Chinese Lunar Calendar: Leap Month, Intercalary Month
What is Chinese Lunar Leap Month?
First, we need to know the difference between the Chinese Astronomical Calendar and the Civil Calendar. The Chinese Civil Calendar is posted by the Chinese government. The Chinese Astronomical Calendar is used by astronomers who work for the government. The Astronomical Calendar is much more accurate than the Civil Calendar. The major difference is the assignment of Leap Months (Intercalary Months). We know a leap month is assigned if a lunar month (new moon to new moon) has no Solar Center Point. But, what do we do when the new moon day and the Solar Center Point fall on the same day?
If the new moon day and the Solar Center Point are on the same day, the lunar month contains that Solar Center Point in the Civil Calendar system. In the Astronomical Calendar, we have to compare the new moon time and the Solar Center Point time (hour, minute, and second). The lunar month contains that Solar Center Point only if the Solar Center Point time comes after new moon time.
The Theory of Chinese Lunar Calendar
Different Time Zones, Different Leap Months
Because they are different, the listing of leap month assignments is different. From the following comparison charts of Beijing Time, PST, and EST from 1979-2001. We find the following issues.
- Leap Month assignments are different between the Astronomical and the Civil Calendar
- Leap Month assignments are different from different time zones' the Civil Calendars
- Leap Month assignments are the same from all different time zones' Astronomical Calendars
- Extra 1985 Leap Month was removed in China time zone
China Beijing Time or Taiwan Standard Time - Longitude East 120th degree
| China Civil Calendar | China Astronomical Calendar | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Pacific Standard Time (PST) - USA Longitude West 120th degree
| Civil Calendar | Astronomical Calendar | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Eastern Standard Time (EST) - USA Longitude West 75th degree
| Civil Calendar | Astronomical Calendar | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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In short, the Leap Month assignments of Chinese Civil Calendar create the confusion from different time. See the example in the page of the Leap Month in Year 2001.
Issues of Leap Months
The Chinese Lunar Calendar system would be perfect if the length of the solar month was always longer than the length of a lunar month. If a lunar month is longer than a solar month, then a lunar month could contain two solar Center Points. We can call such a month the Double-Center-Point Lunar Month.
For example, the lunar month in the December of 2052 contains Winter Solstice 2052/12/21 04:18:32 and Severe Cold 2053/01/19 15:00:35 in the following table of the Chinese Astronomical Calendar.
| Solar or Lunar Date and Time | Center Point or New Moon Time |
| 2052/11/21 14:47:21 | C |
| 2052/12/21 04:16:00 | N |
| 2052/12/21 04:18:32 | C |
| 2053/01/19 15:00:35 | C |
| 2053/01/19 23:13:00 | N |
| 2053/02/18 05:03:15 | C |
| 2053/02/18 16:32:00 | N |
| 2053/03/20 03:48:44 | C |
| 2053/03/20 07:12:00 | N |
| 2053/04/18 18:49:00 | N |
| 2053/04/19 14:31:37 | C |
| 2053/05/18 03:44:00 | N |
| 2053/05/20 13:20:55 | C |
When a lunar month contains two solar Center Points, then there will be a month nearby without any Center Point. Such a month without any Center Point is not a Leap Month, is called a Fake Leap Month. Therefore, the lunar month 2053/03/20 07:12:00in the March of 2053 is not a Leap Month.
Because of this imperfection of the Chinese Leap Month assignment, we need the 5th rule into the Chinese Calendar original rules.
- The new moon day is the first day of a lunar month
- The name of a lunar name is named by the Solar Center Point
- The first lunar month of the year is the 虎 Tiger 寅 month, which contains Rain Water Center Point
- The lunar month without a Center Point is a Leap Month (閏月 Intercalary Month )
- The first Leap Month after a lunar month containing two solar Center Points is a Fake Leap Month and must be removed
All these five rules are good enough to make a nearly perfect Chinese Calendar. However, some articles about the Chinese Calendar provide more rules. such as
- The Winter Solstice should fall in the 11th lunar month
- The Chinese New Year Day must be between January 21st and February 21st
- The 11th, 12th and 1st lunar months shouldn't contain a Leap Month
- If there are two Leap Months in a Lunar Year, then the first Leap Month after the Winter Solstice is considered intercalary.
There are no strong astronomical reasons to make those rules for the Chinese Calendar. Actually, they are only thumb-of-rule to determine the location of Leap Months. When studying the Sun and Moon astronomical data, we can understand more about the issues on the assignments of Leap Month. Also, we will know more about the difference between Astronomical Calendar and Civil Calendar.
Sun and Moon Astronomical Data For Chinese Calendar
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By Master Allen Tsai on January 27, 2023