Winter Corona ~ A wintry sun shines through Northern New Hampshire trees. The pastel hued rings of a corona surround the sun.   Images by freelance photographer Judith Howcroft (Wilderness Light Photography). All images ©Judith Howcroft, shown with permission.

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A corona is a series of multi-coloured rings surrounding a bright central aureole centred on the sun or moon. It is typically a few degrees across. Stretch out your arm and the corona will be the width or less of a clenched hand.   Small water droplets in thin cloud or mist produce it. They individually diffract light to produce the large interference pattern in the sky that we call a corona.

In more detail, to form a corona light incident on a droplet is mostly scattered by its surface. Imagine plane light waves scattered from two points on the rim. Each point produces a set of outgoing spherical waves. Where the waves overlap they interfere. Where both wave crests coincide there is light. The total result a long distance away is the corona pattern we see. The corona is coloured because the diffraction pattern for each colour is a different size and we see the overlapped composite of all colours. The diagram is somewhat fanciful. While in a darkened laboratory a single drop can easily produce a coronal diffraction pattern on a screen, the corona we see in the sky is the sum of contributions of millions of droplets spread across the cloud.