Q: When taking off or landing a light aircraft on a sunny day, its shadow is clearly visible on the ground. When cruising at a height of 450 to 600 metres above ground level the aircraft鈥檚 shadow is no longer visible. However, if you look where the shadow should be in relation to the Sun, a blob of apparently brighter light tracks across the ground at the speed the shadow would be travelling. Why does this occur?
Q: Flying very low on sunny evenings, my powered parachute casts a shadow on the hill which is roughly the shape of a circle segment. Within that segment, and sometimes outside the shadow line as well, the brightness of the light falling on the hillside appears to be enhanced. If I am not close enough to cast a defined shadow, I can see where it should be by the bright patch moving across the hillside. Flying in these conditions may be heavenly, but I refuse to believe I鈥檓 being measured for an angel suit. Any other explanations?
A: In the first question: when an aircraft is high enough to subtend a smaller angle at ground level than the diameter of the Sun, it will not cast a shadow, and the ground beneath it will be uniformly illuminated. At the point on the ground where the Sun is directly behind you, there will be no visible shadows. Elsewhere you see a mixture of reflected light and shadow. Thus the point directly opposite the Sun appears brighter than its surroundings.
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The effect is particularly marked over grassland. You can see the same effect on a small scale if you photograph a plain bath towel using a ring flash on the camera lens.
The enhanced illumination within the shadow area in the second question is a visual illusion closely related to the well-known one where the intersections of a pattern of orthogonal black bars appear to have central light patches. This is thought to be caused by an edge-enhancing mechanism in the retinal pathways of the eye. It is not caused by diffraction. The brighter patch at greater heights is the same phenomenon as in the first question.
A: Vegetation casts shadows on itself and the surrounding ground. If you look at vegetated ground from above, with the Sun directly behind you, your line of sight strikes the ground from the same direction as the sunlight, and you cannot see those shadowed surfaces which are not not reached by the Sun鈥檚 rays. You see only directly illuminated surfaces. But looking from any other angle one can see shadow where your line of sight passes behind an illuminated object. Here the average surface brightness seen is less.
I have noticed this effect from heights of 3000 metres over thin forest, but never over water or flat desert where there are no shadows.