Why are there not 13 months?

Answer 1

The "month" was originally invented in an attempt to split the year up into

slices that match the moon's cycle of phases.

In one year, the moon goes through its complete cycle of phases 12.368 times.

That's closer to 12 than it is to 13. So, even though the system never really

caught on, 12 months in a year still stay closer to paralleling the moon's changes

than 13 shorter months would.

To look at it from a slightly different angle:

-- The moon does a complete set of phases every 29.531days.

-- 12 months in a year would average 30.44 days each ... 0.91 days long.

-- 13 months in a year would average 28.1 days each ... 1.44 days short.

The 12 months come closer to tracking the moon's cycle, even though nobody

cares any more.

You may be interested to know that since there are 12.368 lunar months in a year, if there were 12 lunar months in a year then every 3 years or so there would be the need for an additional month - a kind of lunar Leap month. The Hindu lunar calendar does have several leap days and occasional leap months to allow for the mismatch between the lunar month and the solar year. Unfortunately, the calculations for these additional days and months are not the easiest to understand.