I also use the orbital
series of sine waves that can be projected.
Not exact matches
Girma for a bit
of fun I reconstructed the hadcrut3V global tempreature
series from
sine waves of different frequencies amplitudes and phases (I think it is pretty good fit over 1850 to 2010 — and beyond!).
Any
sine wave with a period longer than the length
of the time
series (in the case
of GISS, about 130 years) will give a fit almost indistinguishable from a straight line fit.
Using spectral analysis
of data
series will give you
sine waves — but does mean that the Sun has a whole lot
of processes going on to give independent cycles
of various periodicity?
I have modeled Milankovitch cycles both as additive
sine waves & as terms
of a Taylor
series approximation.