After the Exodus, God introduced seasonal feasts to Israel...
"This month shall be unto you the beginning of months: it shall be the first month of the year to you." (Exodus 12:2)
"These are the feasts of the LORD which ye shall proclaim in their seasons." (Leviticus 23:4)
The lunar year has only 354 days, which is 11 days short of the solar year of 365 days on which the seasons depend. This means the lunar calendar moves ahead of the seasons by 11 days each year. Autumn feasts would end up in the spring in just 16 years, and vice versa!
Start of a new calendar
God therefore told Israel to start its calendar year in the same season each year, in the harvest season that starts after the spring equinox in Israel. This meant that Israel had to insert an extra month every 2 to 3 years. We call this calendar lunisolar because it is a lunar calendar that keeps in line with the solar seasons by the intermittent addition of these extra months.
Adding extra months keeps the calendar in line with the solar seasons, but the true length of a lunation is 29.53 days, not 29.5, and the true length of an astronomical lunar year is 354.367 days, not 354. This means that every few years an extra day has also to be added at the end of the year to keep in line with the true astronomical lunar year.
Start of the calendar year
The start of the calendar year after the Exodus was determined by the first appearance of the crescent new moon. At the precise time of the true astronomical new moon, the moon and sun are in conjunction (in line). If the astronomical new moon occurs at or near sunset, the moon sets over the horizon at exactly the same time as the sun, in which case you can't see it because there is no visible crescent!
After that the path of the moon falls behind the path of the sun and the moon sets about 45 minutes later each day. The crescent new moon can only be seen:-
- when it grows bigger as its path falls behind that of the sun;
- as the light fades in the dusk after sunset, before the moon sets.
Israel's new day started at sunset, so the crescent new moon could not be seen until after sunset at least one day after the true astronomical new moon depending on the timing of that new moon and the weather conditions. This meant that a new day had already started, so the new month could not start until at least two days after the true astronomical new moon.
Months
Before the Exodus the Civil Calendar had 12 months in the year. The months were alternately 30 and 29 days long and so the lunar year comprised 354 days.
As far as we know month names were first used in Israel's luni-solar calendar after the Exodus, but they underwent some changes after the Babylonian captivity and are not shown. They were as follows, with those pre-captivity names we know shown in brackets:
- Nisan (Abib)
- Iyyar (Ziv)
- Sivan
- Tammuz
- Ab
- Elul
- Tishri (Ethanim)
- Marchesvan (Bul)
- Chislev
- Tebeth
- Shebat
- Adar
- Ve-Adar or Adar Sheni - Intercalary
