\begin{definition}[Definition:Inner Angle of Spherical Triangle]
Let $\triangle ABC$ be a spherical triangle on the surface of a sphere whose center is $O$.
Let the sides $a, b, c$ of $\triangle ABC$ be measured by the angles subtended at $O$, where $a, b, c$ are opposite $A, B, C$ respectively.
Consider the $4$ consecutive parts of $\triangle ABC$:
:$B, a, C, b$
Then $C$ is known as the '''inner angle''' of $B, a, C, b$
\end{definition}