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Chinese Leap Lunar Months - Chinese 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.

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.

China Beijing Time or Taiwan Standard Time - Longitude East 120th degree

China Civil Calendar China Astronomical Calendar
Year New moon Leap month
1979 07-24 09:41 06
1982 05-23 12:40 04
1984 11-23 06:57 10
1985 02-20 02:43 01 (Removed)
1987 07-26 04:37 06
1990 06-23 02:55 05
1993 04-22 07:49 03
1995 09-25 00:55 08
1998 06-24 11:50 05
2001 05-21 10:46 04
Year New moon Leap month
1979 07-24 09:41 06
1982 05-23 12:40 04
    N/A
1985 04-20 13:22 03
1987 08-24 19:59 07
1990 06-23 02:55 05
1993 05-21 22:06 04
1995 10-24 12:36 09
1998 07-23 21:44 06
2001 06-21 19:58 05

Pacific Standard Time (PST) - USA Longitude West 120th degree

Civil Calendar Astronomical Calendar
Year New moon Leap month
1979 06-24 03:58 05
1982 05-22 20:40 04
    N/A
1985 03-21 03:59 02
1987 08-24 03:59 07
1990 06-22 10:55 05
1993 05-21 06:06 04
1995 09-24 08:55 08
1998 07-23 05:44 06
2001 06-21 03:58 05
Year New moon Leap month
1979 07-23 17:41 06
1982 05-22 20:40 04
    N/A
1985 04-19 21:22 03
1987 08-24 03:59 07
1990 06-22 10:55 05
1993 05-21 06:06 04
1995 10-23 20:36 09
1998 07-23 05:44 06
2001 06-21 03:58 05

Eastern Standard Time (EST) - USA Longitude West 75th degree

Civil Calendar Astronomical Calendar
Year New moon Leap month
1979 06-24 06:58 05
1982 05-22 23:40 04
    N/A
1985 04-20 00:22 03
1987 08-24 06:59 07
1990 06-22 13:55 05
1993 05-21 09:06 04
1995 09-24 11:55 08
1998 07-23 08:44 06
2001 05-22 21:46 04
Year New moon Leap month
1979 07-23 20:41 06
1982 05-22 23:40 04
    N/A
1985 04-20 00:22 03
1987 08-24 06:59 07
1990 06-22 13:55 05
1993 05-21 09:06 04
1995 10-23 23:36 09
1998 07-23 08:44 06 06
2001 06-21 06:58 05

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 month as 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 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 this imperfection of Chinese Leap Month assignment, we need the 5th rule into the Chinese Calendar original rules.

All these five rules are good enough to make a nearly perfect Chinese Calendar. However, some articles about the Chinese Calendar provides more rules. such as

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.

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