We can somewhat deduce the nature of the Molad from how it appears in our sources. Every Molad follows the preceding one after 29 days, 12 hours and 793 halakim (where 1080 halakim = 1 hour) have passed. This value is found in the Talmud, Rosh Hashanah 25a:
אמר להם רבן גמליאל: כך מקובלני מבית אבי אבא: אין חדושה של לבנה פחותה מעשרים ותשעה יום ומחצה ושני שלישי שעה ושבעים ושלשה חלקים.
Rabban Gamliel says: thus I learned from the house of my fathers, the reappearance of the New Moon takes place after no less [nor more] than twenty-nine and a half days, two-thirds of an hour [720 halakim] and 73 halakim.
Why did Rabban Gamliel note “פחותה” – no less than? The answer can be found in a reference by Azariah De’ Rossi to a book by Rabbi Abraham Bar Hiyya Hanasi, Sefer ha’Ibur, where the latter remarks that according to Ptolemy, the Egyptian sages originally held that there were only 792 halakim (rather than the 793 held by the fixed calendar).
Rabban Gamliel calls the average period of the moon's revolution “חדושה של לבנה” – the reappearance of the New Moon. Science refers to this same period as the average “synodic month,” with an astronomical value of 29.530587981 days, which equals 29 days, 12 hours and 792.840 halakim. Thus, Rabban Gamliel’s value and the scientific astronomical value diverge by a mere half a second. 793 Halakim is the closest whole number in the numeric system of the Sages. While the Egyptian sages preferred to round down, it seems that, for Rabban Gamliel, it had to be “no less than” 793 halakim.
In fact, the value Rabban Gamliel received as a tradition in his family is identical with that of the Ancient Babylonians – 29;31:50:08:20 days sexagesimal. This number may well have been derived from eclipses which, being cyclical, repeat in the same location every 669 lunar months. The span between two such eclipses, when divided by 669, likely enabled the ancients to achieve such an accurate value.
In fact, the value Rabban Gamliel received as a tradition in his family is identical with that of the Ancient Babylonians – 29;31:50:08:20 days sexagesimal. This number may well have been derived from eclipses which, being cyclical, repeat in the same location every 669 lunar months. The span between two such eclipses, when divided by 669, likely enabled the ancients to achieve such an accurate value.