What is the synodic period of Mercury?

[Helios]

What is the synodic period of Mercury? What is the average synodic period of Mercury? I have combed the internet. Some sources use a number around

115.877484 days and they seem sure of themselves.

I don't own astronomy references. Help please

 

[mgb_phys]

http://nssdc.gsfc.nasa.gov/planetary/factsheet/mercuryfact.html

 

[Helios]

That page just gives

115.88 days

and I have a feeling that this is a rounded-off number.

 

[mgb_phys]

You can calculate the synoptic period from the orbital period of mercury and Earth
E = Earth sidereal year
P = Other planet sidereal year
Then sp = 1/(1/E-1/P)

 

[Helios]

I would calculate this as ( correcting mgb's equation )
P = 87.9691
E = 365.256363
sp = 1/(1/P-1/E) = 115.8772

However, a lot of internet sources, even encyclopedic ones, have a preference for

sp = 115.8776 days

 

[Helios]

and with possibly better values, I would now calculate this as
P = 87.96934
E = 365.256363
sp = 1/(1/P-1/E) = 115.877623

and some other references have concurred with this datum