Saul Kripke: Difference between revisions

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m Saul Kripke is not the uncle of Eric Kripke. I changed the sentence to read that he is the "second cousin once removed" of the notable television....
 
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I'm Chastity (29) from Jaboatao Dos Guararapes, Brazil. <br>I'm learning Danish literature at a local university and I'm just about to graduate.<br>I have a part time job in a post office.<br><br>Also visit my site - [http://goo.gl/cPYUZA funny pranks 2014]
{{about|the mathematical notion|its applications in geodesy|great-circle distance|fictional interstellar organization called Great Circle|Andromeda (novel)}}
 
[[File:Great circle hemispheres.png|thumb|right|A great circle divides the sphere in two equal hemispheres]]
 
A '''great circle''', also known as an '''orthodrome''' or [[Riemannian circle]], of a [[sphere]] is the intersection of the sphere and a [[Plane (geometry)|plane]] which passes through the center point of the sphere, a partial case of a [[circle of a sphere]] where the plane is not required to pass through the center. (A ''small circle'' is the intersection of the sphere and a plane which does not pass through the center.) Any [[diameter]] of any great circle coincides with a diameter of the sphere, and therefore all great circles have the same [[circumference]] as each other, and have the same center as the sphere. A great circle is the largest circle that can be drawn on any given sphere. Every [[circle]] in [[Euclidean space|Euclidean 3-space]] is a great circle of exactly one sphere.
 
For any two points on the surface of a sphere there is a unique great circle through the two points.  An exception is a pair of [[Antipodal point|antipodal]] points, for which there are infinitely many great circles.  The minor arc of a great circle between two points is the shortest surface-path between them. In this sense the minor arc is analogous to “straight lines” in [[spherical geometry]]. The length of the minor arc of a great circle is taken as the distance between two points on a surface of a sphere in [[Riemannian geometry]]. The great circles are the [[geodesic]]s of the sphere.
 
In higher dimensions, the great circles on the [[n-sphere|''n''-sphere]] are the intersection of the ''n''-sphere with two-planes that pass through the origin in the Euclidean space '''R'''<sup>''n''+1</sup>.
 
==Derivation of shortest paths==
{{see also|Great-circle distance}}
To prove that the minor arc of a great circle is the shortest path connecting two points on the surface of a sphere, one has to apply [[calculus of variations]] to it.
 
Consider the class of all regular paths from a point ''p'' to another point ''q''. Introduce [[spherical coordinates]] so that ''p'' coincides with the north pole. Any curve on the sphere that does not intersect either pole, except possibly at the endpoints, can be parametrized by
 
:<math>\theta = \theta(t),\quad \phi = \phi(t),\quad a\le t\le b</math>
 
provided we allow φ to take on arbitrary real values. The infinitesimal arc length in these coordinates is
 
: <math>
ds=r\sqrt{\theta'^2+\phi'^{2}\sin^{2}\theta}\, dt.
</math>
 
So the length of a curve γ from ''p'' to ''q'' is a [[functional (mathematics)|functional]] of the curve given by
 
: <math>
S[\gamma]=r\int_a^b\sqrt{\theta'^2+\phi'^{2}\sin^{2}\theta}\, dt.
</math>
 
Note that ''S''[γ] is at least the length of the meridian from ''p'' to ''q'':
 
:<math>S[\gamma] \ge r\int_a^b|\theta'(t)|\,dt \ge r|\theta(b)-\theta(a)|.</math>
 
Since the starting point and ending point are fixed, ''S'' is minimized if and only if φ'&nbsp;=&nbsp;0, so the curve must lie on a meridian of the sphere φ&nbsp;=&nbsp;φ<sub>0</sub>&nbsp;=&nbsp;constant. In Cartesian coordinates, this is
:<math>x\sin\phi_0 - y\cos\phi_0 = 0</math>
which is a plane through the origin, i.e., the center of the sphere.
 
