A synodic month is the synodic period of the Moon relative to the Sun, about days on average. A lunation is commonly measured from one new Moon to the next. Actual lunations vary, so the constant-speed relation derived in lunar phase is useful for understanding the mean cycle rather than predicting exact dates. [ref]
https://eclipse.gsfc.nasa.gov/SEhelp/moonorbit.html. Fred Espenak, NASA: the synodic month and lunation
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