Microsoft® Visual Basic® Scripting Edition Atn Function 
Language Reference Version 1

Returns the arctangent of a number.
Atn(number)The number argument can be any valid numeric expression.
The Atn function takes the ratio of two sides of a right triangle (number) and returns the corresponding angle in radians. The ratio is the length of the side opposite the angle divided by the length of the side adjacent to the angle.The range of the result is pi/2 to pi/2 radians.
To convert degrees to radians, multiply degrees by pi/180. To convert radians to degrees, multiply radians by 180/pi.
Note Atn is the inverse trigonometric function of Tan, which takes an angle as its argument and returns the ratio of two sides of a right triangle. Do not confuse Atn with the cotangent, which is the simple inverse of a tangent (1/tangent).