The Sphere

The perspective of the sphere is nonexistent: it is simply a circle, larger if close, smaller if far.  The shading is more complicated.  The lessons of the lazy cylinder are useful.  The sphere can be thought of as a cylinder that curves in all directions.  For the sun or a distant light source, the illuminated side is half of the sphere, a hemisphere.  Its terminator is a great circle whose plane is perpendicular to the direction of the light..  A great circle is a circle that encompasses the largest girth of the sphere.  For instance, on the sphere of earth, the equator is a great circle as are any of the latitude or longitude lines.  A great circle can be drawn thru any point on the surface of the sphere.

 The highest value of the illuminated hemisphere is the point on the sphere's surface closest to the light because the light ray is perpindicular to it there.  The value diminishes slightly until the terminator is reached.  


The pattern of dropping value can be understood by imaging a bull-eye pattern applied to the light heminsphere which its center at the point nearest the light and ending at the terminator.  The stripes of the Bulls-eye represent ares of diminishing value away from the point of highest value.  This concept is useful in imaginig the foreshortening effect from the artist's viewpoint on the dimishing value.
A core is on the shaded side of the terminator. Then the reflected light which bounces up from the level surface on which the sphere is balanced comes into play.  The reflected light brightens the shaded side and restrains the core.  Unless the light source is directly over  the sphere, this results in a dark wedge on the underside.

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