A shadow's an area where direct light from a light source can't reach due to obstruction by an object. It occupies all of the space behind an opaque object with light in front of it. The cross section of a shadow's a two-dimensional silhouette, or reverse projection of the object blocking the light. An astronomical object casts human visible shadows when its apparent magnitude's equal or lower than -4. Currently the only astronomical objects able to produce visible shadows on Earth are the Sun,… (More on Shadow)