The 24-hour clock was developed in ancient Egypt and was based upon careful astronomical observations and shadow clocks similar to the modern garden's sundial. Originally the length of these hours varied from season to season due to differing lengths of daylight.
Beginning in the 15th century BCE, the Egyptians designed water clocks for more precise timekeeping. Different water clocks were used for different seasons to keep up with the varying seasonal hours. The classical Greeks and Romans also used the 24-hour system with hours of varying length, and this usage continued until the 14th century AD in Europe with the invention of mechanical timekeeping. A system of hours of equal length was proposed by the Greek Hipparchus in the second century BCE but was only used for calculations.