My husband named his sister’s daughter Sahar (Samech-Hay-Raish) at the Torah when she was born two years ago. It is based off of the word for moon. Now he is questioning whether this is a Hebrew name at all. Can you please advise as to if it is a proper Hebrew name or if we have to pick something else. Her mother is not religious and likes this name.
The name Sahar – סהר is a relatively new name. As you mentioned, it refers to the moon which is also referred to as סהר. See Shir Hashirim (7:3), and also in the Metzudos Dovid and Metzudos Tzion.
Although some suggest that it is not advisable to choose new names which only “sound nice”, certainly after a Hebrew name was already given one should not try to revoke it or change it.
The Rebbe quotes from the Ariza”l (Sefer Hagilgulim in the introduction) that a name that the parents gave to their child is in essence arranged by Hashem, so that the Neshama should receive its proper conduit of vitality. (Sharei Halacha Uminhag vol. 3 p. 295)