We address the issues raised by comparing the geodynamo with some planetary and stellar dynamos. The recently discovered field structure of a fully convective star happens to be more reminiscent of the Earth magnetic field than the Solar magnetic field, despite the fact that the physical and chemical properties of these objects clearly differ. We therefore want to test a simple assumption: the internal geometry of these objects being very different, could the aspect ratio be one of the key parameters controlling some aspects of the magnetic behavior? We report the results of fully three dimensional dynamo simulations which show that varying the aspect ratio of the active dynamo region can yield sharp transition from Earth like steady dynamos to Sun like dynamo waves.