You're likely seeing a lunar halo, not a ring. It's a beautiful optical phenomenon caused by the interaction of moonlight with ice crystals in the atmosphere.

Here's how it happens:

* High-altitude clouds: Lunar halos form when high-altitude clouds, called cirrus clouds, contain tiny ice crystals.

* Refraction and reflection: When moonlight passes through these crystals, it gets refracted (bent) and reflected.

* Circular pattern: Because the ice crystals are hexagonal, the light is refracted at a specific angle, creating a halo around the moon.

Why is it sometimes bigger or brighter?

The size and brightness of a lunar halo depend on the size and shape of the ice crystals and the amount of moisture in the air.

A fun fact: The presence of a lunar halo is often associated with an approaching storm. This is because cirrus clouds are often a sign of a front moving in.

So, enjoy the beautiful halo, but keep an eye on the weather!