What's the reasoning of having 12 months in a year?
The division of the year into 12 months is a practice with roots in ancient civilizations and is primarily based on the lunar cycles and the solar year.
The lunar cycle, which is the time it takes for the Moon to go through all its phases (from new Moon to new Moon), averages approximately 29.5 days.
Early Roman calendars, like many ancient lunar calendars, initially had months that were either 29 or 30 days long, roughly following the lunar cycle. However, these lunar months did not align perfectly with the solar year, the time it takes Earth to orbit the Sun once, which is about 365.25 days. This discrepancy led to seasonal drift over time, meaning the calendar would gradually become out of sync with the seasons.
To address this issue, the Romans, under Julius Caesar in 45 BCE, reformed the calendar to better align with the solar year. This Julian Calendar introduced a 12-month structure that more closely matched the solar year. The 12-month format was likely chosen for practical and cultural reasons. Twelve is a convenient number for division (divisible by 2, 3, 4, and 6), making it easier to divide the year into seasons and to manage agricultural and civic duties.
Moreover, some suggest that the number 12 was favored because of its prevalence in other areas of ancient life, such as the number of cycles of the Moon in a year and the number of constellations through which the Sun appears to move.
Over time, this Julian Calendar was further refined into the Gregorian Calendar, introduced by Pope Gregory XIII in 1582. The Gregorian Calendar made slight adjustments to the leap year system of the Julian Calendar to maintain alignment with the equinoxes. Most of the world today uses the Gregorian Calendar, which retains the 12-month structure.
Thus, the division of the year into 12 months is a result of a combination of lunar cycles, the solar year, and historical and practical considerations that have been honed over centuries. This system has become a globally accepted standard due to its practicality and the gradual spread of Roman and later European influence around the world.