why do we have 12 months in a year

It's been a very long time since I've been on the xkcd forum, but I just stumbled across this thread in the process of checking where my new 13 month dynamic calendar website (
) indexed on Google searches. My calendrical system is the standard 13 month, 28 day system: 13 months with 28 days each and each year having a day zero outside of it to mark the end/beginning of the year. In my system, the leap day occurs at the end of the year on leap years which corresponds to December 20. This is because I used December 21 as day zero as I felt the beginning of the year should have some astronomical significance. The winter solstice made a nice convenient place to move New Year's Day, especially because it is only an eleven day adjustment from the Gregorian calendar. As an added benefit to this move, day 1. 11 becomes January 1 and the Gregorian leap day of February 29 becomes the pi day: 3. 14.


These, among other conversions I'm sure, provide some useful points of reference along the calendar which makes it easier to translate between the two in your head. One of the reasons why past 13 month calendar attempts failed to catch on (in addition to the obvious societal bias against overhauling concepts/systems with lengthy traditional backing), is that they often retained the Gregorian month names. I have dropped this aspect opting for a very simplified numerical date format that resembles the decimal system (year is the biggest measure of time so it comes first, followed by month and day). The problem with Gregorian month names, is that the number months don't actually match the month numbers. September should be the seventh month, but it is the ninth. October should be the eighth month, but it is the tenth. November and December suffer from this problem as well.


Any comments, questions, or suggestions would greatly appreciated. More features are on the way. If it takes the Moon 28 or 29 days to orbit the earth why are most months 30 or 31 days long? This has resulted from a compromise. Initially, months were mostly 29 days long and the average length of a month was 29. 5 days which is the time taken by the Moon to orbit the Earth. However, this resulted in a year of only 354 days while the orbital period of the Earth is 365. 2422 days. As a result, the calender became out of sync with seasons which was bad. This was initially corrected in an arbitrary way by adding a 13th month, but soon the calender was thrown into severe confusion. In 46 B. C. , Julius Caesar reformed the calender by ordering the year to be 365 days in length and to contain 12 months. This forced some days to be added to some of the months to bring the total from 354 up to 365 days.


To account for the extra 0. 2422 days, every fourth year was made a leap year. This made the average length of a year to be 365. 25 days. However, the Julian year still differs from the true year and by 1582, the error had accumulated to 10 days. So, 10 days were dropped from the year 1582 so that October 4, 1582 was followed by October 15, 1582. In addition, a modification was made that century years that were not divisible by 400 would not be considered as leap years. For example, 2000 would be a leap year while 2100 would not. This made the year sufficiently close to the actual year and this calender is called the Gregorian calender. As the year is now set up to follow the seasons accurately, it no longer follows the phases of the Moon. This page was last updated June 28, 2015.

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