Is the biblical new moon the first faint visible crescent or the invisible astronomical lunar conjunction? Here are a number of reasons why we must use the crescent rather than the invisible conjunction:

1.) The ancient Hebrews had no almanacs or telescopes to figure out when the lunar conjunction would occur. The discoveries of Newton were millennia away. They had no choice but to use the visible crescent.

2.) The Hebrew words "yerach" and "lebanah," translated "moon," are never used to describe the first day of the month. The Hebrew word "chodesh" is translated "new moon." In Gesenius' Hebrew/Chaldee Lexicon we find that the term "hodesh" (#2320) which comes from #2318 and means to be new, or to polish a sword. In other words, it appears as a scimitar, or curved sword. It is not invisible.

3.) Psalm 81:3 says, "blow up the trumpet in the new moon, in the time appointed (Heb. "keseh" meaning "full moon"), on our solemn feast day." Since we know that the new moon and feast of the full moon must be the first and fifteenth days of the month respectively, it naturally follows for us to ask whether or not there can be a full moon fifteen days after a crescent. Or must we use the lunar conjunction in order to come out right? The truth is that "The necessary time for full moon varies from 13.73 to 15.80 days after conjunction" (over two days--depending on how the ellipse is oriented with respect to the sun and earth since it is off center) (The Calendars Of Ancient Egypt by Richard A. Parker). So from crescent to full moon would have even greater variability than this. When 200 such months were fed into a computer at random, it was found that the full moon proper occurred the 12th day after the crescent only one; 13th day after, 44 times; 14th day after, 94 times; 15th day after, 60 times, and 16th day after, once. In other words, the full moon proper doesn't always fall on the 15th day, so Psalm 81:3 means just basically round--not hair-splitting.

4.) Revelation 12:1-5 says that "there appeared a great wonder in heaven--a woman clothed with the sun, and the moon under her feet ... And she, being with child, cried, travailing in birth, and pained to he delivered...And she brought forth a man child who was to rule all nations with a rod of iron.'' If an astronomer were to read this passage, he would automatically think of the constellation Virgo at the point in time when the sun's path in the heavens crosses through here body. This event always occurs in mid-October, the exact time of the Feast of Trumpets in many years. If Messiah's first coming was on this feast, as we think his second coming will be, then we know it was a new moon that was visible to John. Trumpets can occur as many as 29 days after September 23rd (Autumnal equinox) or as few as the same day if the equinox and the crescent occur on the same day. Since John saw the moon under her feet, it couldn't have been a lunar conjunction.

5.) The historical precedent is visible crescents. As Hastings Dict. of the Bible, art. "New Moon," p.522 says, "There was no fixed calendar till the 4th century-and the New Moon was declared from actual observation. The eye-witnesses were carefully examined on the 30th day of each month ... If no witnesses were available, then the following day was New Moon." As the Jewish Encyc., art. "New Moon," p.243, says, "The Sanhedrin was assembled in the courtyard ("bet ya'azek") of Jerusalem on the 30th of each month from morning to evening, waiting for the reports of those appointed to observe the new moon." (See also Mishnah R.H.i.7, ii.5-7; Sanh.102) The article "Calendar, History of," p.498 says, "The history of the Jewish calendar may be divided into three periods--the biblical, the talmudic, and the post-talmudic. The first rested purely on the observation of the sun and the moon, the second on observation and reckoning, the third entirely on reckoning." As the Encyclopedia Judaica, art. "New Moon," p.1039 says, "Originally, the New Moon was not fixed by astronomical calculations, but was solemnly proclaimed after witnesses had testified to the reappearance of the crescent of the moon. On the 30th of each month, the members of the High Court (Sanhedrin) assembled in a courtyard in Jerusalem, named Belt Ya'azek, where they waited to receive the testimony of two reliable witnesses; they then sanctified the New Moon. If the moon's crescent was not seen on the 30th day, the new moon was automatically celebrated on the 31st day."

6.) Each month is 29 or 30 days. This is why Saul held a two-day festival (I Sam. 20:27. 34). David and Jonathan knew in advance that there would be a new moon festival the next day (I Sam. 20:5, 18), and the day after that, because of their mentioning waiting until the third day (I Sam. 20:5,12,19). But they didn't know in advance whether the crescent would appear the first day or the second. The very fact that two days were celebrated rather than just one is proof of their inability to predict with certainty the new moon day. But, calculating the lunar conjunction is precise and unambiguous. Only one day is needed using that method, with the help of tables. However, using the conjunction is not biblical.

Reference: Calendar Yahweh Gave To Moses, by Herb Solinsky and Rob Anderson 1982. Biblical Calendar Research by Herb Solinsky