|Upper & Lower Moon Pillars (& Mars)
A scene by Göran Strand (photography) at Östersund, Sweden. Short upper and lower pillars bracket the waning moon. ©Göran Strand, shown with permission
To see both pillars is unusual, the lower pillar requires the moon or sun to be at some height above the horizon. But pillars get fainter and fainter as the light source climbs. The brightest and longest pillars are usually when the sun or moon is at the horizon or just below it. Alternatively, for lower pillars and their close relation subsuns, go to ski slopes when diamond dust throngs the air.
|Reflections – both external and internal – from ice crystals generate pillars. Those at right are too perfect and too regular. Pillar crystals tend to be large, imperfect and most important of all – wobbly.
Precise computer ray tracing calculations pioneered by Robert Greenler et. al. in 1972 showed that – perhaps surprisingly – a sky full of plate crystals wobbling a few degrees from horizontal would reflect sunlight to form two elongated halos apparently stretching upwards and downwards. The distance of the crystals is immaterial and the 'pillars' are at infinity. The pillar illusion hold whether crystals are a few feet away or many miles.