# eli5 What is the concept of months? Why Feb has only 28 while some have 31? When did the concept of 365 days’ year come in people?

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eli5 What is the concept of months? Why Feb has only 28 while some have 31? When did the concept of 365 days’ year come in people?

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The year actually has a basis in reality. Through various methods of measuring the sun and stars people stumbled upon a pattern that we now know is the earth doing an orbit around the sun.

For the months, however, I have no clue. I’ll have to look that one up.

365 days is a year because that’s how many days it takes for the earth to rotate to the same spot in its orbit around the sun. It’s useful to track things this way to aid in farming and you track natural, seasonal phenomena. We have leap year (where we put in 1 extra day) because it’s not exact.

Each month is roughly the time it takes for the moon to make one rotation around the earth (about 12.3 times per year). Again, useful to track things this way for natural phenomena (though less so than years). Some early civilizations had 10 months instead. A blue moon, a special celebration, is when we get two full moons in 1 month. You can’t evenly split 365 into 12, so you get uneven numbers of days, though I’m sure there’s a specific reason for February being so much noticeably shorter.

The short answer, Romans, the medium answer, Julius Caesar + the Egyptians.

The more complete answer is that the Romans had the concept of months since their founding, following a lunar cycle with days added every time the year got too out of sync with the seasons. In the year 46 BCE, Julius Caesar decided that this system was dumb, and easy to manipulate (because he had been the one doing the manipulation for the last 20 years). So he recruited the help of some Egyptian astronomers to revamp the calendar.

Egypt had a 365 day year, divided into 12, 30 day months, + 5 days at the end. (They actually had several different calendars, but this is complicated enough as it is)

Julius coopted the Egyptian system, with a few modifications. First, he made February 28 days, and put those other days into 31 day months. This is because the Romans had a superstition that February was an unlucky month. Then, he took the +5 extra days, and also put them in as other 31 day months. Last change he made was to automatically add an extra day to February every 4 years to keep the alignment with the seasons.

A year’s duration comes from the seasons, which can be tracked by looking at the stars. Seasons are super important for agriculture–you wouldn’t want to plant a crop as winter is about to start–which is what spurred astronomers to develop a method for tracking the seasons accurately. Earth orbits once per 365.24… days, hence a 365 day year that adds an extra day roughly every 4 years.

Months are prompted by a desire to have a smaller unit of time that’s bigger than a day. The moon’s cycle offers a convenient and easily tracked period of about 29.5 days, so it influenced that period. Unfortunately the moon’s cycle is neither a whole number of days nor an even fraction of a year.

To understand the modern calendar it’s best to turn to Rome. The Roman calendar had, at one point, a mere 10 months. Roman culture preferred odd numbers to even ones, so these months had either 29 days (“hollow” months) or 31 (“full” months) (aside: I’m glossing over a *lot* of revisions of calendars here; some had 29 and 30 day months, some keyed directly off the moon, etc). These months were the equivalent to modern day March – December, which explains the names of Sept-December (sept, oct, nov, and dec are latin prefixes for 7, 8, 9, and 10, and these were the 7th through 10th months. Note that initially July and August were named similarly, as quintilis and sextilis, but they were later renamed after Juilus and Augustus. A popular myth is that July and August were inserted, but they were merely renamed).

That calendar’s start date made good sense from a seasonal perspective: it was keyed off of the spring equinox, towards the end of March. Note that the Romans counted dates in terms of the days until certain key dates of a month, as opposed to the modern system of counting up from the first. Even in modern times the spring equinox is in late March.

However, it doesn’t take much math to recognize that a year of 10 months, none with more than 31 days, is never going to get up to the 365.24 days needed for a full year. If you just had those ~300 days then the seasons would slip by a *lot* each year, to the point of making it useless for agriculture. To avoid this problem there was an additional unnamed “intercalary” month (literally “between years”), which was about 60 days long. It was up to certain government officials to determine when this month was over and it was time to declare the new year. Unfortunately, many political positions turned over with a new year, so that led to the duration of the intercalary month being unreasonably short when opponents were in power, or unreasonably long when allies were in power.

That system was tossed in the trash and two new months were added. Not July and August, as many may claim, but January and February. In some calendars these were tacked on as the 11th and 12th months of the year, while in others they were the 1st and 2nd. Having February as the last month of the year explains why it gets “whatever is left over to hit 365” days, which turns out to be 28-29. However, it ultimately settled in as the 2nd month of the year over the course of many calendar reforms.

Those calendar reforms ultimately moved away from Romans’ 29 and 31 day months in favor of 30 and 31 day months, still with February as the month that gets what’s left over. And that’s how we wound up with a calendar that starts on an unremarkable day in the middle of winter, with months that schoolchildren have to learn a mnemonic to remember the duration of, and with the second month of the year being the oddball that has to pick up the slack to keep the year from drifting.

The concept of months comes originally from the phases of the moon.

People could look up into the sky and notice that the moon regularly went through phases and that the time it took for that cycle was the same every time.

The concept of day simply comes from the sun rising and setting and the concept of year comes from the very noticeable change of the seasons.

So people even very far back in time had a natural calender in the sky that they could use to measure the passage of time.

The problem was that none of the natural units they had fit which each other.

A lunar cycle is not a full number of days and a year is not a full number of natural moon cycles or a full number of days.

So people decided to fudge a bit and make a month not a full lunar cycle but something that could be evenly divided into days and did the same for years too.

There were even smaller units of half moons and quarter moons invented by some people which we continue today as weeks and the somewhat obscure except for that popular online game fortnight.

The Babylonians were the ones who codified the basis for our modern timekeeping that we use today and the Romans fixed the calender that ours is based on and gave us the uneven distribution of days per month.

A month is about how long it takes to go from full moon to full moon which is a little less than 30 days (29.53 days). The moon used to be what timed the calendar, but it doesn’t sync up with the Earth’s orbit around the sun (365.2422 days) which is what controls the seasons. You can see this in things that still use the older lunar calendar. The Muslim’s Ramadan slowly moves through the calendar as years go by. Easter does a similar thing except it’s fixed around the spring equinox and can’t go that far. It can still be anywhere between March 22nd and April 25th.

Our uneven months are a way to get the lunar calendar to fit into a solar one. Which months got assigned days is arbitrary. The calendar would work just as well if we alternated 31 and 30 day months with one of the short ones only being 30 days in leap years. The Romans who constructed most of our current solar calendar thought February was an unlucky month, so they made the unlucky month shorter.

Simple answer – months comes from the moon. Originally, a month ran from one new moon -when the moon is first seen after sunset – to the next. That happens a bit under 13 times in a year. Months were normally between 29 and 30 days.

The year is the time it takes the earth to to around the sun. This shows most clearly as the pattern of solstices – when the sun gets as far north or south as it goes each year – and equinoxes, when the days and nights are the same length. They used to keep them in sync by only starting the first month when they saw the spring equinox. So if it was time for month 1 but the equinox hadn’t come, then they repeated month 12. This pattern of not really knowing how long this year is going to be got tiresome, so they instead worked out the pattern of leap years and fixed length months we have now.