It is straightforward to show that $(2\pi)^{-1}\arcsin(\sqrt{1/3})$ is irrational. Indeed, if this equals a rational number $r\in\mathbb{Q}$, then $\sin(2\pi r)=\sqrt{1/3}$. However, $\sin(2\pi r)$ is a sum of two roots of unity, hence an algebraic integer, while $\sqrt{1/3}$ is not.

It is straightforward to show that $(2\pi)^{-1}\arcsin(\sqrt{1/3})$ is irrational. Indeed, if this equals a rational number $r\in\mathbb{Q}$, then $2\sin(2\pi r)=\sqrt{4/3}$. However, $2\sin(2\pi r)$ is a sum of two roots of unity, hence an algebraic integer, while $\sqrt{4/3}$ is not.