Why are there twelve months in a year? Because it is the closest match to the number of times the moon orbits the Earth in a year.
Before there were names for the months, people lived by a lunar calendar. There was no need to know what month it was or what day it was. All people needed to know was how many moons before the end of the season.
The first calendar seems to have been invented by the Sumerians in about 3100 BC. They probably didn’t do it to know what month it was. Astronomers in Sumer tracked the movements of the moon, the planets, and the stars and they made very detailed maps of the sky. They noted down when different constellations were in different places and also the passage of the moon. The moon takes 27 days to orbit the Earth, but the Sumerians had decided that their year had 360 days and that divided nicely by 12, which was roughly how many times the moon went around the Earth. This gave them a 30 day month. The main problem with their calendar is that it doesn’t quite fit the actual year. To compensate, they had to add an extra month in every now and again.
The ancient Egyptians also developed a calendar at roughly the same time as the Sumerians. Their calendar had 365 days and was divided into 3 seasons of 120 days that has four months each of 30 days. They then had an extra thirteenth month of five days at the end of the year to make up the difference. Their calendar also didn’t fit the solar year, so they changed their thirteenth month to have six days every fourth year, probably the first example of a leap year.
There are many calendars in the world, even today, and many of them are based on the moon and the sun. I want to look at the Gregorian calendar, which has 12 months of between 30 and 31 days (except for February, obviously).
The Gregorian calendar comes from the Roman calendar. The Romans had taken calendars from Egypt, Greece, and the Etruscans. They had an eight day week and their months had 30 or 31 days. They decided that a year would have ten months because they based their society on the decimal system. Their months were, in order of first to last:
Mensis Martius – the month of Mars
Mensis Aprilis – the month of Aphrodite
Mensis Maius – the month of Maia
Mensis Iunius – the month of Juno
Mensis Quintilis – the fifth month
Mensis Sextilis – the sixth month
Mensis September – the seventh month
Mensis October – the eighth month
Mensis November – the ninth month
Mensis December – the tenth month
The Latin for the numbers five to ten is 5 – quinque, 6 – sex, 7 – septem, 8 – octo, 9 – novem, 10 – decem. It is clear to see why the last five months were named the way they were.
The Roman calendar had several problems. The first problem was that it was only 304 days long and the year is obviously longer than that. The year started on the spring equinox and the winter months were generally considered as just winter until about 713 BC. The choice was either to make the months longer or add two more months. Making them longer wouldn’t work because they’d go out of sync with the moon. So, King Numa Pomoilius (who may or may not have been real) added the months of Ianuarius – the month of Janus, and Februarius. He also set the lengths of the months at 29 or 31 days because the Romans believed even numbers were unlucky. February was the only month with an even number of days and was considered to be unlucky. He also reordered the months so that January and February came before March, which is why the names for the months from five to ten were out of sequence.
The Romans used this calendar for 700 years, adding in 20 days every year to make the calendar fit the solar calendar. However, the people in charge of adding the extra 20 days sometimes decided not to. If they didn’t like the consul they wouldn’t add the extra time and if they liked the consul they might add more than 20 days. Because of this, when Julius Caesar came to power, the calendar year and the actual year were completely out of sync. Julius Caesar took it upon himself to fix all of this. The senate also changed the name of the fifth month from Quintilis to Julius, in honor of him. Sextilis was also changed to August in honor of Caesar Augustus, the first Roman Emperor.
The calendar underwent several other reforms, the biggest being the Gregorian reform in 1582, but it has always had twelve months. And this is what I learned today.
Sources
https://science.howstuffworks.com/science-vs-myth/everyday-myths/time6.htm
https://en.wikipedia.org/wiki/Calendar
https://en.wikipedia.org/wiki/Egyptian_calendar
https://sites.google.com/site/ishikag610/home/the-calendar-and-the-system-of-irrigation
https://en.wikipedia.org/wiki/Roman_calendar