The Heiligenschein

Today was a good day for looking up at the sky, with dramatic cumulonimbus clouds and a midday thundershower (which caused us to hurry home from our morning outing to the gym to rescue the laundry on the line). And I’ve been reading a sky-oriented book, Out of the blue: a 24-hour skywatcher’s guide, by John Naylor, which discusses all manner of optical phenomena in the sky: rainbows, crepuscular rays, eclipses, and so forth. In this book I finally learned the explanation for a phenomenon I’ve viewed from an airplane window many times, and in a different guise, from the ground as well.

What I notice from the airplane window, usually on descent, is a glowing bright halo on the ground and moving with the plane. The glow is at the antisolar spot —the sun on the opposite side of the plane, and once the plane gets low enough its shadow can be seen at the center of the glow. This phenomenon is a heiligenschein, which means ‘holy glow’, and is caused by a self-shadowing effect. The shadows cast by objects, such as trees seen from the plane, in the antisolar spot are directly underneath the objects themselves, and what one sees is just the light being reflected back by the leaves, rather than a darker mix of light-and-shadow. Hence the glow. Back on the ground, riding my bike, I also frequently notice a glowing spot opposite the sun on the pavement or from road markings, where tiny reflective beads have been incorporated in the paint. It’s the same effect—the light is shining straight back at me.

Remember to look up at the sky, or down at the ground—you never know what marvels of light you may see.