The question is raised as to whether one may complete the recitation of Birkat Ha’Lebana if clouds suddenly covered the moon in the middle of the Beracha. Must one recite the Beracha again from the beginning when the moon is again revealed? The Magen Abraham (Rabbi Abraham Gombiner, Poland, 1637-1682) cited by the Mishna Berura rules that since he started "B’Heter,"when the moon was visible, he can conclude the Beracha with a concealed moon. However, the Magen Abraham adds a caveat and says that if one anticipates that the moon will become concealed in the middle of the Beracha, he should not begin.
The assumption of the Magen Abraham’s restriction is that one should not recite the Beracha unless it seems that it will remain clear for the entire duration. However, at the other end of the spectrum, Rav Chaim of Sanz (Poland, 1793–1876) ruled that even if the moon is concealed for the entire duration of the Beracha, it is permitted to recite the Beracha, as long as he caught a glimpse of the moon, "Toch K’Deh Dibur"-immediately preceding the recitation. This is the minority opinion and is not accepted as Halacha. Nevertheless, the Poskim are willing to rely on his opinion at least to disagree with the Magen Avraham and allow reciting the Beracha when he anticipates that it will become cloudy in the middle of the Beracha. This would be particularly relevant on the last night possible for reciting Beracha, when if he does not recite it now, he will miss the opportunity altogether. In fact, it is possible that even the Magen Avraham would agree in such a case. Hacham Ovadia is apparently lenient even before the last night.
SUMMARY:
One should not recite Birkat Ha’Lebana if the moon is concealed during the entire duration of the Beracha.
If one anticipates that the moon will become concealed in the middle of the recitation, he may rely on the lenient opinions and recite the Beracha. This is certainly true on the last night of recitation.
