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{{Short description|Point on the celestial sphere opposite Sun}}
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[[Image:Subparhelic circle flickr fdecomite.jpg|thumb|250px|A photo centred on the antisolar point featuring various antisolar/subhorizon haloes as viewed from a flight between Brussels and Madrid, August 7, 2006.<br />Photo: Francesco De Comité.]]
[[File:Double-alaskan-rainbow.jpg|thumb|upright=1.3|A [[rainbow]] has 42° centered around the antisolar point, which always coincides with the shadow of the observer's eye/camera, seen here at the bottom of the frame.]]
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The '''antisolar point''' is the imaginary point on the [[celestial sphere]] exactly opposite the [[Sun]] from the viewpoint of an observer.<ref name="timherd">Tim Herd. [http://www.timherd.com/wp-content/uploads/2012/03/angularmeasurements.pdf Angular Measurements in the sky], excerpt from Kaleidoscope Sky, page 27. ISBN 0-8109-9397-X, Abrams, 2007</ref> This means that the antisolar point lies above the [[horizon]] when the Sun is below it, and vice versa. On a sunny day, the antisolar point can be easily found: It is located at the shadow of one's head. Like the [[zenith]] and [[nadir]], the antisolar point does not have a fixed position in three dimensional space, but is defined in relation to the observer: Each observer has their own antisolar point, which moves along with them as they change position.
[[File:Descartes_Rainbow.png|thumb|upright=1.1|Drawing by [[René Descartes]] explaining the formation of a [[rainbow]]. The antisolar point is the center of the rainbow '''M'''. It lies at the end of the straight line running from the sun through the observer's eye '''E'''.]]
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[[File:Subparhelic circle flickr fdecomite.jpg|thumb|Centered on the antisolar point, this photo features various antisolar/subhorizon [[halo (optical phenomenon)|haloes]], as viewed from a plane.]]


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The '''antisolar point''' is the [[vanishing point|abstract point]] on the [[celestial sphere]] directly opposite the [[Sun]] from an observer's [[perspective (graphical)|perspective]].<ref name="timherd">{{cite web |first=Tim |last=Herd |url=https://archive.org/details/kaleidoscopesky0000herd/page/27 |title=Angular Measurements in the sky |work=Kaleidoscope Sky |page=[https://archive.org/details/kaleidoscopesky0000herd/page/27 27] |publisher=Abrams |year=2007 |isbn=081099397X |url-access=registration }}</ref> This means that the antisolar point lies above the [[horizon]] when the Sun is below it, and vice versa. On a [[daytime|sunny day]], the antisolar point can be easily found; it is located within the [[shadow]] of the observer's head. Like the [[zenith]] and [[nadir (astronomy)|nadir]], the antisolar point is not fixed in [[three-dimensional space]], but is defined relative to the observer. Each observer has an antisolar point that moves as the observer changes position.
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The antisolar point forms the [[Centre_(geometry)|geometric center]] of several [[optical phenomena]], including [[rainbow]]s<ref>Les Cowley. [http://www.atoptics.co.uk/rainbows/primary.htm Primary rainbows - Atmospheric Optics], accessed 13 September 2013.</ref>, [[glory (optical phenomenon)|glories]]<ref>http://www.atoptics.co.uk/droplets/glory.htm</ref> and [[heiligenschein]]. Occasionally, during sunset or sunrise, [[anticrepuscular rays]] can be seen to converge at the antisolar point near the horizon.<ref>Les Cowley. [http://www.atoptics.co.uk/atoptics/anti1.htm Anticrepuscular rays], accessed 13 September 2013.</ref> This is an optical illusion, however, caused by [[perspective (visual)]]. In reality, the "rays" (i.e., bands of shadow) run parallel to each other.<ref>http://www.atoptics.co.uk/fz892.htm</ref> Also, on a moonless night away from city lights, it is often possible to see the [[gegenschein]] at that point, arising from the back-scattering of light by interplanetary dust. In [[astronomy]], when the [[full moon|moon]] or [[planet]]s are in [[Opposition (astronomy)|opposition]], they are found close to the antisolar point.


