A lunar halo is created when light is refracted, reflected, and dispersed through ice crystals suspended in cirrus or cirrostratus clouds located at an altitude of 20,000 feet (6,000 meters) and higher, up to 40,000 feet (12,000 meters). 

The shape of these ice crystals focuses light into a halo around the moon or the sun. As ice crystals are usually hexagonal these lunar halos are almost always the same size, with the moon (or the sun) sitting 22 degrees from the other edge of the halo — roughly the width of an outstretched hand at arm's length. 

The uniform 22-degree radius and 44-degree diameter of halos mean that both solar and lunar halos are often referred to as 22-degree halos.

This uniformity in diameter arises because ice has a specific index of reflection and the hexagonal shape of an ice crystal means when its sides are extended it forms a prism with a 60 -degree apex angle. This results in an angle of minimum deviation for light passing through the ice crystal of 21.84 degrees.

These ice crystals also demonstrate a prism effect that separates white light from the sun or is reflected by the moon into various individual colors just like the atmospheric effect that creates a rainbow.

https://www.space.com/moon-halo