 <p>If you were to use arccos you would need the length from one point to the other and the x distance from one point to the other.</p> <p>However with arctan you only need the x distance and the y distance.</p> <p>x and y distances are easy to work out because you just have to minus one coordinate from the other. However the distance between the 2 points is more complicated to work out. This is why it it easier to use arctan instead of arccos, however they can both be used (even arcsin and other inverse trigonometric functions).</p>