Wow, thanks everyone.
BJ: that's nice and simple and unless someone points out a flaw it might well become part of my explanation, but clearly it's not the whole story
BillieBob: is there any chance you could pop a sketch on here because I still can't get my head around what it might look like (KISS is my middle name when teaching
, but not when trying to understand)
Heston: so are you talking about the angle between the plane of rotation and the Earth's surface? I hadn't thought of that, but it might be tricky to explain.
rsuggitt: could you explain your point a bit more? (we all had to learn the formula during CPL/ATPLs, the question now is for a PPL and is about "why")
Everyone else: thanks but we all know that it happens - the question was about a simple *explanation* for a PPL student that is better than the overly-simple one I've used until now
HFD