noun
synodic period
Astronomical term for the time it takes a celestial body to return to the same position relative to the Sun as seen from Earth.
月の会合周期は約29.5日です。
The Moon's synodic period is about 29.5 days.
Compound of 会合 (meeting, conjunction) and 周期 (period, cycle), referring to the period between successive conjunctions of a celestial body with the Sun.