The geometry with which we are most familiar is called Euclidean geometry. Euclidean geometry was named after Euclid, a Greek mathematician who lived in 300 BC. His book, called "The Elements", is a collection of axioms, theorems and proofs about squares, circles acute angles, isosceles triangles, and other such things. Most of the theorems which are taught in high schools today can be found in Euclid's 2000 year old book.

Euclidean geometry is of great practical value. It has been used by the ancient
Greeks through modern society to design buildings, predict the location of
moving objects and survey land.

**1.2 Non-Euclidean Geometry:**
non-Euclidean geometry is any geometry that is different from Euclidean
geometry.
Each Non-Euclidean geometry is a consistent system of definitions,
assumptions, and proofs that describe such objects as points, lines and planes.
The two most common non-Euclidean geometries are spherical geometry and hyperbolic
geometry. The essential difference between Euclidean geometry and these two
non-Euclidean geometries is the nature of parallel lines:
In Euclidean geometry, given a point and a line, there is exactly one line through
the point that is in the same plane as the given line and never intersects it.
In spherical geometry there are no such lines. In hyperbolic geometry there are
at least two distinct lines that pass through the point and are parallel to
(in the same plane as and do not intersect) the given line.

**1.3 Spherical Geometry:**

Spherical geometry is a plane geometry on the surface of a sphere.
In a plane geometry, the basic concepts are points and lines. In spherical
geometry, points are defined in the usual way, but lines are defined such that
the shortest distance between two points lies along them. Therefore, lines
in spherical geometry are *great circles*. A great circle is the largest
circle that can be drawn on a sphere. The longitude lines and the equator are
great circles of the Earth. Latitude lines, except for the equator, are not
great circles. Great circles are lines that divide a sphere into two equal
hemispheres.

Spherical geometry
is used by pilots and ship captains as they navigate around the globe.
Working in spherical geometry has some non-intuitive results. For
example, did you know that the shortest flying distance from Florida to
the Philippine Islands is a path across Alaska? The Philippines are **south**
of Florida - why is flying **north** to Alaska a short-cut? The
answer is that Florida, Alaska, and the Philippines are collinear locations
in spherical geometry (they lie on a great circle). Another odd
property of spherical geometry is that **the sum of the angles of a triangle
is always greater then 180°**. Small triangles, like
those drawn on a football field, have very, very close to 180°.
Big triangles, however, (like the triangle with veracities: New York, L.A.
and Tampa) have significantly more than 180°.

**1.4 Hyperbolic Geometry:**

hyperbolic geometry is the geometry of which the NonEuclid software
is a model. Hyperbolic geometry is a "curved" space, and
plays an important role in Einstein's General theory of Relativity.
hyperbolic geometry is also has many applications within the field of Topology.

Hyperbolic geometry shares many proofs and theorems with Euclidean geometry, and provides a novel and beautiful prospective from which to view those theorems. Hyperbolic geometry also has many differences from Euclidean geometry. The following sections discuss and explore hyperbolic geometry in some detail.

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Next Topic - 2: Using NonEuclid - My First Triangle