Over the last week, I have had good reason to consider the following question - how does the human eye calculate the trajectory of an object propelled at speed in the general direction of the observer ?
I came to the conclusion that the eye uses the deviation in height and width over time from the object's starting position in order to calculate the projected position as the object passes the observer. The closing speed of the object to the observer is clearly of tertiary importance, as a) it can be derived from the other measurements and b) it is more difficult to judge, being a function of the perceived size of the object over time, which will clearly vary less than the other two dimensions.
All of which helps to explain why the damn cricket ball hit me in the head last Sunday before I even picked up where it was going. The ball basically left the bowler's hand and proceeded in a straight line towards my head. Seeing no horizontal or vertical deviation, my brain only registered the increase in perceived size at the very last minute, giving me just enough time to duck my head enough to take the impact on the ear rather than than the bridge of the nose.
Infoholic UK Retired Hurt 1
Conclusion : Wear a helmet - if a quickie chucks a beamer at you, you won't know until it hits you.