What was the calendar based on?

Answer 1

There are many different calendars, all of which are based upon:

1. The day, the time it takes for the earth to revolve completely on its axis, and thus the time between sunrise and sunrise or sunset and sunset.

2. The month, the time it takes the moon to revolve around the earth, and thus the time between full moons, some 28 days or so.

3. The year, the time it takes the earth to revolve around the sun, about the time between solstices, some 365.25 days.

Since the number of days in a month does not divide evenly into the number of days in the year, either you must forget about the months being accurate and focus on having the solstice landing on the same day each year, as the Gregorian calendar does, or forget about the solstices and focus on having the full moon landing on the same day each month, as the Islamic calendar does. Or you can compromise as the Jewish and Chinese calendars do--go by the month usually but add extra days in some years to keep the solstices in line.

A solar calendar like the Gregorian one is useful for those living at higher latitudes, where the difference between midwinter and midsummer is extreme and much more noticeable than the difference between new moon and full moon. Near the equator, the difference between midwinter and midsummer is less, and if people rely on natural light to navigate at night, a lunar calendar like the Islamic one makes more sense.