A shadow is an area where light from a light source is obstructed 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 is a two-dimensional silhouette, or reverse projection of the object blocking the light. Sunlight causes many objects to have shadows at certain times of the day. The angle of the sun, its apparent height in the sky causes a change in the length of shadows. Low-angles create longer shadows.There are three distinct parts of a shadow created by any non-point light source after impinging on an opaque object called the umbra, penumbra and antumbra. For a point source only the umbra is cast. These names are most often used for the shadows cast by astronomical objects, though they are sometimes used to describe levels of darkness, such as in sunspots. An astronomical object casts human-visible shadows when its apparent magnitude is equal or lower than −4. Currently the only astronomical objects able to produce visible shadows on Earth are the sun, the moon and, in the right conditions, Venus or Jupiter.
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