You're likely thinking of the halo around the moon, not a corona.

Here's why:

* Corona: A corona is a bright, outermost layer of a star's atmosphere, often seen during a total solar eclipse. The moon doesn't have a corona.

* Halo: A halo around the moon (or sun) is an optical phenomenon caused by the refraction and reflection of light through ice crystals in the atmosphere.

Here's a breakdown of how halos form:

1. Ice Crystals: High-altitude clouds contain tiny hexagonal ice crystals.

2. Refraction: As sunlight or moonlight passes through these crystals, it bends (refracts) at specific angles.

3. Reflection: Some light also reflects off the surfaces of the ice crystals.

4. Halo Appearance: The refracted and reflected light creates a ring or circle around the sun or moon.

Halos can come in various shapes and sizes, with the most common being a 22° halo, which is a ring with a radius of approximately 22 degrees around the sun or moon.

So, while it's tempting to think of a "corona" around the moon, it's actually a halo, a beautiful atmospheric optical phenomenon.