A Winter Study of Halos: Atmospheric Optics and Omens

17 January 2026

On cold clear days a luminous ring sometimes surrounds the sun or moon, twenty-two degrees across, occasionally flanked by two bright mock suns and crossed by arcs of improbable geometry. These are halos and parhelia, sun dogs, and they occupy a rare and instructive position in our subject: a phenomenon fully explained by physics that has nonetheless carried omen-weight in nearly every culture that recorded the sky. The halo is therefore a perfect specimen for a question this site returns to often: what happens to divination when the mechanism is known?

The physics

Halos are produced by refraction and reflection in hexagonal ice crystals drifting in high cirrus. The common 22-degree ring comes from light passing through alternate faces of randomly tumbling column crystals, the angle set by hexagonal geometry and the refractive index of ice, which is why it never varies. Sun dogs appear when plate crystals fall with faces horizontal, bending light to either side of a low sun and stretching red inward, and the rarer arcs, tangent, circumzenithal, the great 46-degree ring, follow from other crystal habits and orientations, catalogued exhaustively in the standard works of Minnaert and Greenler. The geometry was substantially worked out in the seventeenth century; Descartes treated halos in the essays accompanying the Discourse on Method (1637), and Huygens carried the analysis further. As explanation, the case has been closed for centuries.

The omens

Before the physics there was the reading, and the reading has a documented history of nearly three millennia. Babylonian celestial divination catalogued lunar halos in the omen series Enūma Anu Enlil: a halo around the moon with a planet standing within it portended one thing for the king, with a gate opening in the ring, another; Rochberg's studies show the scribes recording the phenomena with genuine observational care. Aristotle's Meteorologica treats halos as weather signs, which they genuinely are: cirrus sheets run ahead of warm fronts, so the folk rhyme "ring around the moon, rain comes soon" preserves a statistically respectable inference. And the most consequential parhelion in Western history stood over a battlefield: the triple sun seen before Mortimer's Cross in February 1461, which the eighteen-year-old Edward of York read aloud to his frightened army as the Trinity's promise of victory, and took afterward into his heraldry as the Sun in Splendour. The chronicle account is a complete lesson in applied divination: the sign was ambiguous, the interpretation was leadership.

What the halo teaches the diviner

Knowing the ice-crystal optics changes less than one might think. Divination, as our essays on cartomancy and dowsing argue, is a discipline of attention and interpretation, and the sky rewards attention whether or not one asks it questions. The Babylonian scribes, the best astronomers of their age, saw no contradiction between predicting the moon and reading it; the categories "explained" and "meaningful" only became rivals much later. The practitioner who steps outside on a cold night, finds the ring, names the crystals, and still asks what the evening is saying stands in the oldest observatory there is, and is in excellent company.