The day, which is the space between two nights, and the month, which is measured by new moons, were the only units of time available to primitive people. The month has certain importance due to its connection with women's menstrual cycles.
However, the year-a complete orbit of the earth around the sun-is much more influential in human activities because it determines seasons and harvests. Yet, it is very difficult to measure a single year's length. Fig . 1-1: Compilers of a calendar can use the moon's cycle to help predict and record the passage of time.
1-1: Who Invented the Calendar of 365 Days?
It is unclear who first came up with the idea of a 365-day year, but it is believed to have been used by the ancient Egyptians as early as 4236 BCE.
It was later adopted and perfected by the Romans, who named each month and designated specific days for religious observances and government meetings.
The concept of leap years-adding an extra day every four years to account for the extra quarter of a day in the solar orbit-was also introduced by the Romans.