Part 1, What really is an MOA?

I had been guilty of accepting the old 1 MOA is 1" at 100 yards.  I knew it was a sloppy rule but in general it worked.  But, when you start trying to understand your scope and its capability when trying to develop indoor practice methods, it helps to dig a little deeper and truly understand what MOA really is.

I will try to make this as easy as possible.  A Minute of Angle is a geometric unit of measure for an arc.  For those who dislike geometry, let's let look at it this way.  A circle has 360 degrees.  And, each degree has 60 minutes.  For me, that last little factoid must have been presented in class while I was out sick.   This well known detail had not been an area of knowledge for me.

Armed with this new knowledge, we then know there are 21,600 minutes of angle in a circle by the following simple math, 360 x 60 = 21,600.  So, if you are looking at the top of a oil drum, there are 21,600 arcs or MOA that make up the full circle or circumference   Obviously the "length" of those arcs is pretty short but they all add up to equal the circumference which we know as 2(Pi)r.

Assume when you are shooting at 100 yards you are the center of a large invisible circle and the distance between you and your target is the radius.  So, one large imaginary circle, much larger than the oil drum, with a 100 yard radius but still 21,600 MOAs!.  We know then that the circumference of this imaginary circle is 2(Pi)100 yards or 628.3 yards.  Divide the circumference into degrees, 628.3/360 = 1.7 yards for each degree.  But, we need to divide this further into minutes so we divide again by how many minutes there are in a degree which is 60.  1.7/60 is .029 yards in each minute of angle.  A fraction of a yard is meaningless so let's convert to inches.  36 *  .029 is 1.05".  Look familiar?  There is our sloppy rule of thumb that 1 MOA is equal to 1" at 100 yards.  As you can see, this is pretty basic math and now you can calculate what the length of a MOA arc is at any distance.  Below are some common distances.

Click on image for larger view

Part two will look at what this all means for you and your scope which will elaborate on why I have the last two columns to the right.

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