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The antisolar point forms the [[centre (geometry)|geometric center]] of several [[optical phenomena]], including subhorizon [[halo (optical phenomenon)|haloes]], [[rainbow]]s,<ref>{{cite web |url=http://www.atoptics.co.uk/rainbows/primary.htm |title=Primary rainbows |publisher=atoptics.co.uk |first=Les |last=Cowley |accessdate=13 September 2013}}</ref> [[glory (optical phenomenon)|glories]],<ref>{{cite web |url=http://www.atoptics.co.uk/droplets/glory.htm |title=The Glory |publisher=atoptics.co.uk |first=Les |last=Cowley}}</ref> the [[Brocken spectre]], and [[heiligenschein]]. Occasionally, around [[sunset]] or [[sunrise]], [[anticrepuscular rays]] appear to converge toward the antisolar point near the horizon.<ref>{{cite web |url=http://www.atoptics.co.uk/atoptics/anti1.htm |title=Anticrepuscular rays |publisher=atoptics.co.uk |first=Les |last=Cowley |accessdate=13 September 2013}}</ref> However, this is an [[optical illusion]] caused by perspective; in reality, the "rays" (i.e. bands of shadow) run near-parallel to each other.<ref>{{cite web |url=http://www.atoptics.co.uk/fz892.htm |title=Antisolar or anticrepuscular rays |publisher=atoptics.co.uk |first=Les |last=Cowley}}</ref>
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== Anthelic point==

Also around the antisolar point, the [[gegenschein]] is often visible in a moonless [[night sky]] away from [[light pollution|city lights]], arising from the [[backscatter]] of [[sunlight]] by [[interplanetary dust]]. In [[astronomy]], the [[full Moon]] or a [[planet]] in [[opposition (planets)|opposition]] lies near the antisolar point. During a total [[lunar eclipse]], the full Moon enters the [[umbra]] of [[Earth's shadow]], which the planet casts onto [[atmosphere of Earth|its atmosphere]], into space, and toward the antisolar point.

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== Anthelic point ==
The '''anthelic point''' is often used as a synonym for the antisolar point, but the two should be differentiated.<ref name="timherd" />
The '''anthelic point''' is often used as a synonym for the antisolar point, but the two should be differentiated.<ref name="timherd" />
While the antisolar point is directly opposite the sun, always below the horizon when the sun is up, the anthelic point is opposite but at the same elevation as the sun, and is therefore located on the [[parhelic circle]]. There are several [[halo (optical phenomenon)|halo phenomena]] that are centered on or converge on the anthelic point, such as the [[anthelion]], Wegener arcs, Tricker arcs and the parhelic circle itself.<ref>Alexander Wünsche; Jim Foster, [http://epod.usra.edu/blog/2006/04/anthelion-and-anthelic-arcs.html Anthelion and anthelic arcs], 2006</ref><ref>Walter Tape, ''Atmospheric Halos'', ISSN 0066-4634, ISBN 0875908349, American Geophysical Union, 1994, p. 27</ref><ref>Les Cowley. [http://www.atoptics.co.uk/halo/spanti.htm South Pole Halos - Anthelic View - Atmospheric Optics], accessed 13 September 2013</ref>
While the antisolar point is directly opposite the sun, always below the horizon when the sun is up, the anthelic point is opposite but at the same elevation as the sun, and is therefore located on the [[parhelic circle]]. There are several [[halo (optical phenomenon)|halo phenomena]] that are centered on or converge on the anthelic point, such as the [[anthelion]], Wegener arcs, Tricker arcs and the parhelic circle itself.<ref>Alexander Wünsche; Jim Foster, [http://epod.usra.edu/blog/2006/04/anthelion-and-anthelic-arcs.html Anthelion and anthelic arcs], 2006</ref><ref>Walter Tape, ''Atmospheric Halos'', {{ISSN|0066-4634}}, {{ISBN|0875908349}}, American Geophysical Union, 1994, p. 27</ref><ref>Les Cowley. [http://www.atoptics.co.uk/halo/spanti.htm South Pole Halos – Anthelic View – Atmospheric Optics] {{Webarchive|url=https://web.archive.org/web/20150923180132/http://www.atoptics.co.uk/halo/spanti.htm |date=2015-09-23 }}, accessed 13 September 2013</ref>


