<P> The umbra, penumbra and antumbra are three distinct parts of a shadow, created by any light source after impinging on an opaque object . For a point source only the umbra is cast . </P> <P> These names are most often used for the shadows cast by celestial bodies, though they are sometimes used to describe levels of darkness, such as in sunspots . </P> <P> The umbra (Latin for "shadow") is the innermost and darkest part of a shadow, where the light source is completely blocked by the occluding body . An observer in the umbra experiences a total eclipse . The umbra of a round body occluding a round light source forms a right circular cone; to a viewer at the cone's apex, the two bodies are equal in apparent size . The distance from the Moon to the apex of its umbra is roughly equal to that between the Moon and Earth . Because the Earth is 3.70 times wider than the Moon, its umbra extends correspondingly farther, roughly 1.4 million kilometers . </P> <P> The penumbra (from the Latin paene "almost, nearly") is the region in which only a portion of the light source is obscured by the occluding body . An observer in the penumbra experiences a partial eclipse . An alternative definition is that the penumbra is the region where some or all of the light source is obscured (i.e., the umbra is a subset of the penumbra). For example, NASA's Navigation and Ancillary Information Facility defines that a body in the umbra is also within the penumbra . </P>

When the earths shadow completely covers the moon as seen from earth