The ancient Egyptians divided both day and night into 12 hours each, creating the 24-hour day we use today. However, their hours weren't fixed - daytime hours were longer in summer and shorter in winter.
They assigned each hour to a planetary ruler, creating the concept of "planetary hours" still used in astrology today. This system determined auspicious times for different activities.
The Egyptian practice of watching the stars to determine time led to the development of water clocks and eventually mechanical clocks.