Please excuse my ignorance - I am a biologist by training - but this is one of those questions that just keeps bothering me and I can't find the answer with Google/Wikipedia. Take as our example the ocean currents on the earth caused by the Coriolis Effect of the earth turning. When I read up on this accounts quickly jump into the maths of mapping between two reference frames. We talk about sitting in a rotating box and being able to tell it is rotating using a Foucault Pendulum etc. But how does the earth know that it is rotating relative to the universe and not the universe rotating relative to it? If the universe were spinning around the earth our ocean currents would stop. Two clear questions: If spin is always relative to two frames of reference how do the frames of reference decide which one is stationery and which one is spinning? If spin is not always relative is there some absolute frame of reference against which all mechanical spin is ultimately measured? Many thanks for your thoughts on this.