Universal Time UT variations in many magnetospheric state indicators and indices have recently been reviewed by (Lockwood and Milan, 2023). Key effects are introduced into magnetospheric dynamics by the eccentric nature of Earth’s magnetic field, features that cannot be reproduced by a geocentric field model. This paper studies the UT variation in the occurrence of substorm onsets and uses a simple Monte-Carlo model to show how it can arise for an eccentric field model from the effect of the diurnal motions of Earth’s poles on the part of the geomagnetic tail where substorms are initiated. These motions are in any reference frame that has an X axis that points from the center of the Earth to the center of the Sun and are caused by Earth’s rotation. The premise behind the model is shown to be valid using a super-posed epoch study of the conditions leading up to onset. These studies also show the surprising degree of preconditioning required, ahead of the growth phase, for onset to occur. A key factor is the extent to which pole motions caused by Earth’s rotation influence the near-Earth tail at the relevant X coordinate. Numerical simulations by a global MHD model of the magnetosphere reveal the required effect to generate the observed UT variations and with right order of amplitude, albeit too small by a factor of about one third. Reasons why this discrepancy may have arisen for the simulations used are discussed.