Months are also recorded according to the moon. Why do some calendars have an extra month?
Arrange the months of the whole year into strips and see what problems each leap month and leap day solve.
Calendars all say "month", but the number of months in a year is not necessarily the same. The Jewish calendar sometimes schedules a thirteenth month, and the arithmetic Islamic calendar always has a twelvemonth. They all relate to the moon, but use different methods to deal with the relationship between the year and the seasons.
Guess first
If the months are arranged according to the moon, will there always be the same number of months every year?
Show answer and explanation
Not necessarily, some calendars will insert leap months — The Jewish calendar uses leap months to maintain contact with the seasons; the arithmetic Islamic calendar does not insert leap months.
Then connect experience and science
In the same calendar year, open different calendars
After selecting the Gregorian calendar year, take the Jewish calendar year or the arithmetic Islamic calendar year where January 1 of that year is located. The month bar shows the length of each month and marks the Gregorian calendar date of the beginning of the month.
Different calendar years have different starting dates
The Jewish calendar uses leap months to coordinate the moon and seasons
There are 7 leap years in the 19 years of the Jewish calendar, and leap years have one extra month; a year may have 353–355 days or 383–385 days. The length of the month also changes depending on the waning year, the regular year, and the waxing year.
7 leap years in 19 years; 12 months or 13 months
Arithmetic Islamic calendar does not have leap months to make up for seasons
Each year has 12 fixed months, with a month length of 29 or 30 days; in 11 leap years in 30 years, one day is added to the last month. It is 354 or 355 days long, so the months gradually pass through the different seasons.
11 leap years in 30 years; 354 or 355 days
Keep calendar calculations separate from actual holiday announcements
Here you can look up the date of the first month of the month given by the rules, but the arithmetic result is not called a true visual record of the new moon. The actual beginnings of religious months and festival arrangements may depend on the region and the system adopted.
Rule date ≠ measured new moon or local announcement
First fix the calendar, year and month sequence, then accumulate the years and month lengths specified in the calendar, and convert them into the Gregorian calendar through JD. The 7th leap in 19 years and the 11th leap in 30 years belong to different calendars and cannot be combined into one formula.
give it a try
Compare "Jewish Leap Month Year" and "Jewish Calendar Ordinary Year", and then switch to "arithmetic Islamic calendar". After selecting a month, the Gregorian calendar date corresponding to the first day of the month will be listed on the right.
No need to memorize symbols
- 19 year cycle
- Leap Month Cycle in the Jewish Calendar
- 30 year cycle
- Arithmetic Islamic Calendar 11 Leap Year Cycle
- 1 Tishri
- Jewish civil year begins
- 15 Nisan
- Jewish calendar dates for Passover
- AH
- islamic chronology
Algorithm, accuracy and scope of application
Covers Jewish calendar years, month lengths and 19-year leap month rules in Chapter 9 of the second edition, 15 Nisan (Passover) dates, adding 163 days to the next 1 Tishri, as well as arithmetic Islamic calendar month lengths, 30-year leap day rules and date correspondence. The Jewish calendar uses Gauss/Pesach calculations to determine the year boundaries and then arranges the month lengths according to Table 9.A. The limited reference is to the Gregorian calendar 1900–2100. The Islamic calendar uses independent accumulated month/year days to map to JD, and the epoch corresponds to the Julian calendar 622-07-16; the old inverse algorithm of JulianToMoslem that has been marked with AA+ is not used, and the page only queries the start day of the complete month. Differences in sunset, rising, and regional observations are preserved; religious time announcements or all historical systems are not provided. The 13th month clearly shows "non-existence" in the middle of December, and does not silently change to the 12th month.
my understanding check
What is the biggest difference in the number of months between a leap year and a normal year in the Jewish calendar?
one more month — 13 months are scheduled in 7 out of 19 years.
Will there be an extra 13th month in a leap year in the Islamic calendar?
No, just add one day to the last month — It always has 12 months, with 354 days in ordinary years and 355 days in leap years.
Can the beginning of Ramadan calculated in this lesson be directly used as an official announcement in various places?
Can't — While real systems may depend on crescent observation, region, or rules adopted, only arithmetic calendar dates are given in this lesson.
Whether a calendar wants to maintain seasons determines how it arranges leap months or days; the calculated dates also need to be distinguished from actual observations and regional systems.
Jean Meeus · Astronomical Algorithms, 2nd ed. · No. 9 chapter