The major limitation on the EUSO duty cycle comes from the fraction of the time the field of view will be exposed to the sun light or to the moon light. A further limitation could come from the fact that the telescope shutter might have to be closed also when the ISS itself is exposed to either light source. We compute the year averaged duty cycle due to the light-shadow effect under different conditions, taking into account the exact position of the ISS, the sun and the moon minute per minute. This fine time step prediction allows also computing the distribution of the shutter opening times which can be as small as 1 minute.