# How to calculate the limb darkening $\mu$ value

I am reading about limb darkening and I am very confused how to find the value of $$\mu$$.

It states it can be found by $$\mu = \cos(\gamma)$$, but I am lost how $$\gamma$$ is found, or what it represents.

If a planet is, say in mid-transit, would $$\gamma$$ be zero degrees?

I can't seem to visualise the trigonometry to understand it. Hope someone can explain.