Synodic  period is the interval of time which   elapses  between two oppositions or two conjunctions
of a superior planet. In case of inner-planets it is the time  between two conjuntions of the same type whether  they are both inferior or superior.
It can be explained as under:
The  Sun is stationary. The planets (including the  Earth ) are revolving around it. The earth completes
one revolution in approximately 365 days while Mercury com pletes it in 88days.

In the figure 16, let E be the  Earth, M, Mercury and S, Sun. EMS is the inferior conjunction of Mercury. Now the Earth and Mercury start moving. The Mercury completes one revolution in 88 days and when it comes at M, the Earth is not at E but it has moved ahead and the next conjunction takes place when Mercury comes at Mj and Earth at E,. So in moving from M and completing one revolution and after that coming to M3 is its synodic period or the time taken by the Earth in moving from E to E3 is Synodic period of Mercury.
Similarly, for superior conjunction ESM2 and EjSM3, the time taken by Mercury in moving from M2 and completing one revolution and coming to M3 is its Synodic period or the time taken by the Earth
in moving from E to Ej  is the synodic period of Mercury.

The outer planets move slower than the Earth. Earth completes one revolution in 1 year and Jupiter does it approximately in 12 years.
Here S is the Sun, E, and E are the positions of  the Earth and J, and J. positions of Jupiter at the time of opposition, while E2., E3 are positions of the Earth and J,  J.  positions of Jupiter at the time of conjunction.
SE J is the opposition and SE .J.  is the next opposition. The time taken by Jupiter in moving from J to  J, or by the Earth moving from E and completing one  revolution and then coming to E, is the synodic  period  of Jupiter.

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