the tomb of pharaoh Amenhotep I (1525–1504 BC), suggesting that they were first used in ancient Egypt. The ancient Egyptians are also believed to be the inventors of the hourglass, which consisted of two vertically aligned glass chambers connected by a small opening. When the hourglass was turned over, grains of sand fell at a constant rate from one chamber to the other. Another Egyptian method of determining the time during the night was using plumb-lines called merkhets. In use since at least 600 BC, two of these instruments were aligned with Polaris, the north pole star, to create a north–south meridian. The time was accurately measured by observing certain stars as they crossed the line created with the merkhets.
500 BC – 1 BC
Water clocks, or clepsydrae, were commonly used in Ancient Greece following their introduction by Plato, who also invented a water-based alarm clock. One account of Plato's alarm clock describes it as depending on the nightly overflow of a vessel containing lead balls, which floated in a columnar vat. The vat held a steadily increasing amount of water, supplied by a cistern. By morning, the vessel would have floated high enough to tip over, causing the lead balls to cascade onto a copper platter. The resultant clangor would then awaken Plato's students at the Academy. Another possibility is that it comprised two jars, connected by a siphon. Water emptied until it reached the siphon, which transported the water to the other jar. There, the rising water would force air through a whistle, sounding an alarm. The Greeks and Chaldeans regularly maintained timekeeping records as an essential part of their astronomical observations.
Greek astronomer, Andronicus of Cyrrhus, supervised the construction of the Tower of the Winds in Athens in the 1st century B.C.
Ctesibius's clepsydra from the 3rd century BC. Clepsydra, literally water thief, was the Greek word for water clock.
In Greek tradition, clepsydrae were used in court; later, the Romans adopted this practice, as well. There are several mentions of this in historical records and literature of the era; for example, in Theaetetus, Plato says that "Those men, on the other hand, always speak in haste, for the flowing water urges them on". Another mention occurs in Lucius Apuleius' The Golden Ass: "The Clerk of the Court began bawling again, this time summoning the chief witness for the prosecution to appear. Up stepped an old man, whom I did not know. He was invited to speak for as long as there was water in the clock; this was a hollow globe into which water was poured through a funnel in the neck, and