synodic

1. Of or relating to a synod; synodal.
2. Relating to the conjunction of celestial bodies, especially the interval between two successive conjunctions of a planet or the moon with the sun.

[Etymology: Gk: ‘together’ + ‘journey’] astronomy An adjectival qualifier normally referring to the period an orbiting body takes to move between successive conjunctions relative to an observer on Earth, i.e. being effectively in line with Earth and our Sun and in same order. The synodic month, for instance, is the period between successive placements of the Moon in line with Earth and the Sun, either consecutive placements outside Earth's orbit (superior conjunction) else both inside (inferior conjunction). These, of course, are precisely full moon and so-called new moon (i.e. in astronomical usage, the point 180° from full moon, actually lacking any visible Moon, and prior to the like-named visible event of religious and everyday terminology), so the synodic month is the commonly observed period of the Moon, typically measured by the new moon rather than strict inferior conjunction. The synodic periods of planets can be measured directly and from each the sidereal period and true orbital period can be calculated.