The Gregorian calendar, which we use today, is a reformed version of the Julian calendar that was itself a reformed version of the Roman calendar. The number of days in a month was irregular in the Roman calendar due to the orientation and shape of the new moon that determined the midpoint when the moon was half full, and the number of days until that point was considered as the length of the month. The first formal Roman calendar had ten months and approximately 61 days during winter that were not part of any particular month. Numa Pompilius reformed the calendar to avoid having too many months with an even number of days.
When looking at a calendar, one of the first things that pops out is the strangeness that each month has a seemingly random amount of days. Why are the number of days in a month not equal? And why are they distributed as they are? To answer this question, we need to look a little at the history of the modern calendar.
Our calendar, the Gregorian calendar, is a reformed version of the Julian calendar. The Julian calendar, in turn, was a reformed version of the Roman calendar. It was during the reign of Julius Caesar that the calendar was drastically realigned, to make it run a little more smoothly.
Before the Julian reform, the number of days in a month was as follows: Ianuarius 29 days, Februarius 28 days, Martius 31 days, Aprilis 29 days, Maius 31 days, Junius 29 days, Quintile 31 days, Sextilis was 29 days, September was 29 days, October was 31 days, November was 29 days, December was 29 days, and there was an Intercalaris month which lasted 27 days.
In the new Julian calendar, the month of Intercalaris was abolished altogether and the number of days was changed for the shorter months. Those months which already had a length of 31 days remained the same, while Ianuarius, Sextilis and December gained two days each, and Aprilis, Iunius, September and November gained one day each, while Februarius remained the same, though gaining one. extra day every four years, to even out the calendar a bit. Quintilis was later renamed Iulius after Julius Caesar and Sextilis was renamed August after Augustus Caesar.
Looking at the older Roman calendar, it is evident that the number of days in a month was already irregular. This is due to some changes that occurred between the early Roman calendar and the later Roman calendar. Originally, the Roman calendar was a lunar calendar, meaning that the length of each month was roughly based on the moon. The time between two new moons is approximately 29.5 days, so the months were 29 or 30 days long. Those months with 29 days were considered unlucky, those with 30 were considered lucky.
The number of days in a month in the Roman tradition was not calculated in the same way as we calculate it today. Instead of counting from one to 29 or 30, the Romans counted from the time the new moon was first sighted after sunset and, depending on the orientation and shape of that thin crescent, the number of days until midpoint at which the moon was half full would be determined.
The first truly formal Roman calendar had already departed from a true lunar calendar, although the days were much more evenly spaced than in our current calendar. The calendar had only ten months, and the number of days in a month was as follows: 31 days in Martius, 30 days in Aprilis, 31 days in Maius, 30 days in Iunius, 31 days in Quintilis, 30 days in Sextilis, 30 days in in days in September, 31 days in October, 30 days in November and 30 days in December. Additionally, there were approximately 61 days during the winter that weren’t part of any particular month.
Things got even more complicated in the late 8th century BC, due to a Roman superstition that held even numbers to be unlucky. One of Rome’s traditional kings, Numa Pompilius, decided to reform the calendar to make as few months as possible an even number of days. His first reform added two months, January and February, to the end of the calendar, and made all months except February have an odd number of days. This made the calendar roughly alternate in the number of days in a month: 8, 31, 29, 31, 29, 31, 29, 29, 31, 29, 29, 29.
A popular apocryphal legend dating back to the 13th century tries to drastically simplify the reasons behind the number of days each month. It is argued that originally the Julian calendar was regular, with alternating month lengths: 13, 30, 29, 30, 29, 30, 29, 30, 29, 30, 29, 30. The theory holds that Caesar then added a day every month except February, to fill out the calendar. The theory further establishes that during the reign of Caesar Augustus, August was changed to 29 days, to match the length of July. This theory has ample evidence to refute it, including from the earliest irregular Roman calendars. Whatever the ancient Romans’ motives for making their calendar irregular, it was probably unrelated to the hubris of Caesar Augustus.
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