Full Moon Determination

[AFrazier]

Various cultures used various astronomical indicators for the starts of their months, years, and seasons. As it concerns Biblical history, most of that has to do with the Jewish new moon observances, which I am plenty familiar with.

My question concerns the full moon. Visually speaking, it's difficult to say precisely when it's full. When it waxes towards, or wanes away from, the full moon phase, you can visually discern beyond a certain point that it's not quite a full circle. But, there is a point in the waxing and waning just prior, and just after, where it's not a technical full moon, though to the naked eye it would appear to be.

Given that the Jews used full moons, at least in the instance of the Passover, as a landmark (although not a determiner) of the festival, and the Greeks used full moons to determine the Olympic games, and by extension the Olympiads, my question is whether there was a specific methodology for declaring that the moon was full. Jewish months, for example, were determined by the first sighting of the new moon crescent. That's not true new moon. The first sighting is usually at about one day old or later. Similarly, is there a specific protocol for saying, "this moon is now full"?

Up to present, the closest indicator I have found is bRosh Hash. 21a, ". . . When you see the moon ceases shining with daylight, clear away leaven [for Passover]. When does it so shine? On the fifteenth [of the month]. But we clear away leaven on the fourteenth? For them, as they had a clear view, the moon commenced to shine into the day from the fourteenth."

This is the closest visual determiner I have found as far as a fixed protocol is concerned. Does anyone have any information they can add about either Jewish or Greek practices in determining the full moon?

 

[Mockingbird0]

In the Gregorian Easter cycle, the moon is considered to be full on the 14th day of the lunar month. The lunar month begins a day or two after the mean conjunction so it corresponds approximately to the average new waxing crescent. Here are the astronomical and ecclesiastical full moons for 2024:

Astronomical full moon (UT) 2024 / Gregorian ecclesiastical full moon 2024
Jan 25 / Jan 25
Feb 24 / Feb 24
Mar 25 / Mar 25
Apr 23 / Apr 23
May 23 / May 23
June 22 / June 21
July 21 / July 21
Aug 19 / Aug 19
Sept 18 / Sept 18
Oct 17 / Oct 17
Nov 15 / Nov 16
Dec 15 / Dec 15

As you can see the Gregorian approximation is fairly close when only the day is considered.

 

[AFrazier]

In the Gregorian Easter cycle, the moon is considered to be full on the 14th day of the lunar month. The lunar month begins a day or two after the mean conjunction so it corresponds approximately to the average new waxing crescent. Here are the astronomical and ecclesiastical full moons for 2024:

Astronomical full moon (UT) 2024 / Gregorian ecclesiastical full moon 2024
Jan 25 / Jan 25
Feb 24 / Feb 24
Mar 25 / Mar 25
Apr 23 / Apr 23
May 23 / May 23
June 22 / June 21
July 21 / July 21
Aug 19 / Aug 19
Sept 18 / Sept 18
Oct 17 / Oct 17
Nov 15 / Nov 16
Dec 15 / Dec 15

As you can see the Gregorian approximation is fairly close when only the day is considered.

Not quite the information I was looking for, but I appreciate you. My interest is in first century methods of determining when it's full. The Jews, for example, used the first crescent sighting of the new moon to determine "new moon" and the start of a new month. But the full moon doesn't always fall on the fourteenth day of the month by that method. Furthermore, the moon is often full over the course of two days, depending on when it reaches its point of being astronomically full. It remains so for roughly twenty-four hours. So, the full moon can be seen in its full state on two separate days. So, the question is, is there a documented practice for how the Jews determined which visual sighting was "full," since one day before or after is so close that it would be incredibly difficult to discern visually. There must have a been a method or protocol for determining that it was a full moon. In the example I gave, it would suggest that the moon is full when it's first visible as a full moon at dawn. But that's the best I've been able to find.

 

[Bob Crowley]

It seems that from the time of Ezra (400-500BC) to 70AD when the second temple was destroyed, there was an astronomical council which determined the precise date of a new moon, and from that the full moon.

Ancient Jewish Calendar Construction—No. 1

A look at historical, archaeological, and scientific findings.

www.ministrymagazine.org

The Astronomical Council​

Under the second temple—from Ezra to 70 A. D.—the time finally came that the new year was appointed by a judiciary act which was preceded by the hearing of witnesses. According to Schwarz, it is probable that the Great Synagogue established this custom that the new moons—at least of Nisan and Tishri—should be confirmed by observation and the deposition of credible witnesses.10 It became the duty of every Israelite to inform the Jewish court personally if he had seen the new moon. He might even override the Sabbath; and no matter how many witnesses were called before the Sanhedrin, every testimony was heard. On the contrary, no final result was ever to be made public, and for this reason only men who were reticent and trustworthy were summoned?11

The patriarch who presided over the astronomical council had to know whether the witnesses gave a correct report of the moon's position. He must be able to calculate in advance with great precision the direct visibility of the the crescent.12 Sidersky, citing Albiruni, insists that these calculations had been in use since the second century B.C., and that probably they go back much earlier in point of time.13 Zuckermann maintains that the formalities of the ancient tribunal were used merely to confirm the astronomical calculations, and above all, to surround with mystery the deliberation of the council behind closed doors.14

 

[AFrazier]

It seems that from the time of Ezra (400-500BC) to 70AD when the second temple was destroyed, there was an astronomical council which determined the precise date of a new moon, and from that the full moon.

Ancient Jewish Calendar Construction—No. 1

A look at historical, archaeological, and scientific findings.

www.ministrymagazine.org

Thanks Bob. This is mostly concerning the new moon. I have that information already. And truth be told, I have the means to determine full moons. But just as there was a visual methodology for decisively declaring the new moon, I'm wondering if there is something similar for the full moon. I haven't seen anything in the Talmud to definitively suggest if there was or not, except for the vague statement about destroying your leaven if you see the full moon at dawn, for it is the 14th if you do.

BUT, it's not always the 14th when the full moon is seen at dawn. It is often the case at that time of the year. But it's not always the case throughout the year. So, for the sake of other calculations, such as the Olympiads, I'm trying to nail down anything I can find for the ancient world that delineates an ancient method for determining the full moon. It's only technically full for about twenty-four hours. But it looks full for up to three days (the day before, the day of, and the day after). With events that are dependent upon the full moon, like the Olympic games, there must have been a set method for saying, "THIS is the full moon," rather than, "well, it looks full."