synodic

Synodic is an adjective derived from the Greek synodos, meaning a coming together. In science it most often appears in astronomy to describe cycles defined by the relative positions of celestial bodies as seen from Earth, and outside astronomy it can refer more generally to a synod, an assembly of church bishops.

In astronomy, a synodic period is the time required for a celestial body to return to the same alignment with the Sun as seen from Earth. This differs from the sidereal period, which is the time for the body to complete a true orbit relative to the fixed stars. The synodic period S is related to the sidereal period P and the Earth’s orbital period E by the formula 1/S = |1/P − 1/E|. For inner planets (Mercury and Venus), 1/P > 1/E, so the synodic interval corresponds to successive inferior conjunctions with the Sun. For outer planets, 1/P < 1/E, and the synodic period corresponds to successive oppositions as seen from Earth.

A well-known example is the Moon’s synodic month, the time between successive new moons, which is about 29.53 days. This is longer than the Moon’s sidereal orbital period of 27.32 days because Earth itself is moving along its orbit around the Sun.

Beyond astronomy, the term synodic is used to describe matters relating to a synod, a formal assembly of bishops in a church. The word emphasizes a “coming together” or conjunction, in both astronomical and ecclesiastical contexts.