asin() -- returns the arcus sine of its numeric argument

angle=asin(x)

The acos is the arcus sine-function, i.e. the inverse of the sin-function. Or, more elaborate: It Returns the angle (in radian, not degree!), which,fed to the sine -function will produce the argument passed to the asin-function.

print asin(0.5),asin(sin(pi)) 

          
  This will print 0.523599 -2.06823e-13 which is pi/6 and almost 0 respectively.