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==See also==
== See also ==
{{Portal|Astronomy|Space}}
{{Portal|Astronomy|Space}}
* [[Heiligenschein]]
* [[Heiligenschein]]
* [[Opposition surge]]
* [[Subparhelic circle]]
* [[Subparhelic circle]]
* [[Sylvanshine]]
* [[Sylvanshine]]
{{clear}}
{{clear}}


==References==
== References ==
{{reflist|2}}
{{Reflist|30em}}


{{DEFAULTSORT:Antisolar Point}}
[[Category:Spherical astronomy]]
[[Category:Spherical astronomy]]

{{Astronomy-stub}}

Latest revision as of 21:24, 21 December 2024

A rainbow has 42° centered around the antisolar point, which always coincides with the shadow of the observer's eye/camera, seen here at the bottom of the frame.
Drawing by René Descartes explaining the formation of a rainbow. The antisolar point is the center of the rainbow M. It lies at the end of the straight line running from the sun through the observer's eye E.
Centered on the antisolar point, this photo features various antisolar/subhorizon haloes, as viewed from a plane.

The antisolar point is the abstract point on the celestial sphere directly opposite the Sun from an observer's perspective.[1] This means that the antisolar point lies above the horizon when the Sun is below it, and vice versa. On a sunny day, the antisolar point can be easily found; it is located within the shadow of the observer's head. Like the zenith and nadir, the antisolar point is not fixed in three-dimensional space, but is defined relative to the observer. Each observer has an antisolar point that moves as the observer changes position.

The antisolar point forms the geometric center of several optical phenomena, including subhorizon haloes, rainbows,[2] glories,[3] the Brocken spectre, and heiligenschein. Occasionally, around sunset or sunrise, anticrepuscular rays appear to converge toward the antisolar point near the horizon.[4] However, this is an optical illusion caused by perspective; in reality, the "rays" (i.e. bands of shadow) run near-parallel to each other.[5]

Also around the antisolar point, the gegenschein is often visible in a moonless night sky away from city lights, arising from the backscatter of sunlight by interplanetary dust. In astronomy, the full Moon or a planet in opposition lies near the antisolar point. During a total lunar eclipse, the full Moon enters the umbra of Earth's shadow, which the planet casts onto its atmosphere, into space, and toward the antisolar point.

Anthelic point

[edit]

The anthelic point is often used as a synonym for the antisolar point, but the two should be differentiated.[1] While the antisolar point is directly opposite the sun, always below the horizon when the sun is up, the anthelic point is opposite but at the same elevation as the sun, and is therefore located on the parhelic circle. There are several halo phenomena that are centered on or converge on the anthelic point, such as the anthelion, Wegener arcs, Tricker arcs and the parhelic circle itself.[6][7][8]

See also

[edit]

References

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  1. ^ a b Herd, Tim (2007). "Angular Measurements in the sky". Kaleidoscope Sky. Abrams. p. 27. ISBN 081099397X.
  2. ^ Cowley, Les. "Primary rainbows". atoptics.co.uk. Retrieved 13 September 2013.
  3. ^ Cowley, Les. "The Glory". atoptics.co.uk.
  4. ^ Cowley, Les. "Anticrepuscular rays". atoptics.co.uk. Retrieved 13 September 2013.
  5. ^ Cowley, Les. "Antisolar or anticrepuscular rays". atoptics.co.uk.
  6. ^ Alexander Wünsche; Jim Foster, Anthelion and anthelic arcs, 2006
  7. ^ Walter Tape, Atmospheric Halos, ISSN 0066-4634, ISBN 0875908349, American Geophysical Union, 1994, p. 27
  8. ^ Les Cowley. South Pole Halos – Anthelic View – Atmospheric Optics Archived 2015-09-23 at the Wayback Machine, accessed 13 September 2013
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