If there are 13 Lunar cycles in a year why are there only 12 months (moon-ths) on a calendar?

actually, there are slightly more than 12 phase cycles.

because the earth/moon system orbits the sun once, you lose one phase cycle per year, in that lunar cycle.

The moon orbits the earth every 27.3 days, but completes a phse cycle every 29.5 days... because the position relative to the sun has change... the moon needs to orbit "that much farther" to return to the same phase.

when you add all the days on the Gregorian calendar you still get 13 months in a year on a 28 day cycle per month. They added days like 29-30-and some with 31 days making the month 1 to 3 days more. Each season has 13 week cycle. Gregorian still follows the 13 month cycle but the way they have it set up it looks like only 12 months in a year. The only month they have right is Feb. it has 28 days. unless they added the leap year making it 29. Mew moon means new starting on a new month and the last Qrt moon means the last week of the month. If we watch the moon cycle we will always know what week and Month it is with out looking and referring to the Gregorian calendar. It's that simple.

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RE:

If there are 13 Lunar cycles in a year why are there only 12 months (moon-ths) on a calendar?

If a month (moon-th) is based on the cycle of the moon (28 days), why are our months 29-30 days long?

I don't care about the naming of the months or which Caesar moved which days where. I'm curious why we don't have a 13 month, 28 day calendar.

Most lunar calendars must follow an intricate pattern of 12-month and 13-month years, in order to stay in line with the seasons (at least, on average).

The Jewish calendar follows a pre-determined sequence.

The Chinese calendar simply begins the year on the second New Moon after winter solstice (winter = December in this context). This determines whether the previous year had 12 or 13 months.

The Islamic calendar simply uses 12 lunar months. This causes their year to be shorter. Thus, some years, the month of Ramadan is in summer, and roughly 16 years later, it falls in winter.

Still, the beginning of a season (based on equinoces and solstices) could vary quite a lot on such a calendar.

Egypt's economy (and life) depended on agriculture. It was far more important for them to keep a calendar where seasons were fixed.

They used a calendar based on the tropical year (the time it takes for the Sun to cycle from one Tropic line to the other and back). When the Romans borrowed the idea, they fixed it so that the year would begin around the time of Spring (March 1).

Roman culture was based a lot on the older Greek culture, and the Greeks had seen astrologer take the 13 constellations of the zodiac, and turn them into 12 "houses", simply because it is a lot easier to do calculations with 12 rather than with 13 (12 can be divided by 2, 3, 4 and 6).

Combining both, the Romans ended up with 12 months.

By the time IVLIVS CAESAR (ee-ou-li-ous Kay-zar) got to fixing it, the date of Spring equinox fell on March 21 (or thereabouts). So he tried to fix it so that it would remain around there.

Because the pattern of days per month was not regular and because it was one of the very few calendars not based on the Moon, the start of each month was announced (calendae martii = beginning of March) by counting off the days before the start of the next month.

For example, the day after the ides of March (March 13th in our modern way) would be called the 18th day before the calendae of April; then 17th, then 16th and so on.

Actually, there are 12.381 lunar cycles in a year, and that's exactly the reason why our months don't match. The basic cycles by which we tell time: days, months, and years, do not fit exactly into each other, so need to be adjusted. Even mighty intellects like Julius Caesar and Pope Gregory couldn't make them fit without a lot of jiggery-pokery. Nature abhors simple ratios!

You need to do some homework on terminology. The synodic month _is_ based on lunar phases: it is the mean New Moon to New Moon period, 29.530589 days. What you call the "orbital period," astronomers call by several different names for several different definitions (draconic, tropical, and sidereal). I strongly recommend that you study an authoritative source such as the one I use, referenced below.

I hope Raymond chimes in on this. He's the most authoritative person I know on this subject.

[Edit] Ah! I see he has, with his usual lucidity and good humour!

13 Moon Calendar

There is a name for it, pretty lame but it's a 13-Moon Calendar? it's a logical and natural way to count the 365-day year cycle. Instead of 12 months which are (28, 29 -february on leap year), 30, or 31 days long, the year is instead measured into 13 months, each one an even 28 days. 13 moons of 28 days each gives 364 days..then there was this idea that you plus 1 "day out of time," a day of celebration and forgiveness, to acknowledge the passing year and welcome in the new year.

Our universal 12 month calendar corresponds to no natural cycles, the thirteen moon calendar is a "solar-lunar calendar" because 365 days is the measure of the Earth going around the Sun (solar) and 28 days is the average measure of the Moon's synodic and sidereal cycles (lunar).

Most of the world uses the Gregorian calendar, which is derived from the old Roman calendar, because the Roman Empire, and then the Roman Catholic Church, had such dominant influence on European civilization. And maybe we don't have a 13 month calendar because 13 is considered unlucky, or because 12 can be divided by 2, 3, 4 or 6.

"Synodic months deal with the moon's orbital period and not it's phasing which is 28 days."

NO NO NO...that is incorect. No cycle of the moon's is 28 days. (at least not anytime within the span of lifetime for anyone alive today)

I just checked...the closest lunar cycle that there is to rounding to 28 days is the anomalistic month (period of the perigee-apogee-perigee cycle), which is 27.55 days.

The moon's phasing cycle is 29.5 days. PERIOD.

The word synodic means "relating to conjunctions"...and those conjunction events being the full and new moon phases.

Synodic month is BY DEFINITION the period of the illumination phase cycle of the moon.

The cycle of true orbit of the moon is called the SIDEREAL month (pronunced: sigh-DEER-ee-uhl). Sidereal meaning "as measured relative to distant stars".

The sidereal month for Earth's moon's orbit and rotation is 27.3 days in duration.

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The 12 month calendar does not have any correspondance with any natural cycle.

It originally did, but they weren't very accurate, and they wanted to avoid the number thirteen for triskadeckaphobia.

Also, there AREN'T 28 days in a lunar cycle. In the synodic month, the moon cycle about which people are most aware (the illumination phase period)...the synodic month is actually 29.5 days long in duration.

That means, your idealized 28 day month would not really do the trick either.

So, some times there are 12 synodic months per year, and sometimes there are 13 synodic months per year. On an average basis, there are roughly 12.4 moon phase cycles per year. So, 60% of years would have 12 months, and the other 40% of years would have 13 months.

You can try...but your months will not re-synchronize with the new year. In fact, Chinese and Arabic calendars indeed do this..and that is why "Chinese New Year" drifts among the months of January and February rather than remaining on one day of the year.

You read what I read on Yahoo news. That every 19 year cycle that are 7 extra full moons (called blue moon because it gives 13 moons"that's 1 extra in a year that should only have 12 normally). It deals with seasons, time of earth rotation, position of stars, etc. Its not just days per month but so much more.

The moon goes through a full cycle every 29.5 days. Now see if you can devise a calendar out to equal that. Somehow you're going to have to incorporate a day every year that only lasts .37288 of a day. I'll stick with an extra day every 4 years unless you come up with something better.