A special question for people who have studied previously the biblical calendar and its relationship with the moon phases.

Most of us agree that the New Moon (aka First Sliver / First Crescent / Waxing Crescent) occurs immediately after the Dark Moon, which lasts according to my investigation 21-26 hours. I notice now in the Hebrew calendar that it usually assumes the first day of a month being the day after the Dark Moon, while the Islamic calendar varies from 1 to 2 days after the Dark Moon. I do generally distrust Islam, but wonder if the beginning of a biblical month possibly occurred constantly on day 2 after the Dark Moon, considering also that the Jews nearly lost track of their own calendar after Babylon and not always applied the Bible correctly as we are well aware of.

Option 1 · New Month = Dark Moon +1 day · First Crescent has to be predicted and New Month actually starts hours before the New Moon can be observed.

           Dark Moon             First Crescent

Adar 30 ............. ┃ ............. Abib 1 .............┃ ............. Abib 2

Option 2 · New Month = Dark Moon +2 days · First Crescent can be observed the night before the New Month starts.

           Dark Moon             First Crescent

Adar 29 ............. ┃ ............. Adar 30 .............┃ ............. Abib 1

Basis for both options is the sunrise-sunrise rhythm of a biblical day (1Sam 19:11). But even if the erroneous sunset-sunset-rhythm would be assumed, the announcement of the New Year would have to occur in all Israel simultaneously and within minutes after the New Moon becomes visible at night (sacrifices for the new month needed to be prepared and the horn was to be blown).

2 Answers 2


A new moon was recognized at the first sliver of the waxing crescent (see discussion by Ben Dreyfus here). This was done by the Jews by observation until the 4th century, and by calculation thereafter (source).

Because a true lunar month is approx. 29.5 days, an observed lunar month would last either 29 or 30 days. That the beginning of the month was determined by observation in Biblical times did not mean they didn't know when the new moon would be--they could always narrow it down to one of two possible dates (+29 or +30 from the last new moon). So it's not at all unrealistic that people would know the new moon was coming the next day (such as Jonathan in 1 Samuel 20:18).

In fairness to the ancient astronomers, they spent a lot more time looking at the heavens than most of us do, so while error is possible in an observed system, I suggest we give them the benefit of the doubt (in the absence of conflicting evidence) that they knew what they were doing--they saw a waxing crescent (though not quite as quickly as it could have been seen with a telescope) and validated the beginning of a new month. Waiting until dark moon +2 days would generally have been unnecessary.

As noted in the link above to Dreyfus' work, for a time the Jews used signal flares to rapidly transmit--over long distances--the news that a new month had been recognized.

Whether we're counting days from sunrise to sunrise or from sunset to sunset wouldn't make a significant difference to this specific question--the new month began when a sliver of the waxing crescent could be seen & validated--this could be within a few hours of the dark moon.

  • Thank you. 1Sam 20:18 is very interesting, though it could point in both directions, considering the more reasonable translation of 'tomorrow is the New Month' (Strong's 2320), although our translations come with the term 'New Moon'. But yes, a Dark Moon which lasted one day, should have been already a good orientation if the Waning Crescent was observed in time. Commented Aug 1, 2022 at 5:12
  • I probably found the answer - through Elika Kohen's comment in your link. hse.ru/data/2011/01/20/1208886469/… This means that the New Year would have started on day 2 after the Dark Moon (0-Dark Moon; 1-Crescent; 2-Declaration), assuming that this principle can also be applied for the temple period: Commented Aug 1, 2022 at 8:19
  • "The procedure for determination of the new month is described at length in the Mishnah, tractate Rosh ha-Shanah: [...] Whoever sees the new moon must testify, the -NEXT MORNING-, before a rabbinic court consisting of at least three judges. [...] The court interrogate the witnesses to establish whether their testimony is astronomically plausible. If it is, the head of the court declares the new month to be ‘sanctified’: -THAT DAY- is thus rosh ḥodesh, the first day of the month." Commented Aug 1, 2022 at 8:20

It's possible that this question is moot.

  • God defined which days are his holy days:

… This day [Feast of Tabernacles] is holy unto the LORD
— Nehemiah 8:9

  • God appointed these days:

Thou shalt keep the feast of unleavened bread: (thou shalt eat unleavened bread seven days, as I commanded thee, in the time appointed of the month Abib …
— Exodus 23:15

Blow up the trumpet in the new moon, in the time appointed, on our solemn feast day.
— Psalm 81:3

  • God used the Jews to preserve this knowledge:

The secret things belong unto the LORD our God: but those things which are revealed belong unto us and to our children for ever
— Deut. 29:29

  • God forbade the Israelites from using observation to determine special days:

There shall not be found among you … an observer of times …
— Deut. 18:10

Ye shall not … observe times.
— Leviticus 19:26

For these nations, which thou shalt possess, hearkened unto observers of times, and unto diviners: but as for thee, the LORD thy God hath not suffered thee so to do.
— Deut. 18:14

If God had defined which days are holy to him, and had appointed the specific times when they occur, would he have then left it to chance, allowing human observers (which can be unreliable, or impossible due to weather) to choose each day? And if he had expected observation to decide which day begins each month, why would he have then forbidden observing times?

Doesn't it make more sense that God had taught Moses how to calculate the holy calendar (just as the Jews still do in our current time), and that this calculation has been passed down and preserved for 3500 years?

Much of the above is based on The Crucifixion Was Not On Friday — Herman L. Hoeh — 1968

  • Run from Hernan Hoeh. No serious teacher misquotes Deu 18:10 in such a heretical way. "There shall not be found among you one who makes his son or his daughter go through the fire, or one who practices divination, or an interpreter of signs, or an augur, or sorcerer, or one who casts magic spells, or one who consults a spirit of the dead, or spiritist, or one who inquires of the dead." Going by the calculated lunar conjunction also contradicts the command in Deu 16:1: “Observe the month of Abib and keep the Passover…” [“observe” = shamar = “look narrowly for, search”, Strong’s 8104]. Commented Aug 1, 2022 at 6:36
  • @ThomasLorenz, I didn't offer the citation as an authoritative source. I used it as a quick reference for those specific quotations, and thought I should give credit since I didn't do the research myself. Commented Aug 1, 2022 at 13:11
  • @ThomasLorenz, The "observe" in my quotations is "anan", H6049, which is generally used in the sense of "to keep", as in "to keep the law" or "to observe the law". Compare with H104's use in Genesis 17:9 ("keep my covenant"), or Genesis 26:5 ("… Abraham obeyed my voice, and kept my charge, my commandments, my statutes, and my laws."). ¶ Other versions translate Deuteronomy 16:1 as "Set aside the month" or "Keep the month", making it clearer which meaning of "observe" is meant. Commented Aug 1, 2022 at 13:18
  • It says 'an interpreter of signs' in the context of witchcraft, not 'observer of times' in the context of defining the beginning of a month. The exactly same applies for verses Lev 19:26 and Deu 18:14 - also in a totally different context. This is clear heresy. But God can forgive you upon repentance combined with ongoing change. Commented Aug 1, 2022 at 20:15

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