The movements of the Sun, moon, stars etc is pretty predictable regardless, so long as you keep an eye on the seeming size of the moon and the phases you can calculate when the largest/nearest size coincides with a full moon.
In general you don't really need to know why things work in order to calculate the behaviour of future predictable phenomenon, sure you could do it the long way, but any FEer, REer, concave-Earther, toroidal-Earther, rhombicuboctahedral-earther... with eyes and records could figure it out.