why do we have 12 months

Answers to why do we have 12 months

The ancient Egyptians first divided the year on this pattern, around 6000 years ago, based on the observations of their priests. They derived the number from the same golden section which gave us the duodecahedron, and which they believed was the Earthly representation of the Heavens. That is why there are two lots of 12 hours in a day, and why minutes and seconds are all 12 based.

The months were originally introduced to follow the cycles of the Moon, the second most visible body in the sky.  In contrast, both days and years are based on the apparent movement of the Earth around the Sun.  For the ancients, an accurate measurement of the cycle of the Moon was much simpler to make than that of the solar year.  So most of the ancient civilizations used a lunar calendar.  However, the lunar month does not correspond to an exact number of days; it varies continuously and on average lasts about 29 days 12h 44 min 2.8s.  Rapidly, the ancients adopted alternating months of 29 and 30 days, with an extra day every 30 months to account for the 44 min.  The year was then 12 months or 354 days.

But, because this year was 11.25 days short of the solar year, the seasons would shift from one year to the next, making it difficult to plan for sowings and harvests.  The Egyptians were the first to abandon the lunar calendar and to introduce twelve months of 30 days plus five days at the end of the year, to keep the seasons recurring at the same year after year.  Later, the Romans distributed the five days over the year with 12 months of alternatively 31 and 30 days for leap years and February with 29 days otherwise.  However, when the eighth month was dedicated to the emperor August, a 31st day was added to make it the equal of July, dedicated to Julius Caesar, and removed from February which became the short month with only 28 or 29 days that we know today.  Although the solar calendar is used all over the world for civil business, the lunar calendar is still followed in many religions including Islam, Judaism, and Catholicism (for fixing the Easter date).

The debate
We don't need 12 months in a year. They're a terrible complication, since 12 doesn't divide evenly into 365.242199 days.

And basing months on the lunar cycle would be even worse because of the inconvenience to business cycles. A 12-month year can be divided fairly equally into halves & quarters, but an occasional year of 13 months. . . well, let's just stick to the solar year.

To be sure, mariners who want to keep track of the tides, and people like that, could use a good lunar calendar, and if they ever ask me I could provide them with several. But for most of us, keeping up with the seasons is most important.

The civil calendar can't be based on sidereal time. The time that Earth revolves in relation to the fixed stars is not the same as its revolution with respect to the sun. The seasons depend on tropical time: the tropical year is the time from one spring equinox to the next. There is only a few minutes difference in the two years, but a few minutes can add up to an error of a whole day in just a few centuries. (That's what happened to the Julian Calendar, and prompted Pope Gregory what's-his-number to change the leap year formula.)

The Gregorian Calendar probably is here to stay, because of social inertia; and that's unfortunate because it's really an awful, absurdly complicated mess. That every-4-years-except-every-100-years-except-every-400-years rule for the extra day is ridiculous. A much simpler and easier rule, placing the extra day every 1508 days, would make the solstices & equinoxes more regular and would also last longer -- something like 40 000 years before it would go off by a single day.

And if we dumped the months and just numbered the days of the year from 1 to 365, the leap day would be easy to calculate. It would be best not to give it a number of its own; just call it by the number of the regular day it follows. For example: If it comes after Year 4 Day 48, just call it Day 48+ and the day after that would be Year 4 Day 49. So every year would end with Day 365. To get the date of the next leap day, add 4 to the year and 47 to the day, and you find it will come after Year 8 Day 95.

Someone who's a better mathematician than I am could provide a formula where you could locate any leap day just from the number of the year.

Curiously, you could make this calendar even more accurate by placing the leap day at intervals of 51 lunar months + 2 days! That's an average of every 1508.06 days.

There are indeed 12 lunar months in a year, leaving over about 11 days compared to the solar year (354 days as compared to 365). The Gregorian Calendar addressed many problems with previous calendars.

The evidence of the lunar calendar can still be seen though. For example the Muslims celebrate the month of Ramadan according to the lunar cycle (9th lunar cycle) and because of this you have effect that the dates of Ramadan fluctuate wildly over the course of the years and can occur both in winter and summer.

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