From The Collaborative International Dictionary of English v.0.48 [gcide]: Lunation \Lu*na"tion\, n. [Cf. Lunated.] The period of a synodic revolution of the moon, or the time from one new moon to the next; varying in length, at different times, from about 291/4 to 295/6 days, the average length being 29 d., 12h., 44m., 2.9s. [1913 Webster]
From WordNet (r) 3.0 (2006) [wn]:
lunation
n 1: the period between successive new moons (29.531 days) [syn:
lunar month, moon, lunation, synodic month]