Yesterday I was on a flight from Ft. Lauderdale to Atlanta, and when I looked down at the Florida landscape, I saw a bright spot where the shadow of the airplane should be.
We were at about 20,000 ft or more (I’m kinda guessing - I’m not really sure what altitude except ‘very high’). We were also flying roughly northwest, and it was about 10 in the morning, so the sun should still have been pretty low in the east. And I was sitting on the left (western) side of the plane.
So how could I see a bright spot on the ground? Any reflection off the plane’s surface in the sun should have reflected back east… What was I looking at?
There wasn’t a lot of cloud cover, so I’m not sure if it could have been some sort of halo from scattered light hitting the clouds.
I guess it could have been a glory, although it didn’t feature the dark part of the aircraft’s shadow in the middle - the middle was the brightest part of the phenomenon - could a glory have been due to clouds above the aircraft (which I couldn’t have seen) rather than clouds below?
If you look at a point that’s directly in line with you and the Sun, all the shadows are hidden. In astronomy, the brightening of a rough surface under such conditions referred to as the “opposition surge.”
A glory involves colors - it’s a circular rainbow sort of effect (although not all spectral colors are always visible). A bright spot, halo effect is a heiligenschein (from the German meaning “holy light” or halo, if my sources are correct. It’s an optical effect that requires proper angle of the sun along with favorable surface and atmospheric characteristics in order to see. Always appears opposite the sun.
There probably was a very small shadow of the airplane in the middle of it, but given your height above the ground it may have been too small for your eye to pick it out.
My first thought was a glory, too, but that requires cloud. Look for glores around the shadow of your plane when passing over clouds – they look like tight little rainbows surrounding the shadow of the plane. They’re very impressive. They don’t result from the same thing that causes rainbows, but are now believed to be inerferece between counterpropagating beam paths that emerge in the same direction antiparallel to the sun, with very nearly the same path length (since they follow the same path, but in opposite directions).
If there’s no cloud , then one possibility is the heiligenschein, as BroomsticknotesHeiligenschein is caused by retroreflection of light very nearly back along the incident direction. It’s often observed at sunrise on dew-covered grass or with droplets on some other surface. The droplet acts like a crude lens to focus the light oto the surface behind it. Light reflecting from that surface is then sent out along a very nearly antiparallel path back towards the source. If you’re in line with that source (and you are if you’re looking at your shadow), you see a bright spot around your shadow, trailing off the further away you get.
I’ve never seen a heiligenschein from an airplane, and you’d need a big field of something in order to see it. A lot of inexpensive retroreflectors are made from heligenchein-like stuff. Those crossing lines with glass beads in the white paint, for instance.
Another possibility was mentioned on this Board a few months ago – you can get retroreflections from a lot of other things these days – tail-lights, signs, etc.
And one last possibility produces definite, but less dramtic brightening around a shadow – since you’re looking directly at an illuminated surface along very nearly the same direction as the illuminating source, there are very few shadows, and the area immediately around your shadow will seem to be completely and uniformly lit. The farther from your shadow you get, the more you start to see of shadoews not perfectly lit, and so the completeness and uniformity of the illumination decrease as you get farther away.
See M. Minnaert’s classic and indispensable book The Nature of Light and Coor in the Open Air. Also see Greenler’s book Rainbows, Haloes, and Glories, the references in Jearl D. Walker’s The Flying Circus of Physics, and, for mahematical detail, R.A.R. Tricker’s Introduction to Meteorological Optics.
An easy and handy way of seeing that effect is on reflective strips and road signs, the reflective paint contains millions of tiny glass spheres that concentrate and reflect light back to its point of origin; one thing you can do is walk over a zebra or a road line and watch your head´s shadow on the ground, you´ll see a bright halo around it.
It´s the same with water droplets in dew covered ground, clouds or mist.
I see them fairly often - but then, I fly at dawn over farmfields, which are frequently dew-covered, which makes for viewing conditions favorable for this effect
It sounds like the Heiligenschein is the explanation that fits what I saw - and Podkayne’s Mars link is very similar to its appearance (although different in scale!).
That is the correct explaination. The bright spot’s prominance depends on the surface texture of the ground where it is observed. Prarie grasses tend to produce a prominant brightning, pavement virtually none.
This is one of the things I point out to passengers when I take them for glider rides.
Dang! How did I miss Podkayne’s description. Sorry about that. It’s the same one as aqt the end of my post, but she put it up earlier. I first read about it in Minnaert’s book, but it didn’t have the fancy name there.
