Sundials were the earliest clocks for measuring time during the day.

Sundials: An Ancient Timekeeping Device
Sundials are ancient timekeeping devices that use the position of the sun to determine the time of day. They are believed to have been used as early as 1500 BCE in ancient Egypt and Babylon. Sundials consist of a flat plate or disk, called the dial, which is marked with lines or numbers that indicate the hours of the day. A rod or pointer, called the gnomon, is positioned upright on the dial, casting a shadow that moves across the dial as the sun moves across the sky. By observing the position of the shadow, one can determine the time of day.

Importance of Sundials in Ancient Times
Sundials were of great importance in ancient times as they provided a way to measure time during the day. This was particularly useful for activities such as farming, religious rituals, and scheduling events. Sundials allowed people to divide the day into smaller units, enabling better organization and coordination of tasks. They were also used for navigation and determining the seasons.

Advantages and Limitations of Sundials
Sundials have several advantages as timekeeping devices. They are relatively simple to construct and operate, requiring only a flat surface and a gnomon. They are also highly accurate when used correctly, as the position of the shadow can be precisely measured. However, sundials have some limitations. They can only be used during daylight hours, as they rely on the position of the sun. They are also influenced by the Earth's rotation, which causes the apparent motion of the sun to vary throughout the year. Therefore, sundials are not as accurate or reliable as modern clocks for measuring time.

Conclusion
In conclusion, the earliest clocks for measuring time during the day were sundials. They were used in ancient civilizations and relied on the position of the sun to determine the time of day. Sundials played a significant role in organizing daily activities and were an important tool for measuring time before the invention of more accurate timekeeping devices such as pendulum clocks.