Function acos 

pub fn acos(x: f64) -> f64

Returns the arc cosine of x in radians. Use to get the angle of cosine x. x will be clamped between -1.0 and 1.0 (inclusive), in order to prevent [method acos] from returning @GDScript.NAN.

# c is 0.523599 or 30 degrees if converted with rad_to_deg(c)
var c = acos(0.866025)