Sanhedrin 42A

ותרוייהו כרבי יוחנן סבירא להו: הא למיהוי כי יתרא, הא למיהוי כי נפיא.

The Talmud comments: And they both hold in accordance with the opinion of R' Yoḥanan that one can recite the blessing until the flaw of the moon is filled. The dispute is that this one, i.e., Rav Ya’akov bar Idi, who holds one can recite the blessing until 7 days have passed, understands R' Yoḥanan to be referring to the day when the moon will be like the string of a bow. Before that point the moon appeared merely as a bow, and after 7 days it appears like a half-circle, like a bow that has a string. That one, i.e., the rabbis of Neharde’a, who holds one can recite the blessing until 16 days have passed, understands R' Yoḥanan to be referring to the day when the moon will be like a sieve, i.e., a full circle.

אמר ליה רב אחא מדיפתי לרבינא: וליבריך ״הטוב והמטיב״! אמר ליה: אטו כי חסר מי מברכינן ״דיין האמת״, דלבריך ״הטוב והמטיב״? וליברכינהו לתרוייהו? כיון דהיינו אורחיה, לא מברכינן.

§ Rav Aḥa of Difti said to Ravina: And they should bless the blessing of: Blessed are You, Lord our God, King of the Universe, Who is good and Who does good, for the benefit that people derive from the light of moon. Ravina said to him: Is that to say that when the moon is shrinking we bless, as we do for other disasters: Blessed are You, Lord our God, King of the Universe, the true Judge, so that we should conversely bless: Blessed are You, Lord our God, King of the Universe, Who is good and Who does good, when the moon is growing? Rav Aḥa of Difti said to him: You are correct, and we should say them both: The blessing of the true Judge, when the moon is waning, and the blessing of Who is good and Who does good, when the moon is waxing. Ravina answered him: Since this is its nature, we do not bless the moon. The waxing and waning of the moon is not an unexpected occurrence that requires these blessings.