The Jewish calendar is essentially composed of three units that depend on a heavenly body for proper calculation. The month is dependent upon cycles of the moon, and the day and the year are dependent upon cycles of the sun. For the purposes of our discussion, we will focus on the "month" and the "year." ( The following discussion is oversimplified for clarity's sake.)

The Jewish nation was instructed to bless and sanctify each new month. The month is the period of time it takes for the moon to complete a revolution around the earth. On the average, this takes 29.5 days. As we only have complete days in a month, some months contain 29 days, and other contain 30.

As mentioned above, a year is a solar dependent unit of time. It is the period of time it takes the sun to go through a complete cycle of the Zodiac or the time it takes for the sun to repeat the same point in its orbit. A solar month is really just 1/12th of this period of time. A solar month has in it of itself has no specific tie to any cycle of any celestial body.

Why do we maintain two different systems of time calculation?

If we were to calculate a year by equating it to 12 lunar months, we would be faced with a problem. The period of 12 lunar months is shorter than the period of 12 solar months. We know that the seasons of the year are dependent on the position of the sun in relation to the earth. Hence, seasons are also dependent on the sun. As mentioned in post # 52, each holiday has to be in a specific season. Holidays occur on a specific date of a lunar month each year. If we were to ignore the solar calendar and base our years solely on the lunar calendar, after a number of years, the holidays would start moving out of their proper seasons. There is therefore a need to keep our months ( the lunar unit) and years ( the solar unit) in sync, so that the holidays ( which are lunar dependent) will fall out in the proper season of the year (which is solar dependent).

How do we accomplish this? The Jewish calendar has an established system of "leap years." In a leap year, the Jewish year is composed of 13 (lunar) months instead of 12. This way, the difference between a year (12 solar "months') and 12 lunar months is equalized, and the holidays will fall out in their proper seasons. This factor is what differs the Jewish calendar from other calendar systems in use. The secular (Roman) calendar that we use is totally solar-based. A month in the secular calendar, as we said above, is 1/12 of the solar cycle, and has no connection to the lunar cycle. The Moslem calendar takes the opposite approach. A month is connected with a cycle of the moon. However, no attempt is made to reconcile the lunar cycles with the solar year. Therefore, the year is shorter than a solar year, and the months have no connection to seasons.

In summary: By definition, there are two distinct units of time which are not dependent on each other: a year, and a month. A year is a unit of time measured by a cycle of the sun, and a month is a unit of time measured by a cycle of the moon. In the Jewish calendar, we have to reconcile both methods of calculation because we were commanded to sanctify the lunar months, and we were also commanded to celebrate each of the holidays is its appropriate season. In order to accomplish that, we have leap years which contain 13 lunar months instead of 12, to make up the difference of days between a solar year and 12 lunar months.