Lunation's the mean time for one lunar phase cycle (for example, the synodic period of the Moon). It's on average 29.530589 days, or 29 d 12 h 44 min 3 s. The length of this cycle's linked to many phenomena in nature. In a lunar calendar each month corresponds to a lunation. Individual lunations vary in length by several hours, because of the eccentricity of the orbits of the moon and Earth. However, the lunation's never more than 15 hours behind or ahead of a mean lunation.… (More on Lunation)