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A big question on a calendar is when does a new month start?

For calendars that use the moon for determining the start of a new month, there are 4 different ways I've seen people calculate the start of a new month. These are:

New moon (mathematically calculated) not many people seem to be using this since there is no evidence that back in biblical times it was used.

Sliver moon #1 (mathematically calculated - usually 2 days after the new moon) Seems this is more commonly used and specifically in the Jewish calendar

Sliver moon #2 (rabbis actually see the first sliver moon) not many people (today) seem to be using this since you can never calculate a future months start until the new moon actually happens and historically there is no way to know 2000+ years ago when each month actually started

Full moon I've only found a few people who suggest that the Bible says a new month starts on a full moon and not on a new/sliver moon. This would mean that Nisan 14/Passover would be a new moon and then you have the possibility of a full solar eclipse. There is clearly a problem with this since the longest a full solar eclipse has ever been is 4-7 minutes not the 3 hours described in Luke 23:44-46 (NIV)

New moon (mathematically calculated)	not many people seem to be using this since there is no evidence that back in biblical times it was used.
Sliver moon #1 (mathematically calculated - usually 2 days after the new moon)	Seems this is more commonly used and specifically in the Jewish calendar
Sliver moon #2 (rabbis actually see the first sliver moon)	not many people (today) seem to be using this since you can never calculate a future months start until the new moon actually happens and historically there is no way to know 2000+ years ago when each month actually started
Full moon	I've only found a few people who suggest that the Bible says a new month starts on a full moon and not on a new/sliver moon. This would mean that Nisan 14/Passover would be a new moon and then you have the possibility of a full solar eclipse. There is clearly a problem with this since the longest a full solar eclipse has ever been is 4-7 minutes not the 3 hours described in Luke 23:44-46 (NIV)

A problem with using the moon to determine the start of a new month, and subsequently the start of a new year, is that the beginning of a new year could change by almost 30 days from year to year. For example, mapping back to a Gregorian calendar, the beginning of a Jewish new year could be as early as March 9th (or a little earlier) to as late as April 14th (or a little later). As a result of this, the Jewish calendar periodically requiring what are called leap months to resync the calendar back to the sun.

For me, this is problematic since important historical dates, like Passover, are not consistent from year to year. Yes Passover will always be Nisan 14, but relative to the sun it can vary by 30+ day from one year to the next. Imagine having your birthday on Nisan 1 and in some years it still might be snowing outside (March 9th) while in other years the flowers are starting to bloom (April 14th).

This is one reason I personally do not like a lunar calendar since mapping biblical fiests from one year to the other inconsistant. I personally like the static calendar since it is solar based so always starts on the same day regardless of the year.

-- Tait - 2026-02-23

Revision r1 - 2026-02-23 - 22:59:53 - TaitCyrus Edit