Based on FIFA's rule (from franch: Federation Internationale de Football Association), to consider a player in off-side position it is necessary the simultaneous observation of both the position of the player and the ball at the moment of the launch in any position of the ball and the payer in the football field, except in the case of a direct reception of the ball from the goal kick, or lateral reposition of the ball, or corner. The aim of this work is to show that it is not possible to mark correctly the off-side in all cases. This impossibility is due to the fact that the referee responsible to mark the off-side does not have a vision field correspondent to 180 degrees, but approximately 30 to 40 degrees that allows detecting simultaneous events, as discussed in section 2. Based on this discussion one estimates the characteristic time spends by the referee to turn the head approximately 90 degrees to try to observe simultaneous events out of his vision field. Using these intervals of time it was possible to estimate typical distances beyond the off-side line, when in legal position at the moment of the launch.
simultaneity; football; vision field
