Saturday, November 8, 2014

Ring Around the Moon, Rainbow Lunar Halo Tonight

Tonight there was a ring around the moon, a rainbow lunar halo (a 22ยบ halo to be exact). As light passes through hexagonal ice prisms in the upper atmosphere, no light leaves the prisms at an angle less than 22° making a distinct light/dark boundary with a radius of 22° radiating out from the moon.

I believe this is my third time photographing a lunar halo - my first time was most impressive visually but faded when I went back inside to get my camera. My second time was a bit better last November in Brownsburg.

This time my lunar halo wasn't broken up by a contrail, and was clear enough to see the rainbow (or 'moonbow') effect of the light splitting into different wavelengths. These lunar halos can happen throughout the year, because the upper atmosphere can be cold enough for ice crystals even when the temperature below is much warmer - but I've only ever seen it in the colder months.

ring around moon tonight halo
Single frame with Canon T5i, ISO 800, 18mm, f/4, 2.5 sec

halo ring around moon iphone
Lunar halo with iPhone and NightCap app

lunar halo moonbow 55mm
Close-up of the lunar halo moonbow, single frame with Canon T5i, ISO 1600, 55mm, f/5.6, 3.2 sec, saturation and contrast tweaked in iPhoto