Project Genesis

The Calendar and Holidays (incl. Sabbath)

The Approximate Calendar

Why was the Jewish year 5765 (2004-5) a leap year? I thought an extra month was added to prevent Passover from falling before the vernal equinox. But had an extra month not been added that year, Passover would have fallen a couple of days after the vernal equinox.

Well, you caught us! When the calendar was established by direct observation, the leap year was indeed determined by just observing whether it was spring yet. However, when due to oppression the original system became impossible, our fixed calendar was set up in its place. In order to simplify the calculations, the Sages chose a number of approximations. One of them is that they chose an approximate measure for the time between equinoxes. It is very close to the correct value, but has the additional advantage that it is a precise fraction of the length of 19 lunar years of which 7 are leap years and 12 are regular. The result is that the time that the equinoxes fall in the lunar months, and the pattern of leap years, repeats exactly every 19 years, making it easy to calculate the calendar.

Since this approximate equinox is not exactly the same as the real one, over the course of two thousand years there has been a drift of a few days. One of the places that this shows up the most is in the eighth year (like 5765) of the nineteen-year cycle. According to the approximated calculation, the equinox would always be a little more than 16 days and 16 hours after the new moon (molad) of the month of Nisan, so the eighth year is always a leap year. But now, because of the drift due to the slight inaccuracy, it actually ends up before Passover. Though it is convenient, the fixed calendar is not ideal according to the Torah, but it is the best we can do right now. G-d willing, before the drift gets very far, we will be redeemed and again be able to make a calendar based on actual observations.

Best wishes,
Michoel Reach

1 Follow-up »

  1. “G-d willing, before the drift gets very far, we will be redeemed and again be able to make a calendar based on actual observations.”

    What is this concept of redemption, and does it differ among different schools of Judaism? What prevents “actual observations” today?

    Redemption will occur after Moshiach (Messiah) is unveiled. Every true Jew in the world will be ingathered to Israel, the real borders.

    We do not use actual observations today because those observations must be brought before the Sanhedrin (Jewish High Court), and we do not today have a Sanhedrin which is recognized as having this authority. The time is coming however.

    Comment by ATR — June 5, 2007 @ 2:13 pm

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