Corona

A corona occurs when thin clouds composed of water droplets cover the sun or moon, but are thin enough for the light to shine through.

Light is diffracted by the small water droplets in the cloud. The difference here is that coronas usually occur when the cloud is more uniform in terms of thickness and the size of the water droplets. The more uniform the cloud is, the sharper the corona will appear.

A corona usually appears as a circular rainbow coloured pattern around the sun or moon. These patterns will have blue on the inside of the circle, and red on the outside. Since the sun is so bright, when looking for a corona around the sun you may need to instead look at its reflection e.g. in a pool of water, or cover the sun with a small object (without obscuring the circular corona around it). Sometimes, a corona will have another, fainter rainbow-coloured ring around the outside, (or even more rings, each one progressively fainter).

In this image the sun is sinking below the horizon, and there is thicker cloud obscuring it from below, so the corona only showed up as a partial circle.

Image by Sally (@littlemisscontented on Instagram).