[22] Radbaz 1:341 writes that it primarily depends on one’s ability to derive benefit from the moon’s light, and many Aĥaronim cite this position, including ma and mb 426:3. However, Ĥida writes in Moreh Be-etzba §184 that one may not recite the berakha if the moon is covered by even a very thin cloud, and Ben Ish Ĥai, Year 2, Vayikra 23 concurs. Nevertheless, it seems that all agree that technically one may recite the berakha as long as one benefits from the moon’s light. This is the conclusion of Yalkut Yosef 426:5.

Eshel Avraham (Buczacz) explains that one may recite the berakha if the light of the moon allows one to see most of the things that are usually visible when the moon is unobstructed. This is assessed based on the amount of moonlight on the seventh night of the month, or in pressing circumstances, on the fourth of the month. According to this opinion, one may recite the berakha toward the middle of the month (when there is more moonlight) even if the moon is covered by a thicker cloud, and one may recite it toward the beginning of the month (when there is less moonlight) only if the cloud is very thin. The poskim, however, seem to indicate that the distinction depends only on the thickness of the cloud, irrespective of the day on which the berakha is recited. So writes Sefer Kiddush Levana 2:3 (with notes). Therefore, in my humble opinion, it seems that if one can see the outline of the moon, it is considered “visible”, and one may recite the berakha.

Technically, though, the halakha follows the opinion of Radbaz and Eshel Avraham. This is apparent from the words of Terumat Ha-deshen quoted in Leket Yosher: “Once, he saw only a small portion of the moon, because it was partially covered by a cloud, and he nevertheless sanctified it.”

Some wrote that one should look at the moon only briefly. See mb 426:13 and Kaf Ha-ĥayim 426:34.

Many authorities write that one who recited the berakha without looking at the moon has fulfilled his obligation, as long as he could have seen it had he looked. They derive this, by logical inference, from the law of a blind person. Most poskim maintain that a blind person must recite Birkat Ha-levana, because the berakha was instituted in recognition of the moon’s renewal and thus applies whether or not one can see it. In addition, even blind people benefit from the moon, as others use its light to escort them. This is the viewpoint of Maharshal, ma, Eliya Rabba, and Pri Ĥadash. However, Maharikash maintains that a blind person should not recite the berakha, as he does not derive pleasure from the moon. In practice, a blind person should not recite the berakha, because of the uncertainty surrounding the matter. Nevertheless, it is preferable that he hear the berakha from someone else (see mb 426:1; bhl 426, s.v. “nehenin”; Kaf Ha-ĥayim 426:2).

[23] See Sefer Kiddush Levanah (chap. 2, n. 9) where the author quotes Rav Pe’alim (vol. 3, Orach Chaim 68) who is unsure how to rule in a case where the moon will disappear in the middle of the blessing, but no days remain on which to recite Birkat HaLevanah. He concludes, “It is possible to say that under such dire circumstances everyone agrees that one should say the blessing, but further investigation is required.” The author of Halichot Shlomo (Tefillah 15:12) writes that if one is concerned that immediately after beginning the blessing, the moon will be covered, it is permissible to say the blessing, b’diavad.