What causes the variation in Mercury's synodic period?

[niki999]

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.

 

[phyzguy]

The 116 days is an average, assuming circular orbits. Since Mercury's orbit is far from circular, the synodic period will vary depending on where it is in its orbit. According to Wiki, the synodic period can vary from 105 to 129 days.

http://en.wikipedia.org/wiki/Mercury_(planet)#Orbit_and_rotation

 

[niki999]

Ah, right, I have missed that :-)

Thanks phyzguy !