Why do we have only 12 months, when there are 13 lunar months in a year?

The lunar month is 29.53 days. If you have 13 lunar months that gives you 383.89 days in the year. If you have 12 lunar months you get 354.36 days. Consequently if you want to divide the year into lunar months you get 12 months plus a 12 day unnamed period or, you can add a day or two to each month to fill the year with 12 months. And that is what has been done.

the 365.25 wouldn't change, so the dates of solstices etc wouldn't change either. different than now, but just curious if anyone has an explanation as to why the people who set up this calendar didn't use a 13.36 calendar. where a few (4)months like now would have an extra day. Each month would coordinate with the moon. Every woman's monthly cycle would be easier to track if they were off cycle.

Julius Caesar's astronomers explained the need for 12 months in a year and the addition of a leap year to synchronize with the seasons. At the time, there were only ten months in the calendar while there are just over 12 lunar cycles in a year.

The months of January and February were added to the calendar and the original fifth and sixth months were renamed July and August in honour of Julius Caesar and his successor Augustus.

These months were both given 31 days to reflect their importance, having been named after Roman leaders.

the lunar month has a mean period of 29.53059 days, and a circle has 360 degrees. So if you round off the lunar cycle to 30 days and divide it into 360 degrees, you come up with 12 months.

The 0.47 day round of error for each month, times the 12 months, accumulates to give us the other 5 1/2 days in a year, or 365 1/2 days.

"Because we use a solar calendar, not a lunar one" (as someone said above) is just parroting. Then why the hell we have months too? And like de Saade cut the month's feet to fit the year as its cot? Ridiculous.

In India Lunar Calender is followed in two ways: one is sidereal month, but it is not termed a month; rather it is observed as Lunar transit through the 27 Asterisms one a day(Nakshatras; or Lunar Mansions as per Chinese).

The synodal month between two successive FullMoons or NewMoons (or any of the 30 phases) is called the month. I think you should consider this. It falls short of a year by 11 days. Then, every three years when the stagger in the Nakshatras goes skewed an extra month is introduced like extra January when the FullMoon returns to the assigned Nakshatra, But then it gets in excess to make up thirty days after 10 years. Then a regular month just gets wiped out as emptied month. It is all complicated.

The Indians follow Lunar Calender for routine work but correct to bring it on par with Solar Year without breaking the rhythm of Lunar phases. Solar event for instance, like entry into a new Zodiacal sign (=Sankranthi) is noted & some of them like Makar Sankranthi (entering Capricorn; then considered as Winter Solstice, which now has moved to Sagittarius). Similarily Maesha Sankranthi (to Aries) that was Spring Equinox then is celebrated as New Year in some parts of India.

If you want Calender reform to fit month too, there are enough number of models already to choose from. But can you (or those) educate yourself?

It takes 365 1/4 days for the Earth to make one rotation around the sun. If you use a lunar calendar, things will not occur on the same calendar date every year--for example, the equinoxes and solstices wouldn't be on the same date every year.

Take something like Chinese New Year. It doesn't occur on the same date every year because it is based on the moon. It seems to me that things would get very out-of-sync if they were based on a lunar calendar.

the moon's orbital period is 27.3 days. a lunar month, the time between full moons, is 29.5 days. these are different things! While the moon is orbiting the earth, the earth is also orbiting the sun, so starting from full moon, the moon must travel for slightly longer than one orbital period to return to full moon. nobody is going to use the moon's orbital period in a calendar, it's far too obscure compared to the easily observable lunar month.

It all began with Julius Ceasar and Ceaser Augustus wanting a summer month named after each of them.

Before that, the Romans had a ten month year. (Confusing or what?) Named after Januar, Februar, Mars, Avril, Mai, Sept (7), Oct (8), Nov (9) and Dec (10).

With Pope Gregory realising that Epithany was occurring later every year, he decided something needed to be done he decided to drop two days in February and add keep his former Emporers' months at 31days.

Just to keep up with our sidereal year we add leap years.

the month was originally based on the phases of the moon (hence the name), but adjustments to the calendar over the centuries have changed the gregorian calendar to a solar calendar. the months are convenient subdivisions without astronomical significance.

Many people consider 13 unlucky.