The following application compares a text presentation on the Jewish calendar with a multimedia presentation of the same material based on the principles outlined by Richard Mayer. This page describes the workings of the Jewish Calendar. The text is adapted from a presentation in Hebrew that can be found on the Da’at website at http://www.daat.ac.il/daat/shabat/luach/month3.html

The Rotations of the Heavenly Bodies

The earth rotates on its axis (an imaginary line that runs through its center) from east to west, one complete revolution per day. More precisely: one revolution of the earth on its axis constitutes a day. This movement of the earth is noticeable in that it establishes day and night. Because of the rotation, the sun appears to rise in the east and set in the west.

The moon revolves around earth once per month. More precisely, one complete revolution of the moon around the earth constitutes a month. The location of the moon in relation to the sun and the earth causes the moon as we see it to change its shape during the course of the month. In the beginning of the month, the moon appears to be very thin, while in the middle of the month, the moon appears to be full. The moon seems to disappear at the end of the month. According to the Rabbis, the average length of the month is 29 days, 12 hours, and 793/1080 parts of an hour (44 minutes and 3.33 seconds).

The earth completes one revolution around the sun in the course of a year. More precisely, one complete revolution of the earth around the sun constitutes a year. The earth revolves around the sun in 365.2422 days. This motion creates the four seasons – winter, spring, summer, and fall.

The Christian, Moslem, and Jewish Calendars

Jews, Moslems and Christians differ in the calendars that they use.

The Christian calendar is a solar calendar, based on the 365 day year which represents one revolution of the earth around the sun. As a result, Christian holidays always fall on the same date and in the same season. While the Christian calendar is divided into months, these months are arbitrary and have no real significance.

The Moslem calendar is a lunar calendar, based solely on the revolution of the moon around the earth. The Moslem year consists of twelve months reflecting twelve revolutions of the moon around the earth corresponding to approximately 354 days. Thus, the Moslem year is 11 days shorter than that of the Christian calendar. As a result, Moslem holidays, such as Ramadan, occur annually on different dates in the solar calendar and in different seasons.

The Jewish calendar is primarily a lunar calendar, but is based on both the moon and the sun. As a lunar calendar, it is similar to the Moslem calendar with 354 days. However, in order to ensure that Pesah falls in the spring and that Sukkot falls at the time of the harvest, a thirteenth month is periodically added to the year. As a result, Jewish holidays such as Pesah will fall on different dates in the solar calendar, but will always fall in the same season of the year.

Setting the Jewish Calendar

As mentioned earlier, the length of the month in the lunar calendar is approximately 29.5 days. As a result, the length of the month varies between 29 and 30 days. In the time of the Mishna, the beginning of the new month was determined in a process called Kiddush haHodesh. Witnesses testified to the Bet Din that they had seen the molad, the moment that the moon begins to reappear at the beginning of the month. The molad takes place when the moon is close to the line connecting the earth and the sun. At this point, the moon is not visible to the eye because the light side of the moon is totally facing the sun.

In the time of the Mishna, leap years were also set by the Bet Din when they determined that an additional month was needed to ensure that the holidays of Pesah and Sukkot would fall at the appropriate time of the year.

Today, the length of the months and the leap years are set by predetermined calculations.

The Significance of Kiddush haHodesh

The use of the lunar calendar gives a message to the Jewish people. According to some commentators, the mitzva of Kiddush haHodesh was the first commandment given to the Jewish people, even before they received the Torah (see Shemot 12: 1-2, and Rashi). Samson Raphael Hirsch explains that Kiddush haHodesh is a cornerstone of our national existence because it serves as a symbol of self-renewal and repentance.

The Da’at website, www.daat.ac.il, of the Israeli Ministry of Education is an excellent resource for educational articles and materials in Hebrew. Our thanks to the director of the site, Dr. Yehuda Eisenberg, for his permission to make use of this material.