Why Are There Two Extra Suns in the Sky? Ice Crystals Acting as Prisms

High cloud contains ice crystals, and a particular form of those crystals is a flat six-sided plate. Such plates falling slowly through still air settle into a horizontal orientation, the way a leaf falls flat rather than edge on, so an enormous number of them hang in the sky aligned the same way. Light passing through one of these plates enters one side face and leaves another, with the two faces at sixty degrees to each other, which bends it by a minimum of about twenty two degrees. Since the crystals are aligned, the bent light arrives from points to the left and right of the sun rather than from all around it, producing two bright patches at that separation.

Several features distinguish them from anything else in the sky:

• •Two patches, one either side of the sun at the same height
• •A separation of about twenty two degrees, roughly a spread hand at arm's length
• •Red on the side nearest the sun and blue or white further out
• •Frequently a faint white tail extending outwards
• •Best seen when the sun is low, since the effect weakens as it rises
• •Thin high cloud present, often the kind that precedes a change in weather

These patches are one member of a large group of optical effects produced by ice, and the others are worth knowing since several appear together. A circle of the same radius around the sun is the most common of all and is produced by randomly oriented crystals rather than aligned ones. A bright arc curving above the sun, frequently intensely coloured, comes from light entering the top face of a plate and leaving a side. Pillars of light above or below the sun come from reflection off crystal faces rather than refraction through them. White circles running right around the sky at the sun's height, and arcs touching the common halo above and below, complete the commoner set. Elaborate displays showing many of these at once occur in polar regions.

Capturing these effects is easy once a few things are understood and frustrating otherwise. The sun must be blocked, since including it in the frame ruins the exposure and produces flare that swamps everything, so photographers hide it behind a building, a post or a raised hand. A wide lens is needed, because the separation from the sun is about twenty two degrees and capturing both patches plus the sun requires a field of view of nearly fifty degrees. Underexposing slightly saturates the colour. A polarising filter deepens the sky and can strengthen or weaken the effect depending on its angle. And the display changes within minutes as the cloud moves, so photographing immediately matters more than photographing well.

Before the optics were understood these appearances were read as significant events and the records are historically useful. Chronicles describe multiple suns in the sky as portents, and several such observations are attached to specific battles and deaths, with one appearing in an English account of 1461 where a commander reportedly turned the sight to his advantage by presenting it to his troops as a favourable sign. Comparable accounts survive from China, Japan and elsewhere, generally recorded because they were thought to mean something. Since the effects require specific conditions and appear only briefly, and since the accounts frequently give dates and places, they occasionally serve as evidence about the weather at a documented moment.

Flat six-sided ice plates settle horizontally as they fall, so light bent by about twenty two degrees through their side faces arrives from two points either side of the sun. They show red on the inner edge, are best seen when the sun is low, and belong to a family that includes the common halo, arcs and pillars. Chronicles recorded them as portents, which occasionally makes them evidence about weather on a known date.