What hides between Shadow and Light?

[mmammel]

Well, Excel is limited to about 15 digits,

You can just do MOD 9 on EACH value that you calculate in the fibonacci sequence -- you don't need to let the sum get large. in excel you could put the seeds in A1 and A2 then in A3 put:
=MOD(A1+A2,9)
Then just copy that formula down the whole column.