A Minute of Angle (MOA) or minute of arc or arcminute, is a unit of angular measurement equal to one sixtieth (1⁄60) of one degree. Since one degree is defined as one three hundred and sixtieth (1⁄360) of a rotation, 1 minute of arc is 1⁄21,600 of the same. It is used in those fields which require a unit for the expression of small angles, such as astronomy, navigation and marksmanship.
The exact measure of MOA is 1.0471996" at 100 yards of distance. Most American made telescopic sights used by hunters and marksmen are equipped with MOA adjustment knobs which provide the shooter with incremental MOA adjustment. Depending on the telescopic sight, the adjustments are measured in "clicks" of the knob which equate to a manufacturer set incremental adjustment. For example, one click may change the point of impact by ½ MOA for every click while another telescopic sight may have ¼ MOA adjustments, which is considered to be a finer MOA adjustment.
Minute of Angle is a linear effect. That is: twice the distance will result in twice the deviation. The value of 1 MOA @ 100 yds is 1.04719758 inches. In linear progression, 1 MOA is equal to 0.52359879 inches @ 50 yds and to 2.09439516 inches @ 200 yds. It is basically the measure of the opposite side of a right triangle with a 1 MOA (1/60th of 1 deg) "A" angle.