Closely Related:
Judaism.SE/How was Rosh Chodesh determined, if a New Moon Cannot be Seen?
Calculating months and weeks in the Mosaic Law
Astronomy.SE/How Soon Could a Waxing Crescent Moon Be Seen?

1. Question:

Apart from Rabbinical commentary, (in the Talmud) - is there any Scriptural or literary evidence explaining how a New Moon was determined - especially since it is not visible?
Or, was it the case that the Waxing Crescent Moon was taken "As and For" the New Moon?

2. The Text/Context:

NASB, 1 Samuel 20:18 - Then Jonathan said to him, Tomorrow is the new moon, and you will be missed because your seat will be empty.

If a New Moon cannot be seen, then it seems that the first visible confirmation of a New Moon is actually the Waxing Crescent Moon, which can only be observed after the New Moon - minimally - after 12.1 hours with a telescope, and 15.5 hours without. (See Astronomy.SE/How Soon Could a Waxing Crescent Moon Be Seen?).

  • Voted to close as off-topic because 1) the question is about a Biblical subject but does not cite a particular verse. That is, the question does not arise from the verse cited any more than any other verse such as Exodus 12:2. 2) There is an anachronistic assumption behind question that there was a need for a "method". But the need for a method only arose in later times. 3) Whatever the answer, it makes no difference for our understanding of any Biblical text.
    – user17080
    Commented Apr 7, 2017 at 14:13
  • You could ask a question about the meaning of מחר חודש. Does it imply prediction, or does it simply mean "in a while" similar to mañana in American English. There isn't any computation of time in the OT other than וספרתם לכם ממחרת השבת and similar trivial examples.
    – user17080
    Commented Apr 7, 2017 at 14:53
  • The word מחר doesn't necessarily mean "tomorrow" in this verse. And the word חדש doesn't necessarily mean "new moon". That is a common translation but it is not a necessary translation. The simple meaning of the verse can be read, "At the coming new moon feast your place will be empty and your absence will be noted". In general, there is no calculation of time, weights or measures in the MT. The notions of precision that we have today are both post OT and "off-topic" as far as the OT is concerned. Besides that, there is no evidence of a single set calendar or even a need for one in the OT.
    – user17080
    Commented Apr 7, 2017 at 15:57
  • 1
    I think that the correct question to ask is "Does I Sam 20:18 imply that Jonathan knew when the new moon would be, and if so, how would he know?". I think that you should ask that question and close this one.
    – user17080
    Commented Apr 8, 2017 at 17:03
  • 1
    related: hermeneutics.stackexchange.com/questions/35457/…
    – bach
    Commented Oct 28, 2021 at 23:56

3 Answers 3


Genesis 1:14 Then God said, “Let there be lights in the expanse of the heavens to separate the day from the night, and they shall serve as signs and for seasons, and for days and years;

Psalm 104:19 He made the moon for the seasons; The sun knows the place of its setting.

When the New Moon occurred seems to have been totally natural to determine and non of the prophets that is mentioning New Moon as Moses, Samuel, Ezra, Nehemiah, Isiah, Ezekiel, Hosea & Amos even tell us how it is done, the answer might be found in Jubilees that state that; "1:13-14 I will hide My face from them. I will deliver them into the hand of the Gentiles. They will be captured like prey for their eating. I will remove them from the land. I will scatter them among the Gentiles. 14 And they will forget My law and all My commandments and all My judgments. They will go astray regarding the observance of new months, and sabbaths, and festivals, and jubilees, and laws."

The Book of Jubilees

This is the history of how the days were divided and of the days of the law and of the testimony, of the events of the years, and of the weeks of years, of their Jubilees throughout all the years of the world, as Yah spoke to Moses on Mount Sinai when he went up to receive the tablets of the law and the commandment, according to the voice of Elohim when he said to him, “Go up to the top of the Mount.”

