Lunation is the mean time for one lunar phase cycle (i.e., the synodic period of the Moon). It is on average 29.530589 days, or 29 d 12 h 44 min 3 s. The length of this cycle is 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 is never more than 15 hours behind or ahead of a mean lunation.
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