The non-zero vectors are ${\overrightarrow a ,\overrightarrow b }$ and ${\overrightarrow c }$ are related by ${\overrightarrow a = 8\overrightarrow b }$ and ${\overrightarrow c = - 7\overrightarrow b \,\,.}$ Then the angle between ${\overrightarrow a }$ and ${\overrightarrow c }$ is :