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Number of days in a lunar month


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#1 agedstudent

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Posted 13 December 2008 - 09:58 PM

To find the number of days in a certain month in the lunar calendar I have to look up the almanac. Unlike the western calendar I always know which month is the long month and which is shorrt month. I can't do the same with the lunar month. Can anybody tell me how the number of days in a lunar month is determined?

#2 William O'Chee

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Posted 14 December 2008 - 12:07 AM

A lunar month is determined by a complete cycle of the phases of the moon. Although the moon orbits the Earth in a little under 28 days, the synodic period is about 29.5 days. The synodic period is the time it takes for the moon to return to the same point in the sky, and therefore to exactly the same phase as previously. The difference between the orbital period and the synodic period is accounted for by the fact the Earth has moved roughly one twelfth of the way through its own orbit around the sun in this time, and hence that affects the apparent position of the moon in the sky.

Lunar calendars attempt to show this in terms of days, but because there is a fraction of a day in the synodic period, they are approximations only, since the same phase will not occur at the same time each day.

#3 Jaak

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Posted 20 February 2009 - 04:55 PM

A lunar month is determined by a complete cycle of the phases of the moon. Although the moon orbits the Earth in a little under 28 days, the synodic period is about 29.5 days. The synodic period is the time it takes for the moon to return to the same point in the sky, and therefore to exactly the same phase as previously. The difference between the orbital period and the synodic period is accounted for by the fact the Earth has moved roughly one twelfth of the way through its own orbit around the sun in this time, and hence that affects the apparent position of the moon in the sky.

Lunar calendars attempt to show this in terms of days, but because there is a fraction of a day in the synodic period, they are approximations only, since the same phase will not occur at the same time each day.

Yes, but how is the approximation made?

In the Julian and Gregorian calendar, there is one month, February, which is sometimes 28 and sometimes 29 days.

In a lunar calendar, there have to be some years which have 12 months and some which have 13 months. Usually, the 13th month is at the same time of year. This was the case in China till Emperor Wudi. But the calendar of emperor Wudi enacted that any month of the year could be 13th month.

Since the synodic month is about 29,53 days, there is no need for an especially short month like February. It is quite possible to have just 29 and 30 day months. Approximately each second month should be a hollow month, and the other a full month.

But since this is only approximate, there have to be extra full, 30 day months. Sometimes there have to be two or more 30 day months in succession.

When do the Chinese have their extra full months?

#4 Naoko

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Posted 29 March 2009 - 04:25 AM

Since the synodic month is about 29,53 days, there is no need for an especially short month like February. It is quite possible to have just 29 and 30 day months. Approximately each second month should be a hollow month, and the other a full month.

But since this is only approximate, there have to be extra full, 30 day months. Sometimes there have to be two or more 30 day months in succession.

When do the Chinese have their extra full months?

The synodic month is about 29.53 days, but that is average. Actual lunar months vary from 29.27 to 29.83. From the 5th century AD the Chinese calendar has used actual lunar month. Long or short months are put when they are calculated to occur, whatever the pattern, and it can be very irregular. There can be as many as 4 long months or 3 short months in a row.

As to how the new moons are determined, the method changed little by little over the century as they understood the variations in the sun and moon. The Mongol-period 授時 calendar was a great calendar. The Ming and Qing calendars were just minor variations on it.

#5 hanhuang

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Posted 12 May 2009 - 08:22 AM

Beginning of each lunar month coincides with the eclipse moon.

One rule which ties the lunar calendar to the solar year is winter solstice must occur on the 11th month of the lunar calendar. If there happens to be 13 new moons between 2 winter solstices, a leap month is added to that year.




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