Unless this is a remote unmanned station in general I don't think it's worth automating the second axis. You can adjust it manually every 1-2 months.
Using any type of light-based detection has issues which need to be handled, like what if the sky is a constant brightness due to cloud? You need to apply sanity tests to make sure the thing doesn't hunt all over the place.
Personally I would use an RTC and a small lookup table of sunrise/set times for the year. Probably just one entry per week or even less.
Rob