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Sunset Pillar, Canary Islands ~ Roberto Porto pictured this pillar in a turbulent sky over the island of Tenerife.

All images ©Roberto Porto, shown with permission

One the first simulations. The 'sun' is a few degrees above the computer's 'horizon'.

It revealed a large diffuse lower pillar and a diminutive upper one.

The situation changed as Greenler and his students tried other sun elevations and crystal tilts.

Upper sun pillars are at their best when the sun is at the horizon or below it. the latter can be followed for some time after sunset, creeping along the horizon and following the hidden sun.

The arm waving explanation for sun pillars is that they are direct reflections from millions of plate-shaped ice crystals wobbling a few degrees from horizontal.   Their glints towards the eye look collectively like an upward pillar of light.

But is that convincing?   Why would wobbly crystals give that shape rather than a much wider blur?    Why are they only seen near sunset or sunrise?  Should the pillar not be brighter nearer the sun? Should the lower pillar not be brighter than the one above the sun?

Faced in the early 1970s with these and more questions from a student, Robert Greenler decided to test the explanation by direct computation.  

They ray traced reflections from computer representations of wobbly mirrors.   Each reflected ray of light formed a little black dot on the ‘sky’ of an ancient plotter. The thousands of dots formed sun pillars like those observed. Greenler and his students went on to investigate whether other crystals could form pillars.  They then moved on to investigate other halos.  

This was the birth of computer simulation of ice halos by ray tracing.    Arm waving is not sufficient for science.