How to fix this equation
\begin{eqnarray}
\frac{1}{2} (1-\varepsilon^2)\left(\partial_\tau \phi\right)^2
&= \frac{1}{2} \left(1-\epsilon ^2\right)\left(
-\frac{S \epsilon \sin\tau }{\sqrt{\lambda }}
-\frac{g_2 S^2 \epsilon ^2\sin2\tau }{3 \lambda }\right.\\
&\quad\left. +\epsilon ^3 \left(-\frac{\left(-\frac{1}{54} g_2^2 S
\left(32+19 S^2\right) \lambda +Z \left(
\frac{35 g_2^4}{27}
-\frac{7 g_2 g_4}{4}+\frac{5 g_5}{8}
-\frac{g_2^2 \lambda }{6}
+\frac{\lambda ^2}{24}\right)\right) \sin\tau }
{\lambda ^{5/2}}\right.\right.\\
&\quad\left.\left.-\frac{S^3 \left(4 g_2^2-3 \lambda \right) \sin3\tau }
{24 \lambda ^{3/2}}\right)\right)^2 \\
&= \frac{S^2 \epsilon ^2 \sin^2\tau }{2 \lambda }+\frac{2 g_2S^3 \epsilon ^3 \cos\tau \sin^2\tau }{3 \lambda ^{3/2}} \\
& \quad +\frac{1}{216 \lambda ^3}S \epsilon ^4 \Bigl( 280 g_2^4 Z-378 g_2 g_4 Z+135 g_5 Z-128 g_2^2 S \lambda -16 g_2^2 S^3 \lambda \\
& \quad -36 \text{g2}^2 Z \lambda -108 S \lambda ^2-27 S^3 \lambda ^2+9 Z \lambda ^2+6 S^3 \left(16 g_2^2-9 \lambda \right) \lambda \cos2 \tau \Bigr) \sin\tau
\end{eqnarray}
The problem I got that equation crossed the margin and equation number. How to fix it?
eqnarray
to begin with; see the link in the first answer to eqnarray vs. align