Atmospheric
Optics
Greek Halos Over two thousand years ago right here in Greece, Anaxagoras (c500-428BC), Aristotle (384-322BC) and others probed the nature of the world. Amongst their optics questioning they reasoned that halos and rainbows were bundles of rays in particular directions rather than corporeal objects. They explained their formation as scattering and reflection by objects in the atmosphere. They were correct - a triumph of observation and rational thinking.
Imaged over Athens by Kosmas Gazeas of the University of Athens.
The Greeks would have loved the geometries of ice halos. Crystals in near perfect polyhedral forms and arcs of geometric precision scribed on the sky.
Hexagonal prisms formed this display. Long prisms with end faces vertical, short prisms (plates) with end faces horizontal and some prisms randomly oriented.
Nearest the sun is the common 22° halo from the randomly oriented crystals. Touching it above and below the sun is the circumscribed halo from column crystals.
Twice as far from the sun there are two more halos. One is just visible next to the Greek flag in the top image. They can be better seen in the two images at lower right one of which is enhanced by colour subtraction. There is a section of a strong almost circular arc and tangent to it a much weaker arc extending further from the sun. The stronger arc might be interpreted as a 46 degree halo. However, there is no sign of it above the sun and the 22 degree halo formed from the same crystals is comparatively weak. An infralateral arc formed by rays passing between the side and end faces of horizontal columns fits the images. It also appears naturally in the HaloSim ray tracing at right using just enough horizontal columns of medium length to make the circumscribed halo and parhelic circle. The weaker arc touching the infralateral (it appears on two images but we cannot be wholly certain of its existence) is the usually more frequent circumhorizon arc made by horizontal plates.
Plate crystals of course give sundogs or 22° parhelia but they did not appear over Athens because the sun was too high to allow them to form.
A star of the show is the small parhelic circle seen best in the image below. It was mostly formed by reflections from the end faces of the plentiful horizontal columns. The weakness of the circumscribed halo suggests a scarcity of plates, the more usual major source of the parhelic circle.
