Try it … you'll be surprised!!  And it is not due to your "blind spot", where the optic nerve comes into the retina ...

          From an ex-American Airlines Captain ...

 This is a great illustration of what we were taught about scanning outside the cockpit when I went through training back in the '50s.  We were told to scan the horizon for a short distance, stop momentarily, and repeat the process.  I can remember being told why this was the most effective technique to locate other aircraft.  It was emphasized repeatedly to not fix your gaze for more than a couple of seconds on any single object.  The instructors, some of whom were WWII veterans with years of experience, instructed us to continually "keep our eyes moving and our head on a swivel" because this was the best way to survive, not only in combat, but from peacetime hazards (like a midair collision) as well.

 We basically had to take the advice on faith (until we could experience for ourselves) because the technology to demonstrate it didn't exist at that time. Click on the link below for a demonstration ...

         http://www.msf-usa.org/motion.html