16.1: Euclidean space

Recall that Euclidean space is the set $\mathbb{R}^3$ of all triples $(x,y,z)$ of real numbers such that the distance between a pair of points $A=(x_A,y_A,z_A)$ and $B=(x_B,y_B,z_B)$ is defined by the following formula:

$AB := \sqrt{(x_A-x_B)^2+(y_A-y_B)^2+(z_A-z_B)^2}.$

The planes in the space are defined as the set of solutions of equation

$a\cdot x+b\cdot y+c\cdot z+d=0$

for real numbers $a$, $b$, $c$, and $d$ such that at least one of the numbers $a$, $b$ or $c$ is not zero. Any plane in the Euclidean space is isometric to the Euclidean plane.

A sphere in space is the direct analog of a circle in the plane. Formally, sphere with center $O$ and radius $r$ is the set of points in the space that lie on the distance $r$ from $O$.

Let $A$ and $B$ be two points on the unit sphere centered at $O$. The spherical distance from $A$ to $B$ (briefly $AB_s$) is defined as $|\measuredangle AOB|$.

In spherical geometry, the role of lines play the great circles; that is, the intersection of the sphere with a plane passing thru $O$.

Note that the great circles do not form lines in the sense of Definition 1.5.1. Also, any two distinct great circles intersect at two antipodal points. In particular, the sphere does not satisfy the axioms of the neutral plane.