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Why Is It That a Year Has 12 Months?

The ancient Romans (as, in many things, borrowing from the Greeks) had a 10-month calendar, dating to 738 B.C. The names of those months, in order, were these:

  • Martius
  • Aprilis
  • Maius
  • Junius
  • Quintilis
  • Sextilis
  • September
  • October
  • November
  • December.
Roman calendar

The Romans chose to have the length of a month roughly matching a cycle of the Moon. One lunar cycle was not an exact number of days; rather, it took the Moon 29.5 days to go once around Earth. Some ancient cultures assigned 29 days to a month; other cultures chose 30. (The sophisticated Babylonians used a mixture of the two.) For the Romans, Martius began the new year, with the first day of that month coinciding with the Spring Equinox.

Even with 30 days in a month, a total of 10 months lasting 30 days would be 300 days, and one orbit of Earth around the Sun lasted longer than that, at least 60 days longer.

Later on, the Romans added two months to the end of the year: Januarius and Februarius. That made it 12 months of 30 days, so 360. Even with a few of the months lasting 31 days, it wasn't an exact science.In 46 B.C., Julius Caesar took the initiative to revise the calendar. He kept the idea of 12 months and assigned 30 or 31 as the number of days for 11 of those months; the one exception was Februarius, which lasted 29 days. Every fourth year, Februarius had one more day, bringing that month's total up to 30.

Caesar also moved the first of the year from Martius to Januarius. This explains why Leap Day, or the last day of February every fourth year, is not at the beginning of the year. (It used to be.)

Two further points about Caesar(s): The Romans, after Julius Caesar's death, renamed Quintilis as Julius. Also, the following month, Sextilis, became Augustus, after Augustus Caesar, Rome's first emperor.

The Julian Calendar was the law of the land for a very long time. When the Gregorian Calendar superseded the Julian Calendar, the idea of 12 months in a year did not disappear.

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