Early Egyptian water clock, 1415-1380 BC (c) Science Museum / Science & Society Picture Library
Keeping track of the passage of time was crucial to the ancient Egyptians, especially to the astronomers and priests who were responsible for determining the exact hour for the daily rituals and sacrifices. They divided the day into two equal periods of twelve hours each. However, due to the revolution of the earth around the sun, the length of the hours in summer was not the same as the length of hours in winter. Summer daylight hours were much longer than nighttime hours, and this was reversed in winter.
A sundial from about 1500 BCE shows this division into 12-hour parts, but the sundial was no use at night. A water clock was invented.
There is documentation of a water clock on a tomb inscription from the 16th century BC, but the oldest water clock we have physical evidence of dates to c. 1417-1379 BCE, during the reign of Amenhotep III where it was used in the Temple of Amen-Re at Karnak.
These simple clocks were stone vessels with sloping sides and a small hole near the bottom that allowed water to drip at a nearly constant rate. The water could drip into one of twelve separate columns. Each column had twelve consistently spaced markings on the inside to mark twelve hours, and each column represented a different month. The length of hours for each month varied slightly due to the seasonal difference.
These clocks were critical at night but may have been used in daylight as well.