# Doubt in the equation of trajectory of partical moving under central Force

In the book by Goldstein (Third Edition, Page 95) on classical mechanics, Goldstein derives equation of the trajectory of particle moving under center force. The equation he derives is given by:

$$r= \frac{a(1-e^{2})}{1+e\cos(\theta-\theta')}.$$

My question is how $\theta$ is measured in the given equation i.e w.r.t which position $\theta$ is measured?

It should be clear from inspection that $r$ has it's smallest value when $$\theta = \theta' + 2 n \pi \;,$$ for $n \in \mathbb{Z}$ (that is $n$ is any integer) and likewise has it's largest value for $$\theta = \theta' + (2 n + 1) \pi \;.$$
Which tells you that in coordinate system $\theta'$ is the angular position of periapsis and $\theta' + \pi$ is the angular position of apoapsis.