I didn’t see the moon (yet) tonight (Leil Yud daled). Tomorrow night I will be flying. What should I do if I see the moon on the plane, should I do it then or can I / should I push it off until the next night?
If you see the moon on the plane, you should do Kiddush Levana, and not risk the chance of missing Kiddush Levana altogether.
The Minhag is that Kiddush Levana is not performed under a roof (OC 426:4). There are two reasons for this Minhag:
- According to the Bach (OC 426), it is disrespectful when one does not make the effort to leave one’s home and go out to greet the king; how much more so when greeting the Shechinah. [It might be argued that this reason does not apply when in an airplane, where it is impossible to “go out and greet the king”, and staying inside is therefore not an act of disrespect.]
- According to the Maharil (cited in Magen Avraham OC 426:14), the concern is that there might be Tumah within the building, and it is appropriate that one should greet the Shechinah in a place which is Tahor.
In any case, the Bach rules that this Minhag applies only where practical. In cases where it is not practical to go outdoors, such as an (even mildly) ill person who is housebound, or someone who is frightened of the reactions of the Gentiles, may recite Kiddush Levana at the window. The Bach’s ruling is brought down by the Taz (OC 426:4) and Magen Avraham (ibid) and other Achronim. The Magen Avraham provides another (more obvious) example – when the outdoors is filthy and not befitting the Bracha.
There is a further discussion in the Poskim whether one may view the moon from behind a (transparent glass or plastic) windowpane. The Pri Megadim (Mishbetzos Zahav OC 426:4) rules that one may do so when it is not possible to open the window, such as in your situation. The Birchei Yosef (OC 426:4) seems to conclude that it is unnecessary to open the window at all. [This particular issue is a far-ranging one in Halacha, and also concerns vision through glasses, but the common practice is to be lenient in this matter, especially when the pane is completely transparent.]