This paper studies spin glass dynamics by using an equation derived from Sherrington-Kirkpatrick’s model which normally conduces to ageing processes. The model here is considerably simpler in the sense that it is spherical and thus has a sphere as its state space. The article mainly analyzes the convergence of the solutions of a stochastic differential system as the order of the system increases and proves the so-called large deviation principle with good rate functions. As pointed out by the authors themselves, the main advantage of such simple model is that it allows to obtain rigorous proofs and a first mathematical understanding of the ageing phenomenon.