- Spherical geometry
Spherical geometry is the
geometryof the two- dimensional surface of a sphere. It is an example of a non-Euclidean geometry. Two practical applications of the principles of spherical geometry are navigationand astronomy.
plane geometrythe basic concepts are points and line. On the sphere, points are defined in the usual sense. The equivalents of lines are not defined in the usual sense of "straight line" but in the sense of "the shortest paths between points" which is called a geodesic. On the sphere the geodesics are the great circles, so the other geometric concepts are defined like in plane geometry but with lines replaced by great circles. Thus, in spherical geometry angles are defined between great circles, resulting in a spherical trigonometrythat differs from ordinary trigonometryin many respects (for example, the sum of the interior angles of a triangle exceeds 180 degrees).
Spherical geometry is the simplest model of
elliptic geometry, in which a line has no parallels through a given point. Contrast this with hyperbolic geometry, in which a line has two parallels, and an infinite number of ultra-parallels, through a given point.
An important related geometry related to that modeled by the sphere is called the
real projective plane; it is obtained by identifying antipodes (pairs of opposite points) on the sphere. Locally, the projective plane has all the properties of spherical geometry, but it has different global properties. In particular, it is non-orientable.
* [http://www.math.uncc.edu/~droyster/math3181/notes/hyprgeom/node5.html Spherical Geometry] UNCC
* [http://math.rice.edu/~pcmi/sphere/ The Geometry of the Sphere]
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