How do I shade the outside of a semicircle perpendicular to the x-axis in such a way that the color fades as one moves away from the semicircle? Thanks for the help!
\begin{tikzpicture}
\draw[blue] (-1,0) .. controls (-1,0.555) and (-0.555,1) .. (0,1)
.. controls (0.555,1) and (1,0.555) .. (1,0);
\draw (-2,0) -- (2,0);
\end{tikzpicture}
