Bi-Curious: New Moon

New Moon

Contrasted with the astronomical definition of the new moon, which is not visible to the naked eye, the new moon in the Hebrew Calendar is marked by the day and hour that the new crescent is observed.’  The ‘new moon’ is also referred to as the ‘dark moon’, since, although a tiny piece can sometimes be observed, it is mostly unseen. Rosh Hashanah is designed to cover both meanings. (cf. note xiii below.)

http://en.wikipedia.org/wiki/Rosh_Chodesh