Clocks can range from watches, to more exotic varieties such as the Clock of the Long Now. The English word clock actually comes from French, Latin, and German words that mean bell. The passage of the hours at sea were marked by bells, and denoted the time (see ship's bells). The hours were marked by bells in the abbeys as well as at sea. They can be driven by a variety of means, including gravity, springs, and various forms of electrical power, and regulated by a variety of means such as a pendulum. There are also a variety of different calendars, for example the Lunar calendar and the Solar calendar, although the Gregorian calendar is the most commonly used.

A chip-scale atomic clock

A "chronometer" is a portable timekeeper that meets certain precision standards. Initially, the term was used to refer to the marine chronometer, a timepiece used to determine longitude by means of celestial navigation. More recently, the term has also been applied to the chronometer watch, a wristwatch that meets precision standards set by the Swiss agency COSC. Over 1,000,000 "Officially Certified Chronometer" certificates, mostly for mechanical wrist-chronometers (wristwatches) with sprung balance oscillators, are being delivered each year, after passing the COSC's most severe tests and being singly identified by an officially recorded individual serial number. According to COSC, a chronometer is a high-precision watch capable of displaying the seconds and housing a movement that has been tested over several days, in different positions, and at different temperatures, by an official, neutral body (COSC). Each movement is individually tested for several consecutive days, in five positions and at three temperatures. Any watch with the denomination "chronometer" is provided with a certified movement.

The most accurate type of timekeeping device is currently the atomic clock, which are accurate to seconds in many thousands of years, and are used to calibrate other clock and timekeeping instruments. Atomic clocks use the spin property of the caesium atom as its basis, and since 1967, the International System of Measurements bases its unit of time, the second, on the properties of caesium. SI defines the second as 9,192,631,770 cycles of the radiation which corresponds to the transition between two electron spin energy levels of the ground state of the ¹³³Cs atom.

Today, the GPS global positioning systems in coordination with the NTP network time protocol can be used to synchronize timekeeping systems across the globe.