TikZ plot of the Andromeda "paradox" in special relativity

Andromeda v1.tex

\documentclass[crop=true, border=10pt]{standalone}
\usepackage{comment}
\begin{comment}
:Title: Attack from Andromeda
:Author: J A Miller (millerja@uah.edu), 2020/02/04

The Andromeda "paradox". Spacetime diagram for aliens launching an attack on Earth, according to three observers: one on the ground in the same frame as the aliens, and two walking past the first in opposite directions with the same speed.

References:
1. https://en.wikipedia.org/wiki/Rietdijk%E2%80%93Putnam_argument
2. https://en.wikipedia.org/wiki/Eternalism_(philosophy_of_time)

Plot style and colors from McNees
https://gist.github.com/mcnees/45b9f53ad371c38ba6f3759df5880fb1
\end{comment}



\usepackage{xcolor}

\usepackage{tikz}
\usetikzlibrary{arrows.meta}
\usetikzlibrary{decorations.markings}

\usepackage{pgfplots}
\pgfplotsset{compat=1.16}

% colors
\definecolor{plum}{rgb}{0.36078, 0.20784, 0.4}
\definecolor{chameleon}{rgb}{0.30588, 0.60392, 0.023529}
\definecolor{cornflower}{rgb}{0.12549, 0.29020, 0.52941}
\definecolor{scarlet}{rgb}{0.8, 0, 0}
\definecolor{brick}{rgb}{0.64314, 0, 0}
\definecolor{sunrise}{rgb}{0.80784, 0.36078, 0}
\definecolor{lightblue}{rgb}{0.15,0.35,0.75}



\begin{document}

% the speeds of the walking people
\newcommand*\vrel{0.25}
\pgfmathparse{1/sqrt(1-\vrel*\vrel)} \let\gammarel\pgfmathresult
\pgfmathparse{\vrel*\gammarel} \let\etarel\pgfmathresult
% some ancillary calculations
\pgfmathparse{1/\vrel - \vrel} \let\fac\pgfmathresult


\begin{tikzpicture}[scale=1,domain=-3:9]
    \tikzstyle{axisarrow} = [-{Latex[inset=0pt,length=8pt]}]

    % Draw the grid.
    \draw [cornflower!30,step=0.2,thin] (-3,-3) grid (9,9);
    \draw [cornflower!60,step=1.0,thin] (-3,-3) grid (9,9);

    % Clip all lines that would fall outside the grid, if any
    \clip(-3,-3) rectangle (9,9);
    
    % Draw Axes
    \draw[thick,cornflower,axisarrow] (0,-3) -- (0,9);
    \node[inner sep=0pt] at (0.25,8.7) {$t$};
    \draw[thick,cornflower,axisarrow] (-3,0) -- (9,0);
    \node[inner sep=0pt] at (8.7,-0.25) {$x$};
     
    % Draw the worldline of Andromeda
    \draw[plum,thick,axisarrow] (8,-3) -- (8,9);
    
    % Draw the primed coordinate system
    \draw[thick,axisarrow,scarlet] (-3*\vrel,-3) -- (9*\vrel,9) node (tp) {};
    \node[inner sep=0pt] at (2.5,8.7) {$t^\prime$};
    \draw[thick,axisarrow,scarlet] (0,0) -- (9,\vrel*9) node (xp) {};
    \node[inner sep=0pt] at (8.7,1.9) {$x^\prime$};
	
    % Draw the double primed coordinate system (make \vrel negative)
    \draw[thick,axisarrow,chameleon] (0,0) -- (-9*\vrel,9) node (tpp) {};
    \node[inner sep=0pt] at (-1.8,8.7) {$t^{\prime\prime}$};
    \draw[thick,axisarrow,chameleon] (0,0) -- (9,-9*\vrel) node (xpp) {};
    \node[inner sep=0pt] at (8.7,-2.5) {$x^{\prime\prime}$};
	
    % Lines of simultaneity
    \draw[dashed,thick,scarlet] (8,0) -- (-8*\vrel/\fac,-8/\fac) {};
    \draw[dashed,thick,chameleon] (8,0) -- (-8*\vrel/\fac,8/\fac) {};
	
    % Label some points
    \node[circle,draw=cornflower!70,fill=cornflower!20,inner sep=2pt,label={left:T}](T) at (-8*\vrel/\fac,-8/\fac)  {};
    \node[circle,draw=cornflower!70,fill=cornflower!20,inner sep=2pt,label={left:A}](A) at (-8*\vrel/\fac,8/\fac)  {};
    \node[circle,draw=cornflower!70,fill=cornflower!20,inner sep=2pt,label={above left:S}](S) at (0,0)  {};
    \node[circle,draw=cornflower!70,fill=cornflower!20,inner sep=2pt,label={above right:L}](L) at (8,0)  {};
    
    % Label the worldlines
    \node[inner sep=0pt,rotate=90] at (7.7,5.2) {Andromeda worldline};
    \node[inner sep=0pt,rotate=90] at (-0.3,5.2) {Stationary observer worldline};
    \node[inner sep=0pt,rotate=76] at (1.6,5.2) {Toward Andromeda};
    \node[inner sep=0pt,rotate=-76] at (-1.6,5.2) {Away from Andromeda};

\end{tikzpicture}  

\end{document}