יהוה said to Moses and Aaron in the land of Egypt:
This month shall mark for you the beginning of the months; it shall be the first of the months of the year for you.
§ Rabbi Shimon ben Pazi raises a contradiction between two verses. It is written: “And God made the two great lights” (Genesis 1:16), and it is also written in the same verse: “The greater light to rule the day, and the lesser light to rule the night,” indicating that only one was great. Rabbi Shimon ben Pazi explains: When God first created the sun and the moon, they were equally bright. Then, the moon said before the Holy One, Blessed be He: Master of the Universe, is it possible for two kings to serve with one crown? One of us must be subservient to the other. God therefore said to her, i.e., the moon: If so, go and diminish yourself.
§ Having cited a statement of Rabbi Aḥa bar Ḥanina, citing Rabbi Asi, citing Rabbi Yoḥanan, the Gemara cites another statement in his name: Rabbi Aḥa bar Ḥanina says that Rav Asi says that Rabbi Yoḥanan says: Until how many days into a new month may one recite a blessing on the month, i.e., Birkat HaLevana? Until the flaw of the moon is filled, when it no longer appears deficient. And until how many days is that? Rav Ya’akov bar Idi says that Rav Yehuda says: Until seven days of the month have passed. The Sages of Neharde’a say: Until sixteen days of the month have passed.
Moon blessing and its time - 4 subsections.
The one who sees the moon in its newness blesses "Who at Their Word created heavens, and at the breath of Their mouth all their hosts; a law and a time They gave unto them, that they would would not change their duty". Note: one should only sanctify the month at night, at the time that the moon shines and we benefit from its light (Agur).
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And we benefit from its light - that is, at the time that its shining is visible over the earth. If the moon is covered with clouds, if the cloud is thin and weak so that the moon is seen from within it and it's possible to benefit from the moon, bless, and if the cloud is thick, do not bless. And if one begins to bless and then the moon is covered by clouds, finish the blessing, except in the case where one estimates that the moon will be covered by cloud in the middle of one's blessing, for in this case one shouldn't begin the blessing. And see Biur Halacha, and this is the ruling of there is a screen dividing one and the moon - the ruling is this too, like clouds, that if the screen is translucent so that one benefits from the moonlight through it, and it's possible to see things in the light of the moon, one can bless over it, and if the screen is thick and one does not benefit from the moonlight, do not bless over it, for that would be a blessing in vain, and see what we write at the end of the siman.
Under a roof - rather, under the dome of the heavens. And the reason is that sanctification of the moon is like greeting the face of the Shechina, and this is not honourable to stand under a roof. For this reason, we exit from under the roof to the street, the way one would go to greet kings. And in any case all of this is only lechatchila, but one who has any worry that they would be unable to go outside, such as the place being unclean or living amongst non-Jews, should sanctify in their home via a window or an open door by the moon.