# ACOS

The ACOS function returns the arccosine of a given number, expressed in radians.

## Syntax

`ACOS(number)`

### Parameters

#### number

This is a number between -1 and 1. If the parameter is `null`, the function will also return `null`.

## Examples

In this example, the ACOS function calculates the arccosine of 0.9.

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

```esql
ROW b = -0.5
| EVAL acos_b = ACOS(b)
```
