You're likely referring to a halo, which is a luminous ring around the Sun or Moon, not a ring "in the sky" as it appears to be surrounding the celestial body.

Here's how halos form:

* Cause: Halos are caused by the refraction and reflection of light from the Sun or Moon through ice crystals in high-altitude clouds called cirrus clouds.

* Mechanism: These ice crystals act like prisms, bending the light and creating the halo. The shape of the ice crystals determines the specific type of halo observed.

* Appearance: Halos often appear as a white, circular ring around the Sun or Moon, but they can also take on other forms, such as arcs, patches, or pillars.

How halos differ from other celestial occurrences:

* Cause: Halos are atmospheric phenomena caused by ice crystals, while other celestial occurrences like rainbows are caused by water droplets.

* Location: Halos are observed around the Sun or Moon, while rainbows appear opposite the Sun in the sky.

* Shape: Halos are typically rings, while rainbows are arcs.

* Color: While most halos are white, they can sometimes show color, particularly on their inner edge. Rainbows, on the other hand, are known for their vibrant colors.

Other celestial occurrences:

There are other celestial occurrences that you might be mistaking for a ring in the sky:

* Corona: A corona is a ring of colored light around the Sun or Moon, often seen through thin clouds. It is caused by diffraction of light, where light waves bend around small water droplets or ice crystals.

* Moon Illusion: The moon illusion is a visual phenomenon where the Moon appears larger near the horizon than when high in the sky. It's not a real change in size but a perceptual effect.

Let me know if you have any further questions!