Monday, November 1, 2010

Is it a sun-dog? Is it a parhelic? No, it's a 22° halo!

Between about 9 and 11 this morning, an eerie glowing ring appeared around the sun, and brought Wits (and Joburg) to a halt as they gazed upwards in wonder.

The 22º Halo in the sky over the Wits Physics and Biology buildings.
The physics department was disgorged onto the concourse to stare at the spectacle.
And the dangers of looking into the sun be damned.
'A 22° halo is a circle at 22° around the sun, or occasionally the moon. It forms as sunlight is refracted in hexagonal ice crystals suspended in the atmosphere. As the light beam passes through two sides of the prism, the angle of minimum deviation is almost 22°. This wavelength-dependent variation in refraction causes the inner edge of the circle to be reddish while the outer edge is bluish.'

Apparently, Venus was visible as a tiny red dot passing over the sun during the halo.
I couldn't see it, and a camera can only do so much, so here's a strelitzia passing in front of the sun instead.
Floating in the sky above the Biology and Oppenheimer Life-sciences buildings
Above the Wits Great Hall.
A classic halo shot.
This one gives you a true sense of how large and looming the halo was. Pretty cool to watch people when they first saw it - a moment of shock, a stumble to halt, and then wonder and wasted hours of ruined retinas.

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