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A penumbra is an area of slight darkness — not full dark, not full light. The picture science teachers draw is an eclipse: the umbra is the deep, fully dark heart of the shadow; the penumbra is the paler ring around it, where the light is only partly blocked. That is the sense you need in physics class. Writers also stretch it, in careful prose, to mean a grey zone around a clear rule or a clear idea — the fuzzy edge. It is a learned word. In daily speech you would say a bit of shade or a partial shadow. You will almost never hear it from a child pointing at a tree.
半影（相對「本影」；物理課講日食、月食時用）
組成From New Latin pēnumbra, from Latin paene (“almost”) + umbra (“shadow”): almost-shadow.
同家族的字
- umbrellaboth go back to Latin umbra, “shadow”
- umbragesame Latin umbra, “shadow”
- penultimateboth start from Latin paene, “almost”
原始詞典資料6 筆記錄
以下是外部詞典記錄的用法，原文照列，我們還沒有寫成教學內容。上面的說明才是我們自己寫的；有些用法可能上面已經講過了。
penumbranoun/pəˈnʌm.bɹə/
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A partially shaded area around the edges of a shadow, especially an eclipse.
- [A]ny penumbral fringe that may be detected cannot be well distinguished from the effects of the true penumbræ; […]
- The other places see the penumbra of the moon's shadow fall on the earth, so the eclipse is partial, and part of the sun's disc is still visible.
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astronomynatural-sciencesA region around the edge of a sunspot, darker than the sun's surface but lighter than the middle of the sunspot.
- The atmosphere was very clear, evidenced by the steadiness with which the mottling of the sun’s surface and the penumbræ of several spots were visible.
- The velocities found in the penumbræ of spots generally increase with the distance from the centre of radiation, the motion being zero at some point in the umbra, and accelerating outwards.
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An area of uncertainty or intermediacy between two mutually exclusive states or categories.
- These firms or businesses are not illegal in the strict sense, but there is a shadowy penumbra within which they live, and it is often convenient for the government to look the other way.
- […] God chose to descend into the realm of human imperfection, where the light of truth is spare and must exist in the penumbra of partial knowledge mixed with partial ignorance.
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An area that lies on the edge of something; a fringe.
- Thank God we are not all cowards, we have not all a low ambition, which would make men shades, pœnumbræ^([sic]) of their fellows.
- Whilst the orthodox, de-charismatized churches steadily lose influence and support and the new cults develop, in the religious penumbra there have persisted, during the last century, echoes of charisma.
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Something related to, connected to, and implied by, the existence of something else that is necessary for the second thing to be full and complete in its essential aspects.
- The foregoing [United States Supreme Court] cases suggest that specific guarantees in the Bill of Rights have penumbras, formed by emanations from those guarantees that help give them life and substance.
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medicinesciencesA region of the brain that has lost only some of its blood supply, and retains structural integrity but has lost function.
資料來源：英文維基詞典 English Wiktionary（CC BY-SA 4.0）。 原文照列，未經改寫。