# The synodic period of Mercury

## Main Question or Discussion Point

Hi people, there is something that I try to understand: according to all my sources, the synodic period of Mercury is about 116 earth days. We can calculate this, since we know that the orbital period of Mercury around the sun is about 88 earth days, therefore:

SynodicPeriod ≈ 1/(1/88 - 1/365) ≈ 116

BUT, on 22-MAR-2012 we have a inferior conjunction and the next one is on 29-JUL-2012. This is 128 days !!

The above dates are given by software programs (Celestia, Stellarium and others). Can somebody enlighten me? It seems hard to find sources on this.