# ACOS

Returns the arccosine of a number as an angle, expressed in radians.

## Syntax

`ACOS(number)`

### Parameters

#### `number`

- A number between -1 and 1.  
- If `null`, the function returns `null`.

## Examples

```esql
ROW a = .9
| EVAL acos = ACOS(a)
```

Calculate the arccosine of the value `0.9` and store the result in a new column named `acos`.