Editing Eclipse (section)

== Umbra, penumbra and antumbra ==
{{Main|Umbra, penumbra and antumbra}}
{{more citations needed|section|date=November 2017}}

[[File:Umbra01.svg|thumb|upright=1.3|Umbra, penumbra and antumbra cast by an opaque object occulting a larger light source]]
For any two objects in space, a line can be extended from the first through the second. The latter object will block some amount of light being emitted by the former, creating a region of shadow around the axis of the line. Typically these objects are moving with respect to each other and their surroundings, so the resulting shadow will sweep through a region of space, only passing through any particular location in the region for a fixed interval of time. As viewed from such a location, this shadowing event is known as an eclipse.<ref name=westfall2014>{{citation | title=Celestial Shadows: Eclipses, Transits, and Occultations | volume=410 | series=Astrophysics and Space Science Library | first1=John | last1=Westfall | first2=William | last2=Sheehan | publisher=Springer | year=2014 | isbn=978-1493915354 | pages=1–5 | url=https://books.google.com/books?id=W9mLBQAAQBAJ&pg=PA1 | postscript=. }}</ref>

Typically the cross-section of the objects involved in an astronomical eclipse is roughly disk-shaped.<ref name=westfall2014/> The region of an object's shadow during an eclipse is divided into three parts:<ref name=glossary>{{cite web | last=Espenak | first=Fred | date=September 21, 2007 | url=http://sunearth.gsfc.nasa.gov/eclipse/SEhelp/SEglossary.html | title=Glossary of Solar Eclipse Terms | publisher=NASA | access-date=2008-02-28 | url-status=dead | archive-url=https://web.archive.org/web/20080224074526/http://sunearth.gsfc.nasa.gov/eclipse/SEhelp/SEglossary.html | archive-date=February 24, 2008 }}</ref>
* The ''umbra'' (Latin for 'shadow'), within which the object completely covers the light source. For the [[Sun]], this light source is the photosphere.
* The ''antumbra'' (from Latin ''ante'', 'before, in front of', plus ''umbra'') extending beyond the tip of the umbra, within which the object is completely in front of the light source but too small to completely cover it.
* The ''penumbra'' (from the Latin ''paene'', 'almost, nearly', plus ''umbra''), within which the object is only partially in front of the light source.

[[File:Solar eclipse types.svg|thumb|upright=0.9|Sun-Moon configurations that produce a total (A), annular (B), and partial (C) solar eclipse|alt=|left]]
A '''total eclipse''' occurs when the observer is within the umbra, an '''annular eclipse''' when the observer is within the antumbra, and a '''partial eclipse''' when the observer is within the penumbra. During a lunar eclipse only the umbra and penumbra are applicable, because the antumbra of the Sun-Earth system lies far beyond the Moon. Analogously, Earth's apparent diameter from the viewpoint of the Moon is nearly four times that of the Sun and thus cannot produce an annular eclipse. The same terms may be used analogously in describing other eclipses, e.g., the antumbra of [[Deimos (moon)|Deimos]] crossing [[Mars]], or [[Phobos (moon)|Phobos]] entering Mars's penumbra.

The ''first contact'' occurs when the eclipsing object's disc first starts to impinge on the light source; ''second contact'' is when the disc moves completely within the light source; ''third contact'' when it starts to move out of the light; and ''fourth'' or ''last contact'' when it finally leaves the light source's disc entirely.

For spherical bodies, when the occulting object is smaller than the star, the length (''L'') of the umbra's cone-shaped shadow is given by:

:<math>L\ =\ \frac{r \cdot R_o}{R_s - R_o}</math>

where ''R<sub>s</sub>'' is the radius of the star, ''R<sub>o</sub>'' is the occulting object's radius, and ''r'' is the distance from the star to the occulting object. For [[Earth]], on average ''L'' is equal to 1.384{{e|6}}&nbsp;[[kilometer|km]], which is much larger than the Moon's [[semimajor axis]] of 3.844{{e|5}}&nbsp;km. Hence the umbral cone of the Earth can completely envelop the Moon during a [[lunar eclipse]].<ref>{{cite book 
|first=Robin M. |last=Green |date=1985 
|title=Spherical Astronomy |publisher=Oxford University Press 
|isbn=978-0-521-31779-5 }}</ref> If the occulting object has an atmosphere, however, some of the luminosity of the star can be [[refraction|refracted]] into the volume of the umbra. This occurs, for example, during an eclipse of the Moon by the Earth&mdash;producing a faint, [[red|ruddy]] illumination of the Moon even at totality.

On Earth, the shadow cast during an eclipse moves very approximately at 1&nbsp;km per sec. This depends on the location of the shadow on the Earth and the angle in which it is moving.<ref>{{cite web|url=http://www.sciforums.com/threads/speed-of-eclipse-shadow.53722/|title=Speed of eclipse shadow? - Sciforums|website=sciforums.com|date=31 March 2006 |url-status=live|archive-url=https://web.archive.org/web/20150402094757/http://www.sciforums.com/threads/speed-of-eclipse-shadow.53722/|archive-date=2015-04-02}}</ref>