What is the origin of the idiom keep your eye on the ball?

Aircraft have an instrument called a "Slip and Turn Indicator". The "slip" part in a turn shows a measure of how much the aircraft is slipping towards or away from the direction of turn (this should be zero in a properly balanced turn, as there should be enough rudder input to keep it in check). The slip indicator is a simple ball in a U-Shaped tube in the vertical plane, facing the pilot and with the base of the U being towards the feet. In the early days of aviation, and nowadays with student and newly trained pilots, one cause of crashing on the "turn-to-finals" is letting the aircraft enter a spin. This would not happen if the aircraft did not slip and thus the term "keeping your eye on the ball" refers to keeping the ball in the middle of the tube such that there is little or no slip and the chances of entering a spin are hugely reduced. "Keeping your eye on the ball" is commonly thought to have origins in football, although this is disputed and almost certain to be a falacy.