I have seen so many people ask about this that I whipped up a diagram to illustrate it.  Here it is:

Actually when dealing with firearms a MOA is considered to be by defination a linear measurement equaling 1" on a target 100 meters away from the muzzle.  It approximates the geometric.  The truly quasi-scientific folks will spend hours explaining this and confuse themselves and others in the process.  A true waste of time.

1MOA is 1" at 100 yards; 2" at 200 yards; etc.

Trying to make it scientific, or show how it's derived just gives folks a headache.  

-- Chuck

Chuck,  I'm afraid you're pretty much completely wrong on this one.  And it's spelled "definition".

The "one inch at 100 meters" notion describes, approximately, the sides of a right triangle with an interior angle that subtends 1 MOA.

about 1.1" at 100 yards (Art Pejsa). usually rounded to an even 1".

Karl

For the math;

1) 1 minute of angle=1/60th of a degree

2) 100 yards=3,600 inches

3) sin of 1 degree/60=.0002909


3,600(.0002909)=1.047" or 1" rounded

Class dismissed.

coyote3