Contact · What a penumbra is
The edge of a shadow.
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01 / 07
Light travels in straight lines. An object in its path stops some of it. Behind the object lies a region the light does not reach. That region is the shadow.
02 / 07
A shadow has two parts. The first is the umbra, from the Latin word for shadow. It is the region where the object hides the whole light source, so no direct light reaches it. The base of this plant, beneath the leaves, is the everyday version.
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The second part is the penumbra. The word joins paene, Latin for almost, to umbra: the almost-shadow. It is the region where the object hides only part of the light source. A viewer standing there sees some of the light and not all of it, so the region is neither dark nor fully lit.
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The penumbra exists because lights have size. A point of light would cast a shadow with a hard edge and no penumbra. A wider light casts a wider soft edge. Move the slider and watch the screen on the right.
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The same geometry holds at the scale of planets. When the Moon passes in front of the Sun, a viewer inside the Moon’s umbra sees the Sun fully covered. A viewer inside its penumbra sees it partly covered. The Sun is not a point, so the penumbra is wide.
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This leaf is lit from behind. It is not opaque, so it blocks some of the light and passes the rest. Strictly, its glow is light transmitted through tissue, and a penumbra is the geometric effect of a light with size. The two mechanisms differ. They share a result: a surface that is neither dark nor fully lit.
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A penumbra is the region where part of the source is visible. Any inquiry has a similar region, where some evidence reaches us and some does not. This publication reports from that region. Where a question is unresolved, the report says so.
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Corrections and pitches.
The Penumbra values the growth of knowledge. Corrections are posted, not quietly edited. If a report is wrong, write and say where. If you have a finding that belongs here, write and send it.
For a correction, include the sentence and the source that contradicts it. For a pitch, include the finding and a link to the original study or document.