The penumbra is a part of a shadow cast by an object. As seen from the penumbra, part of the perimeter of the light source is obscured, and part of the light source is visible.

For an object to cast a penumbral shadow, the light source must be larger than a point - obviously, since a point cannot be partly obscured, it is either visible or it is not.

Assuming your monitor is a light source: Hold a small pad of paper between your eye and your monitor so that it covers your view of the right half the monitor. Your eye is now in the penumbral shadow of the paper.

The penumbra of the moon on the Earth is the area where a partial solar eclpise can be seen. The penumbra of the Earth on the moon causes a partial lunar eclipse.

The penumbra totally encloses the umbra and the antumbra - i.e. when the light source is larger than a point, any path from outside the shadow to either the umbra or the antumbra must pass through the penumbra.

Pe*num"bra (?), n. [NL., fr. L. paene almost + umbra shade.]

1.

An incomplete or partial shadow.

2. Astron.

The shadow cast, in an eclipse, where the light is partly, but not wholly, cut off by the intervening body; the space of partial illumination between the umbra, or perfect shadow, on all sides, and the full light.

Sir I. Newton.

⇒ The faint shade surrounding the dark central portion of a solar spot is also called the penumbra, and sometimes umbra.

3. Paint.

The part of a picture where the shade imperceptibly blends with the light.

 

© Webster 1913.

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