Shadow/Illumination on ground.

[propilot1983]

Anybody know why when you get up to a certain altitude you loose the shadow of your airplane but gain some sort of illumination on the ground instead. Is this an illusion or what? I've always noticed this but never asked. Anybody know?

Dilbert

 

[GravityHater]

Huh - neat Q. I will post, if only to solicit the correct answer.

My guess is the shadow becomes smaller than the resolution limits of the human eye as you climb so that is why it disappears..
The bright spot is a concentration of light caused by refraction (bending) of light all around the surface of the airplane, to focus at a point directly opposite the sun.
The bright spot only appears when it does (after the shadow is gone) because the focal point is much longer than the distance at which a shadow is still visible.

 

[EagleRJ]

I always thought it was the reflection of the Sun off the leaves that happen to be facing you. You only see the effect on a sunny day, and only when flying over deciduous trees. If there are a lot of trees that have glossy leaves, there will appear to be a shiny "spot" on the ground- a reflection of the Sun.

...or maybe it's a chemtrail generator, an EMP gun, or a monorail station! ;-)

 

[propilot1983]

Looked again today and it still made an illumination over a plowed field. I cant figure it out. Maybe i'm bioluminecent (sp).

 

[Andy Neill]

I'll take a stab at the shadow part of the question. First, it's a fair assumption that the sun is larger than the average plane. Because that is so. The light rays leaving one edge of the sun and barely missing one edge of the plane will eventually meet the rays leaving the opposite edge of the sun barely missing the opposite edge of the plane. The farther the plane is from the sun (about 93 million miles?) the more the angle of converging lines approach parallel. If the pane was very close to the ground, the shadow cast by no part of the sun being seen behind the plane would be the umbra and the part that was diminished because part of the sun was blocked by the plane would be the paenumbra.

The shadow will disappear when the plane of x feet wide is at a height h feet above the earth where
x = h sin(angle of convergence) or
h = x/sin(angle of convergence)

If the plane was 40 feet wide and the angle of convergence of light rays was 2 degrees, then the altitude at which there would be no shadow would be 40/sin(2) = 1146 feet.

I like GravityHater's point about light rays bending (Einstein discussed this in his notion of light being part material with mass being affected by gravitational pull), but I'm not readiy to buy into this as the major player in the absence of shadow.