==Applications==
Some examples of great circles on the [[celestial sphere]] include the [[celestial horizon]], the [[celestial equator]], and the [[ecliptic]]. Great circles are also used as rather accurate approximations of [[geodesics on an ellipsoid|geodesics]] on the [[Earth]]'s surface (although it [[shape of the Earth|is not a perfect sphere]]), as well as on spheroidal [[celestial bodies]].
 
==See also==
* [[Rhumb line]]
 
==External links==
* [http://mathworld.wolfram.com/GreatCircle.html Great Circle – from MathWorld] Great Circle description, figures, and equations. Mathworld, Wolfram Research, Inc. c1999
* [http://www.greatcirclemapper.net The Great Circle Mapper] Displays Great Circle flight routes on a map and calculates distance and duration
* [http://www.gcmap.com/ Great Circle Mapper] Interactive tool for plotting great circle routes.
* [http://williams.best.vwh.net/gccalc.htm Great Circle Calculator] deriving (initial) course and distance between two points.
* [http://www.acscdg.com/ Great Circle Distance] Graphical tool for drawing great circles over maps. Also shows distance and azimuth in a table.
* [http://demonstrations.wolfram.com/GreatCirclesOnMercatorsChart/ Great Circles on Mercator's Chart] by John Snyder with additional contributions by Jeff Bryant, Pratik Desai, and Carl Woll, [[Wolfram Demonstrations Project]].
* [http://www.spigolo.altervista.org/ortho/OrthoFirstProblem.html 3D First Problem] {{it icon}} 3D javascript interactive tool ([[Google Chrome]], [[Firefox]], [[Safari (web browser)]]).
* [http://www.spigolo.altervista.org/ortho/OrthoSecondProblem.html 3D Second Problem] {{it icon}} 3D javascript interactive tool ([[Google Chrome]], [[Firefox]], [[Safari (web browser)]]).
 
[[Category:Elementary geometry]]
[[Category:Spherical trigonometry]]
[[Category:Riemannian geometry]]

Revision as of 10:10, 19 January 2014

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A great circle divides the sphere in two equal hemispheres

A great circle, also known as an orthodrome or Riemannian circle, of a sphere is the intersection of the sphere and a plane which passes through the center point of the sphere, a partial case of a circle of a sphere where the plane is not required to pass through the center. (A small circle is the intersection of the sphere and a plane which does not pass through the center.) Any diameter of any great circle coincides with a diameter of the sphere, and therefore all great circles have the same circumference as each other, and have the same center as the sphere. A great circle is the largest circle that can be drawn on any given sphere. Every circle in Euclidean 3-space is a great circle of exactly one sphere.

For any two points on the surface of a sphere there is a unique great circle through the two points. An exception is a pair of antipodal points, for which there are infinitely many great circles. The minor arc of a great circle between two points is the shortest surface-path between them. In this sense the minor arc is analogous to “straight lines” in spherical geometry. The length of the minor arc of a great circle is taken as the distance between two points on a surface of a sphere in Riemannian geometry. The great circles are the geodesics of the sphere.

In higher dimensions, the great circles on the n-sphere are the intersection of the n-sphere with two-planes that pass through the origin in the Euclidean space Rn+1.

Derivation of shortest paths

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Consider the class of all regular paths from a point p to another point q. Introduce spherical coordinates so that p coincides with the north pole. Any curve on the sphere that does not intersect either pole, except possibly at the endpoints, can be parametrized by

provided we allow φ to take on arbitrary real values. The infinitesimal arc length in these coordinates is

So the length of a curve γ from p to q is a functional of the curve given by

Note that S[γ] is at least the length of the meridian from p to q:

Since the starting point and ending point are fixed, S is minimized if and only if φ' = 0, so the curve must lie on a meridian of the sphere φ = φ0 = constant. In Cartesian coordinates, this is

which is a plane through the origin, i.e., the center of the sphere.

Applications

Some examples of great circles on the celestial sphere include the celestial horizon, the celestial equator, and the ecliptic. Great circles are also used as rather accurate approximations of geodesics on the Earth's surface (although it is not a perfect sphere), as well as on spheroidal celestial bodies.

See also

External links