A large variety of devices have been invented to measure time. The study of these devices is called horology.
An Egyptian device dating to c.1500 BCE, similar in shape to a bent T-square, 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.
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.
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). Waterclocks were used in Alexandria, and then worldwide, for example in Greece, from c. 400 BCE. They could be used to measure the hours even at night, but required manual timekeeping to replenish the flow of water. Plato is said to have invented a water-based alarm clock. 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). 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.
The hourglass uses the flow of sand to measure the flow of time. They were used in navigation. Ferdinand Magellan used 18 glasses on each ship for his circumnavigation of the globe (1522). 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.
Incense sticks and candles were, and are, commonly used to measure time in temples and churches across the globe. 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.