A timepiece that can be used without sunlight enters the scene
Sundials only tell time when the sun is shining, and so water clocks were invented by devising various means of controlling the flow of water to a constant rate. Water clocks were already being used to tell the time at night in Egypt by around 1550 B.C. The oldest existing water clock has a hole in the bottom so that as the water leaked out, the time could be measured by the level of water remaining. To allow the user to read time by a temporal hour system, seasonal time system according to the changes in the length of nighttime through the seasons, the vessel has different scales for each month marked on the inside.