# Put node on ellipse in tikz

I'm trying to make a simple drawing in tikz consisting of:

• An ellipse
• Some label inside the ellipse
• Another label near the curve
• An outer normal vector to the ellipse, again with a label

Producing the ellipse and the first two labels is no problem. However, I can't figure out how to put a node on the boundary of the ellipse. How to do this?

\documentclass{article}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
\filldraw[fill=black!20!white](0, 0) ellipse (100pt and 50pt);
\end{tikzpicture}
\end{document}

• Please supply how you draw the ellipse (i.e. a MWE), is the ellipse a node, or a manual \draw command, for instance? – zeroth Feb 11 '13 at 10:31
• @zeroth: currently I produce the ellipse with \filldraw[fill=black!20!white](0, 0) ellipse (100pt and 50pt) – Martijn Feb 11 '13 at 10:32
• Please add that to the question (with a full \documentclass ... \usepackage it will make it a lot easier for people to supply an answer), i.e. no need to re-create an ellipse. Kind regards and thanks! :) – zeroth Feb 11 '13 at 10:38
• @zeroth: added the info. Please note that I'm not having trouble with errors or bugs, but need help using TikZ – Martijn Feb 11 '13 at 10:54
• I am well aware of that. :) Thanks for supplying it to the question. – zeroth Feb 11 '13 at 11:53

For drawing the normal vector, you can use the approach described in How to draw tangent line of an arbitrary point on a path in TikZ. It allows you to type

\filldraw[fill=black!20!white, tangent=0.1](0, 0) ellipse (100pt and 50pt);
\draw [blue, ultra thick, use tangent, -latex] (0,0) -- (0,-2) node [pos=0.5, anchor=east] {Normal vector};


to get: \documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary{decorations.markings}

\tikzset{
tangent/.style={
decoration={
markings,% switch on markings
mark=
at position #1
with
{
\coordinate (tangent point-\pgfkeysvalueof{/pgf/decoration/mark info/sequence number}) at (0pt,0pt);
\coordinate (tangent unit vector-\pgfkeysvalueof{/pgf/decoration/mark info/sequence number}) at (1,0pt);
\coordinate (tangent orthogonal unit vector-\pgfkeysvalueof{/pgf/decoration/mark info/sequence number}) at (0pt,1);
}
},
postaction=decorate
},
use tangent/.style={
shift=(tangent point-#1),
x=(tangent unit vector-#1),
y=(tangent orthogonal unit vector-#1)
},
use tangent/.default=1
}

\begin{document}

\begin{tikzpicture}
\filldraw[fill=black!20!white, tangent=0.1](0, 0) ellipse (100pt and 50pt);
\draw [blue, ultra thick, use tangent, -latex] (0,0) -- (0,-2) node [pos=0.5, anchor=east] {Normal vector};
\end{tikzpicture}

\end{document}