what-is-synodic
WHAT IS A SYNODIC PERIOD?
When a table of astronomical numerical data refers to an orbital period or a rotation period, it means (respectively) the SIDEREAL revolution period and the SIDEREAL ROTATION PERIOD. That is to say:- these periods as would be observed from a fixed star (sidereal means relating to a STAR). A SYNODIC revolution period or a SYNODIC rotation period is The APPARENT revolution period or rotation period, as viewed from Earth, without factoring in the fact that Earth is moving in an orbit round The Sun. Alternatively, you can have a SYNODIC revolution or rotation period, AS VIEWED FROM MARS (or as viewed from any other planet).
The image below is a scan from Collins Dictionary of Astronomy, by Daintith and Gould, 2006. This scan verifies the formula for calculating synodic periods.