 Real projective line

Just a circle.

Take ${\mathbb{R}}^2$ with a line at $x=0$. Identify all the points that an observer

The real projective line, denoted as \(\mathbb{RP}^1\), is a fundamental concept in projective geometry. It can be understood as the space of all lines that pass through the origin in \(\mathbb{R}^2\). Each line corresponds to a unique direction in the plane, and projective geometry allows for a more compact representation of these directions.