The knowledge that we seek to increase, to inform our understanding of visible reality, is based on the physics of light, the examination of human anatomy, the study of perspective, the conceptualization of bodies in space and in motion, and the mathematics of geometry.  These are widespread fields of study, and the artist may only need to consider very narrow and specific portions within each discipline.  This blog will aim to cover those aspects that relate specifically to purposeful application by the artist, and these will be covered in due course.  For now, we will start with the foundational groundwork .
First, in order to have a common understanding, it is necessary to have a common vocabulary, to discuss both the most basic principles and finer points.
Classical Realists are concerned with the creation of an illusion of reality, specifically the illusion of a three-dimensional world on a two-dimensional surface. In order to achieve a masterful drawing, the realist needs to understand and apply the principles of the Classical Order of Light.
Without light, nothing is visible.  What we aim to create on paper is our closest approximation to what the eye sees.  We therefore need to understand how light works, and how we, as humans, see.
We need to turn to the study of physics to understand exactly how light works, and in future posts, we’ll be examining different aspects of the science of light.  For now, we’ll be looking at the Classical Order of Light.

– Classical Drawing Atelier, Juliette Aristides
Light travels in straight lines and when it hits an opaque object, such as a sphere, it will not pass through that object.  Indeed, a part of the sphere will receive light and a part of the sphere will not receive light.
That part of the object where direct light cannot reach is the shadow.  Specifically, it is the form shadow (the shadow within that form).
Additionally, that form, our sphere in this case, is casting a shadow on another form, in this case, the ground plane, and that is the cast shadow.
These concepts are so essential to grasp and remember, it is worth repeating and internalizing them, so that we won’t confuse the shadow area with other tones on the light side that might describe undulations, furrows and other details.
The part that receives light from the direct light source belongs to the family of the lights.
The part that does not receive light from the direct light source will belong to the family of the shadows.
If in doubt about what one is seeing, it is worth testing whether a “darker” area is shadow or light.  One should first identify where the light source is located in relation to the object one is observing and use a process of logic to arrive at the right answer.  If the light source is in the top left, then it stands to reason that whatever is facing away from the light source (to the right, and downwards) would be the form shadow.  Any plane that is facing away from the light, a down plane, would most likely be part of the shadows.
A form shadow will also have a bed-bug line or terminator.  This is a boundary that completely “fences off” the perimeter of the shadow from the light.  We can look for such a bed-bug line to determine whether that is shadow or not.
A cast shadow will be on another plane where there is an impediment, obstructing the path of the light from falling on that other plane/object.  A cast shadow will have its own outline or perimeter clearly visible.
The third test one could apply is to actually place something solid like a knitting needle in the area which one is testing.  If a cast shadow from the needle is visible in that area, then it is not part of the shadow.  If that area is indeed part of the shadow (no light is falling there in the first instance) there will be no cast shadow visible from the needle.

This photo illustrates that the shadow being cast by the knitting needle doesn’t continue into the shadow below the bottom lip.  This means that entire area under that lower lip is shadow, albeit very light shadow, including reflected light.