The Jewish Calendar this year is a Leap Year. Why?

Why does the Jewish calendar have 13 months this year?

Short answer:

The modern Jewish calendar has 7 leap years containing an extra 13th month every cycle of 19 years.

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Longer, historical answer:

There are two main types of calendars: A lunar one and a solar one.

The Lunar Year

The lunar calendar is where each year is a number of lunar cycles. The Muslim religious calendar is based on this.

This lunar calendar is easy to observe. The downside of using this type of calendar however is that it does not stay synchronized with the solar calendar. Twelve lunar months of around 29.5 days each equals a lunar year of around 354 days. This means that each lunar year will be out of sync with the solar orbit of ~365.25 year by around 11 days.

Thus using a lunar year of 12 lunar months means the seasons will shift through the calendar from year to year. For instance, the Muslim fast of Ramadan can occur in any season depending on the year.

The Solar Year

Don’t think that using a solar year doesn’t also has its challenges. A solar orbit is precisely 365.25. Thus, a solar orbit still means that a 365-day year loses a day every 4 years.

Efforts to synchronize the number of days of the year to the solar orbit has been going on since Sumerian times. Efforts to synchronize the lunar year to the solar year were undertaken in ancient China and Egypt as well, though apparently the latter didn’t change the yearly calendar but merely added five days of feasts to the end of every year.

The Romans “invented” the solar leap year of an extra day every four years. This was the famous Julian Calendar - in use since 46 BCE.

By the 1570’s, by the way, those extra 11 minutes of each solar year not accounted for by the Julian Calendar were threatening to put Easter in a different season and the equinoxes were 11 days out, so Pope Gregory XIII introduced the Gregorian Calendar with its refined leap year calculations. Oh and he changed the date by 11 days to catch up to the real solar cycle. There are urban myths that this caused Calendar Riots all over Europe as people accused the Pope of stealing 11 days of their lives.

The Jewish Calendar

The Jewish calendar uses a lunar year as per a Torah Commandment. The command to sanctify the New Moon and calculate the Jewish calendar was the very first law (Ex 12) given to the Jewish people.

However, the Jewish Calendar also synchronizes to the Solar Calendar by introducing leap months. This is to ensure that the Jewish festivals such as Passover and Succot occur in their allotted seasons (also mandated by the Torah).

Thousands of years ago the greatest Rabbis, (some of whom were great astronomers would decide a leap year.

Hillel II (4th century CE) calculated the details of the Jewish calendar and devised a perpetual system of 19 year cycles. Within each cycle of 19 years there are 7 leap years (years with 13 lunar months instead of 12). The extra month is Adar 2

Thus every 2 or 3 years, the Jewish people have a 13-month Jewish year instead of 12.