Changes

Some of the earliest timekeeping devices involved the use of water, or what were called water clocks or <i>clepsydrae</i> as called by the Greeks. This involved either using outflow of water or inflow. Effectively, as water drained out or filled in, the rate was seen as relatively constant an based on marking on containers would then tell how much time had passed as the water drained or filled in. Such devices were known to have existed in many of the earliest complex society in Eurasia, including in Egypt, Mesopotamia, the Indus, China, and likely other regions, including ancient Greece and Rome. In China during the Han dynasty, water clocks were critical for astronomical observations and became an increasingly complex set of tanks that also had secondary tanks that would provide additional water to keep water moving to measure time as pressure changes could mean that the rate of which water moved changed, thus potentially invalidating any time measure.

Other early devices include candle clocks, which were probably only used indoors. These were also throughout much of Eurasia from the Bronze Age and throughout the Medieval period. Candle clocks were simple, where the rate of burning would be constant and the candle would be pre-measured so that different levels in which the candle melted as it burned would help tell time. The sundial is likely another ancient device. Both the Babylonians and Egyptians used the sundial at least by the 2nd millennium BCE if not earlier. The sexagesima system we use in our own clocks today was already used and invented by the Sumerians and Babylonians, which was also likely used for initial sundials. The oldest sundial appears to date from around 1500 BCE, but sundials are likely much older.  The Greek philosopher Philo in the 3rd century BC and in China, under development by the mathematician Yi Xing by the 7th century, escapement clocks were built, which use a mechanical device that releases a gear which then allows the rate of release to be used as a timekeeping element. This is similar to our mechanical clocks and such clocks would begin to signal the increasing realization that mechanical timeclocks would be the only way to keep time more consistent.