# Day 22 | LThMath Advent Calendar

The following image has been circulating on the internet and I think it is such a fun way of understanding the importance of axis.

__Axis and a Christmas Tree__

A **Cartesian coordinate system** in a plane is a coordinate system that specifies each point uniquely by a pair of numerical **coordinates**, which are the signed distances to the point from two fixed perpendicular oriented lines, measured in the same unit of length. Each reference line is called a *coordinate axis* or just *axis* (plural *axes*) of the system, and the point where they meet is its *origin*, at ordered pair (0, 0). The coordinates can also be defined as the positions of the perpendicular projections of the point onto the two axes, expressed as signed distances from the origin.

The **Spherical coordinate system** is commonly used in *physics*. It assigns three numbers (known as coordinates) to every point in Euclidean space: radial distance *r*, polar angle *θ* (theta), and azimuthal angle *φ* (phi). The symbol *ρ* (rho) is often used instead of *r*.