The ACOS() function returns the arc-cosine (inverse cosine) of "Number". "Number" is the cosine of a radians.
The result of the ACOS() function is a value between 0 and PI, or between 0 and 180 degrees if the result is converted to degrees.
The DEGREES() function can be used to calculate the angle from a radians.
The function returns NULL if "Number" is not in a valid cosine range -1 to 1.