ATN Function Details
 
Syntax
  ATN(numeric-expression)
 
The numeric-expression can be of any numeric type.
 
ATN is evaluated by default in single precision. If numeric-expression is
a double-precision value, ATN is evaluated in double precision.
 
The result is given in radians and is in the range -PI/2 to PI/2 radians,
where PI = 3.141593. PI/2 radians equals 90 degrees.
 
You can convert an angle measurement from degrees to radians by
multiplying the degrees by PI/180, where PI = 3.141593.
 
To convert a radian value to degrees, multiply it by 57.2958.