The ACOS() function returns the arc cosine of input n, the input n must be in the range of -1 to 1. The function will return a value in the range of 0 to pi, expressed in radians.

ACOS function examples

SELECT ACOS(.2) FROM DUAL;
-- output
1.36943840600456582777619613942212803186
SELECT ACOS(-.4) FROM DUAL; 
-- output
1.98231317286238463861605958925708704694
SELECT ACOS(-1) FROM DUAL;  
-- output
3.1415926535897932384626433832795028842
SELECT ACOS(1) FROM DUAL;   
-- output

Pass value 1.2 which is not in range -1 to 1, and notice the error.

SELECT ACOS(1.2) FROM DUAL;
-- output
ORA-01428: argument '1.2' is out of range
01428. 00000 -  "argument '%s' is out of range"

References

1. ACOS Function :- Oracle official docs