Back in the day it was really common to speed up renders by building custom direct lighting rigs for illustrations like this. So it may be the result of a workaround to optimize render times, or the result of moving the workaround through the scene to various places and forgetting to move one of the lights in the rig, etc.
Or, changing the cloud materials, or recreating one of the cloud materials in one location, and forgetting to turn off "accept shadows" for example, which is a common toggle to see in 3D rendering software.
It's kind of fun to remember all the different ways you could hack the lighting alone, to say nothing of using procedural textures to trick the eye, various modeling tricks, and so on...
What ever object he used for the sky is set at the height of the Chrysler Building. If it had been moved up higher the shadow would either not be seen or there would be a gap between the shadow at the top of the building. Generally (if memory serves) the sky in povray was a sphere, you would model you scene inside the sphere and map the sky texture on the inside of the sphere but it does not look like he did this, that shadow looks very flat but I can not see any telltale signs of what he did use.
Back in the day it was really common to speed up renders by building custom direct lighting rigs for illustrations like this. So it may be the result of a workaround to optimize render times, or the result of moving the workaround through the scene to various places and forgetting to move one of the lights in the rig, etc.
Or, changing the cloud materials, or recreating one of the cloud materials in one location, and forgetting to turn off "accept shadows" for example, which is a common toggle to see in 3D rendering software.
It's kind of fun to remember all the different ways you could hack the lighting alone, to say nothing of using procedural textures to trick the eye, various modeling tricks, and so on...