Umbra, penumbra and antumbra#Antumbra
{{Short description|Distinct parts of a shadow}}
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File:Diagram of umbra, penumbra & antumbra.png
File:Kernschatten und Halbschatten.svg
The umbra, penumbra and antumbra are three distinct parts of a shadow, created by any light source after impinging on an opaque object. Assuming no diffraction, for a collimated beam (such as a point source) of light, only the umbra is cast.
These names are most often used for the shadows cast by celestial bodies, though they are sometimes used to describe levels, such as in sunspots.
Umbra
{{Redirect|Umbra}}
The umbra ({{langnf|la||shadow}}) is the innermost and darkest part of a shadow, where the light source is completely blocked by the occluding body. An observer within the umbra experiences a total occultation. The umbra of a round body occluding a round light source forms a right circular cone. When viewed from the cone's apex, the two bodies appear the same size.
The distance from the Moon to the apex of its umbra is roughly equal to that between the Moon and Earth: {{cvt|384402|km|mi}}. Since Earth's diameter is 3.7 times the Moon's, its umbra extends correspondingly farther: roughly {{convert|1.4|e6km|mi|abbr=unit}}.{{cite web |url=http://www.astronomy.ohio-state.edu/~pogge/Ast161/Unit2/eclipses.html |title=Lecture 9: Eclipses of the Sun & Moon |work=Astronomy 161: An Introduction to Solar System Astronomy |first=Richard |last=Pogge |publisher=Ohio State University |access-date=July 16, 2015}}
Penumbra
{{redirect|Penumbra}}
The penumbra ({{etymology|la|{{wikt-lang|la|paene}}|almost, nearly||{{wikt-lang|la|umbra}}|shadow}}) 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.[http://naif.jpl.nasa.gov/pub/naif/misc/toolkit_docs_N0061/Tutorials/pdf/individual_docs/45_event_finding_preview.pdf Event Finding Subsystem Preview] Navigation and Ancillary Information Facility.
File:Earth umbral cone (partial).png, showing how the umbral cone extends beyond the orbit of the Moon (The Moon is indicated by the yellow dot.)]]
File:Partial Lunar Eclipse 2019-07-16.jpg
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Antumbra
File: Transit Of Mercury, May 9th, 2016.png in front of the Sun, an extreme version of an annular eclipse. Mercury is visible as a black dot below and to the left of the center. The dark area above the center of the solar disk is a sunspot.]]
The antumbra ({{etymology|la|{{wikt-lang|la|ante}}|before||{{wikt-lang|la|umbra}}|shadow}}) is the region from which the occluding body appears entirely within the disc of the light source. An observer in this region experiences an annular eclipse, in which a bright ring is visible around the eclipsing body. If the observer moves closer to the light source, the apparent size of the occluding body increases until it causes a full umbra.{{cite web|url=https://www.timeanddate.com/eclipse/antumbra-shadow.html |title=Eclipses: What Is the Antumbra? |work=timeanddate.com |access-date=26 May 2019 }}
See also
References
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