The blurry edges of everyday shadows are caused by the light sources being extended objects. Light from different parts of the source are able to travel into the shadow area because they emanate from different points in space. The darker, inner part of the shadow where no light can reach is called the "umbra". The lighter, outer part of the shadow, where some light can reach is called the "penumbra". The parts where all the light can reach are unshadowed. Everyday objects illuminated by sunlight, such as trees and fences, can cast shadows composed of an umbra and penumbra just like astronomical objects.
