[29] The Biblical Year is a Whole number of Biblical Months, 12 or 13

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A tropical year is the average time from one vernal equinox to the next vernal equinox, or equivalently, from one autumnal equinox to the next autumnal equinox. In ordinary speech this is also called the solar year, and it approximates the agricultural year without drifting away.

Since a biblical month averages about 29.5 days, a 12-month period will contain about 354 days and a 13-month period will contain about 384 days. But a tropical year contains about 365.2422 days, which is about 11 days more than 12 biblical months.

Leviticus 23 is the most concentrated single area of the Tanak dealing with calendric aspects of the festival days. Upon reading through Lev 23 it should be noted that months are never mentioned by name in this chapter, but always by numbered occurrence through the year. Thus once the first month is determined, all the other months are determined because they follow sequentially by number. The first month maintains a fixed relationship to the festivals. But now it will be shown that the festivals maintain a fixed relationship to the agricultural year in Palestine. Ex 34:22 shows that the Feast of Weeks approximates the wheat harvest. Ex 23:16 shows that the Feast of Ingathering approximates a harvest time of the year. Deut 16:13 shows that the Feast of Booths approximates a harvest time of the year, but a comparison of Ex 23:14-17 and Deut 16:16 shows that the Feast of Ingathering is the same as the Feast of Booths. Since there is no harvest in Palestine during late autumn and winter, the festivals must maintain an approximately fixed relationship to the agricultural year. Therefore, the first month must maintain an approximately fixed relationship to the agricultural year and hence the tropical year. Technically this is expressed by saying that the biblical calendar is lunar-solar in nature.

The Bible has an example of a year with 13 months, showing that the biblical year was not an exact tropical year. Here is the example. The time difference between Ezek 1:1-2 and Ezek 8:1 is the difference between month 4 day 5 in the 5th year of King Jehoiachin's exile and month 6 day 5 in the 6th year of his exile. This is 14 or 15 months depending on whether the 5th year of his exile had 12 or 13 months. If the difference is 14 months, this is about 29.5 times 14 (= 413) days with an overestimate of 30 times 14 (= 420) days. The overestimate of 420 days is 17 days short of the known events because Ezek 3:15 accounts for 7 days and Ezek 4:4-6 accounts for 390 plus 40 days, the total being 437 days. Thus the difference must have been 15 months, which is about 29.5 times 15 (= 442.5) days, just five or six days more than the known events of that time period.

If one should claim that the 5th year of the king's exile was a tropical year, and an overestimate of 366 days (“leap” year) plus 60 days (two extra months) is allowed, the total is 426 days, which is still far short of the 437 days for the known events. Thus, although the biblical year maintains an approximately fixed relationship to the agricultural year, the example with 13 months shows that the biblical year is not an exact tropical year.

It will now be shown that a biblical year consists of a whole number of biblical months rather than a smaller subdivision such as days. A biblical reason for this is that Num 28:14 has the Hebrew expression chodesh bh chadshoh lh chadshay ha shanah, meaning “month by month for months of the year”, but idiomatically “each month throughout the year”. Also, I Chr 27:1 has the Hebrew expression chodesh bh chodesh lh col chadshay ha shanah, meaning “month by month for all months of the year”, but idiomatically “each month throughout the whole year”. The above example of a year with 13 months is further biblical evidence that a year consists of a whole number of months.

A biblical year cannot contain fewer than 12 months because Est 9:20-23, 26 maintains that each year on the 14th and 15th days of the month Adar the Jews are to celebrate the festival called Purim. Est 8:12 states that Adar is the 12th month. If a year could only have 11 months, then the Jews would be unable to celebrate Purim that year. Further evidence of a requirement of at least 12 months in the year comes from I Ki 4:7 and I Chr 27:1-15. Hence a biblical year contains 12 months or 13 months, or approximately 354 days or 384 days. This is an illustration of the fact that the modern cultural concept of a year always having 365 or 366 days need not necessarily be practiced in some ancient societies.

In ancient Egypt, from some time onward, their civil calendar always had 365 days, which was divided up into 12 months of 30 days each plus five extra days (see page 28 of the reference by Ronald Wells). The time of the establishment of the 365-day Egyptian civil calendar has not been convincingly proved. However, from writings that have survived from Elephantine, Egypt during Persian rulership over Egypt, the double dating scheme that equates certain dates in the Egyptian calendar with dates in the Babylonian calendar unquestionably demonstrates that from 471 BCE onward into the Middle Ages this Egyptian calendar was used (see Horn and Wood 1954, Parker 1955, and Porten 1996). Since this calendar loses about 1/4 of a day each tropical year, in 120 years it would lose about 30 days. The Egyptians certainly realized that this calendar would continuously lose time in comparison to the agricultural year, but it did not stop them from using it anyway.

Furthermore, this Egyptian calendar became the preferred calendar by which the best Greek astronomers in Alexandria recorded their astronomical observations, although they knew it fell short of the tropical year, which they measured quite accurately.

The main point in all this is to emphasize that any practical ancient calendar may have a concept of a year associated with that calendar, so that such a calendar year need not equal the tropical year. As long as a society considers a calendar year sufficiently practical for its use, it may use such a year for centuries regardless of its lack of accuracy compared to the tropical year. For ease of computation in whole numbers and payment for months worked, it is convenient to use 12 months of 30 days each and thus use a civil calendar of 360 days. The existence of such a calendar year does not provide evidence that a tropical year ever actually contained 360 days. The only way that such a claim could be proved is if there was historical evidence that the agricultural year actually averaged 360 days over many years, or if surviving archaeological statements associated with astronomical cycles claimed or directly implied that a tropical year equaled 360 days. This question of whether there is any known evidence in man’s history for a 360 day tropical year has come up twice on the web site for discussions on the history of astronomy, HASTRO-L, since I became a member in 2000, and thereby received all its emails since then. HASTRO-L is the only on-line discussion group exclusively devoted to the history of astronomy on the Internet.

HASTRO-L has many active contributors who are professors of history and professors of astronomy. There is no historical evidence that a tropical year ever equaled 360 days, although there is evidence for an ancient calendar having 360 days in certain areas of the ancient Middle East.

Some people have conjectured that during the time of the biblical flood in the days of Noah, a tropical year or a biblical year had 360 days. This remains unproved speculation. Chapters 7 and 8 of Genesis do not claim that each of the periods of time mentioned are non-overlapping, and do not claim that the sum of these time periods fully cover one exact year. The belief that a tropical year at the time of Noah had exactly 360 days is mere speculation.