Vector Algebra

Let $\overrightarrow a ,\overrightarrow b$ and $\overrightarrow c$ be non-zero vectors such that $\left( {\overrightarrow a \times \overrightarrow b } \right) \times \overrightarrow c = {1 \over 3}\left| {\overrightarrow b } \right|\left| {\overrightarrow c } \right|\overrightarrow a \,\,.$ If $\theta$ is the acute angle between the vectors ${\overrightarrow b }$ and ${\overrightarrow c },$ then $sin\theta$ equals :