II · The atlas
Which crystals make each one, how high the Sun has to be, and where to look. Every sky here is computed; where we have a photograph of the real thing, it sits beside it.
ring, moon ring, winter halo
Light crossing two faces of a hexagon that meet at 60° bends at least 21.84°. It piles up there, and none falls inside, so the middle looks darker.
parhelia, mock suns
The plates act as 60° prisms for light passing sideways. The higher the Sun, the further out the dogs move: 21.8° at the horizon, 24.8° at 30°.
Sunlight reflected off the upright faces of plates and columns. It is white because reflection does not split colours.
upside-down rainbow, Bravais arc
Light enters a plate’s flat top and leaves through a side: a 90° prism, so the colours separate cleanly, violet on top.
lower 46° plate arc
Light enters a side face and leaves through the bottom. The Sun never gets that high north of 55°, so Stockholm never sees one; London gets about 140 hours a year.
circumscribed halo when the Sun is high
A sharp V over the Sun when it is low; as the Sun climbs past 29–32° the upper and lower arcs join into one oval round the ring.
Light enters a side face and leaves through an end face, a 90° turn. Larger and fainter than the 22° ring, with its outer edge washed out.
Each plate is a tiny mirror tilted a little. Only the ones in the vertical line between you and the Sun send its light to you.
over street lights
The same mirrors as a sun pillar, but the light is close, so the pillars look much taller. The pillar sits between you and the lamp, not over it.
paraselenae (moon dogs) with plates
The 22° halo again, in moonlight. Folklore called it a storm ring; the cirrostratus that makes it often runs ahead of a front.
named for William Edward Parry, 1820
Columns that keep their axis horizontal and a pair of side faces level: the “Parry orientation”. Why they float that way is still studied.
named for Tobias Lowitz, 1790
Lowitz drew them in St Petersburg in 1790. They were doubted for two hundred years, until small cameras caught them around 1990. How they form is still debated.
Measuring
A hand stretched out at arm’s length spans about 22°: thumb on the Sun, little finger on the ring. Two hands reach the circumzenithal arc and the 46° halo. That is all the instrument most reports need.
