Note:  Do not rely on this information. It is very old.

Cosineofan Angle

Cosine of an Angle, in Geometry, means the number by which any length on the one bounding line must be multiplied in order to give the length of its projection on the other. If the angle is zero between two lines, they are parallel, and the projection is equal to the original length. Hence the cosine of 0° is unity. Similarly we see that the cosine of 90° is zero, and, taking regard of the sense or direction in which the bounding lines are drawn, it follows that the cosine of an angle between 90° and 180° must be negative, diminishing to -1 at 180°. For no angle can it exceed unity. For cotangent, cosecant, and other such functions, see Trigonometry.