5

I am trying to draw the celestial sphere and can't figure out how to draw arcs around the outer surface of my sphere. I want to draw a red arrow starting from the vernal equinox towards right under the sun directly at the edge of the equatorial plane. Furthermore I want to draw the declination, that is the arc from the right hand side of the sphere towards the position of the sun. Both arcs are indicated with my superb photoshop skills. What would be the approach to draw arbitrary values for the right ascension and the declination?

Celestial sphere

This is a MWE

\documentclass{scrartcl}
\usepackage{tikz}
\begin{document}
    \begin{tikzpicture}[
        dot/.style = {outer sep = 0, inner sep = 0, shape = circle, label = {#1}},
        dot/.default =,
        small dot/.style = {minimum size = .2cm, dot = {#1}},
        small dot/.default =,]
        % Equator
        \draw[thick] (-6,0) arc (180:360:6cm and 1cm);
        \draw[thick, dashed, opacity=.6] (-6,0) arc (180:0:6cm and 1cm);
        % Ecliptic of the sun (back)
        \draw[thick, dashed, rotate=23.5, color=blue, opacity=.6] (-6,0) arc (180:0:6cm and 1cm);
        % Celestial sphere
        \draw[thick] (0,0) circle (6cm);
        \shade[ball color=blue!10!white,opacity=0.20] (0,0) circle (6cm);
        % Earth
        \draw[thick] (0,0) circle (0.5cm);
        \shade[ball color=blue!60!white,opacity=0.80] (0,0) circle (0.5cm);
        % Sun
        \draw[thick] (5.5024,2.392) circle (0.5cm);
        \shade[ball color=yellow!60!white,opacity=0.8] (5.5024,2.392) circle (0.5cm);
        % Ecliptic of the sun (front)
        \draw[thick, rotate=23.5, color=blue] (-6,0) arc (180:360:6cm and 1cm);
        % Vernal equinox
        \node[fill = black, draw = black, small dot = {right: }] at (0.2,-1) {};
    \end{tikzpicture}
\end{document}
8

You can draw the arrows using the information you already have and with the help of arc.

Output

example image

Code

\documentclass{scrartcl}
\usepackage{tikz}

\usetikzlibrary{bending}
\begin{document}
    \begin{tikzpicture}[
        dot/.style = {outer sep = 0, inner sep = 0, shape = circle, label = {#1}},
        dot/.default =,
        small dot/.style = {minimum size = .2cm, dot = {#1}},
        small dot/.default =,]
        % Equator
        \draw[thick] (-6,0) arc (180:360:6cm and 1cm);
        \draw[thick, dashed, opacity=.6] (-6,0) arc (180:0:6cm and 1cm);
        % Ecliptic of the sun (back)
        \draw[thick, dashed, rotate=23.5, color=blue, opacity=.6] (-6,0) arc (180:0:6cm and 1cm);
        % Celestial sphere
        \draw[thick] (0,0) circle (6cm);
        \shade[ball color=blue!10!white,opacity=0.20] (0,0) circle (6cm);
        % Earth
        \draw[thick] (0,0) circle (0.5cm);
        \shade[ball color=blue!60!white,opacity=0.80] (0,0) circle (0.5cm);
        % Sun
        \draw[thick] (5.5024,2.392) circle (0.5cm);
        \shade[ball color=yellow!60!white,opacity=0.8] (5.5024,2.392) circle (0.5cm);
        % Ecliptic of the sun (front)
        \draw[thick, rotate=23.5, color=blue] (-6,0) arc (180:360:6cm and 1cm);
        % Vernal equinox
        \node[fill = black, draw = black, small dot = {right: }] at (0.2,-1) {};


        \draw[red, very thick, ->] (.2,-1) arc (-90:0:5.8 and 1.02);
        \draw[green, very thick, ->] ++(6cm,0) arc (0:23:6cm);
    \end{tikzpicture}
\end{document}
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