Chapter 1:7-17 Write all these words for yourself which I speak to you today, for I know their rebellion and their stubbornness, before I brought them into the land of which I swore to their fathers, to Abraham and to Isaac and to Jacob, saying, “Unto your offspring will I give a land flowing with milk and honey. 8 They will eat and be satisfied, and they will turn to strange deities, to deities that cannot deliver them from any of their tribulation, and this witness shall be heard for a witness against them. 9 They will forget all My commandments, even all that I command them, and they will walk in the ways of the Gentiles, and after their uncleanness, and after their shame, and will serve their deities, and these will prove to them an offence and a tribulation and an sickness and a trap. 10 Many will perish and they will be taken captive, and will fall into the hands of the enemy, because they have forsaken My laws and My commandments, and the festivals of My covenant, and My sabbaths, and My pure place which I have made pure for Myself in their presence, and My tabernacle, and My sanctuary, which I have made pure for Myself in the midst of the land, that I should set My name on it, that it should reside there. 11 They will make themselves high places and places of worship and graven images. Each will worship graven images of his own making, Thus they will go astray. They will sacrifice their children to demons, and to all errors their hearts can work. 12 I will send witnesses to them that I may testify against them, but they will not hear. They will kill the witnesses. They will persecute those who seek the law, and they will abolish and change everything (in the Law) so as to work evil before My eyes. 13 I will hide My face from them. I will deliver them into the hand of the Gentiles. They will be captured like prey for their eating. I will remove them from the land. I will scatter them among the Gentiles. 14 And they will forget My law and all My commandments and all My judgments. They will go astray regarding the observance of new months, and sabbaths, and festivals, and jubilees, and laws. 15 After this they will turn to Me from among the Gentiles with all their heart and with all their soul and with all their strength, and I will gather them from among all the Gentiles, and they will seek me. I shall be found by them when they seek me with all their heart and with all their soul. 16 I will allow them to see abounding peace with righteousness. I will remove them, the plant of uprightness, with all My heart and with all My soul, and they shall be for a blessing and not for a curse, and they shall be the head and not the tail. 17 I will build My sanctuary among them, and I will dwell with them, and I will be their Elohim and they shall be My people in truth and righteousness.

Chapter 2:13-14 I will hide My face from them. I will deliver them into the hand of the Gentiles. They will be captured like prey for their eating. I will remove them from the land. I will scatter them among the Gentiles. 14 And they will forget My law and all My commandments and all My judgments. They will go astray regarding the observance of new months, and sabbaths, and festivals, and jubilees, and laws.

Chapter 6:32-38 Command you the children of Israel that they observe the years according to this counting, three hundred and sixty-four days, and these will constitute a complete year, and they will not disturb its time from its days and from its feasts; for every thing will fall out in them according to their testimony, and they will not leave out any day nor disturb any feasts. 33 But if they neglect and do not observe them according to His commandment, then they will disturb all their seasons and the years will be dislodged from this order, and they will neglect their established rules. 34 And all the children of Israel will forget and will not find the path of the years, and will forget the new months, and seasons, and sabbaths and they will wrongly determine all the order of the years. 35 For I know and from now on will I declare it to you, and it is not of my own devising; for the book lies written in the presence of me, and on the heavenly tablets the division of days is ordained, or they forget the feasts of the covenant and walk according to the feasts of the Gentiles after their error and after their ignorance. 36 For there will be those who will assuredly make observations of the moon and how it disturbs the seasons and comes in from year to year ten days too soon. For this reason I command and testify to you that you may testify to them; for after your death your children will disturb them, so that they will not make the year three hundred and sixty-four days only, and for this reason they will go wrong as to the new months and seasons and sabbaths and festivals, and they will eat all kinds of blood with all kinds of flesh.

Jubilees PDF

In Leviticus 23 all expect The New Moon is mentioned;

Lev 23:2 Yah's appointed times which you shall proclaim as set apart convocations—My appointed times are these: The Sabbath (Lev. 23:3) The Passover (23:5) The Feast of Unleavened bread (23:6-8) The Feast of First fruits (23:10-14) The Feast of Weeks (23:15-22) The Feast of Trumpets (23:24-25) the Day of Atonement (23:27-32) The Feast of Tabernacles (23:35-37).

Just as if it was to be a hidden day, concealed, veiled for a period of time until The New Moon ones again will be observed (Colossians 2:16-17, Isaiah 66:23, Ezekiel 45:17, 46:1-6).

  • 1
    Your statement that even the prophets didn't clarify this lends to a "common-sense" definition, (taken for granted), and probably evidence that they didn't think it needed to be clarified. But, there were a lot of changes to "tradition", (in my opinion), caused by Babylonian influence—so there may have been a change. But, your citations from Jubiliees, (and if I recall correctly, Ezekiel as well, that this is lost information does serve as an answer, (at least a moot/unknowable answer). So, I have chosen to accept this until someone offers a quantifiable source/reference. Thanks! Commented Jan 15, 2022 at 2:24

I am unaware of any Biblical text that explicitly answers this question, but we can answer what was done in practice: a new moon was recognized at the first sliver of the waxing crescent (see discussion by Ben Dreyfus here).

As noted in the first link you cited, this was done by the Jews by observation until the 4th century, and by calculation thereafter.

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 Jonathan would know the new moon was coming.

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.


The conjunction is the darken moon the Evening before the New Moon. This was the sign seen between the end and the beginning of a new Month. My take is that there was total darkness before the light in the beginning 6 days of creation. Surely we at least can understand that much. So is that where the answers are? I feel we can trust Jubilees as I feel it is worthy in my heart.

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