# Comparing and Contrasting Euclidean, Spherical, and Hyperbolic Geometries

**Topics:**Geometry, Pythagorean theorem, Euclidean geometry

**Pages:**5 (1815 words)

**Published:**May 9, 2005

The first issue that I will focus on is the definition of a straight line on all of these surfaces. For a Euclidean plane the definition of a "straight line" is a line that can be traced by a point that travels at a constant direction. When I say constant direction I mean that any portion of this line can move along the rest of this line without leaving it. In other words, a "straight line" is a line with zero curvature or zero deviation. Zero curvature can be determined by using the following symmetries. These symmetries include: reflection-in-the-line symmetry, reflection-perpendicular-to-the-line symmetry, half-turn symmetry, rigid-motion-along-itself symmetry, central symmetry or point symmetry, and similarity or self-similarity "quasi symmetry." So, if a line on a Euclidean plane satisfies all of the above conditions we can say it is a straight line. I have included my homework assignment of my definition of a straight line for a Euclidean plane so that one can see why I have stated this to be my definition. My definition for a straight line on a sphere is very similar to that on a Euclidean Plane with a few minor adjustments. My definition of a straight line on a sphere is one that satisfies the following Symmetries. These symmetries include: reflection-through-itself symmetry, reflection-perpendicular-to-itself symmetry, half-turn symmetry, rigid-motion-along-itself symmetry, and central symmetry. If we find that a line on a sphere satisfies all of the above condition, then that line is straight on a sphere. I have included my homework assignment for straightness on a sphere so that one can see why a straight line on a sphere must satisfy these conditions. Finally, I need to give my definition of a straight line on a hyperbolic plane. My definition of a straight line on a hyperbolic plane must satisfy the following symmetries. These symmetries include: reflection-in-the-line symmetry, reflection-perpendicular-to-the-line symmetry, half-turn symmetry, rigid-motion-along-itself symmetry, central-symmetry, and self-symmetry. If a line on a hyperbolic plane satisfies these conditions then we can say that it is straight. I have included my homework of my definition of a straight line on a hyperbolic plane so that one can see why these conditions must be satisfied.

The next issue that I will address for these three geometries is the definition of an angle on all three surfaces. The definition that I will give applies to all three surfaces. There are at least three different perspectives from which we can define "angle". These include: a dynamic notion of angle-angle as movement, angles as measure, and angles as a geometric shape. A dynamic notion of angle involves an action which may include a rotation, a turning point, or a change in direction between two lines. Angles as measure may be thought of as the length of a circular arc or the ratio between areas of circular sectors. When thinking of an angle as a geometric shape an angle may be seen as the delineation of space by two intersecting lines. I have provided my homework assignment on my definition of an angle so that one can see the reasoning of my definition for all three surfaces. However, my homework assignment does not ask to define an angle on a hyperbolic plane. This is because a region on a hyperbolic plane can be looked at...

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