How to calculate Longitude from Right Ascension?
In other "words", the connection between the time of transit $t_\mathrm{tr}$ of an object, its right ascension $\alpha$ and the geographical longitude of an observer $l$ is the (apparent) siderial time at Greenwich $\theta_0$ (if you know your local siderial time $\theta$, you don't need $\theta_0$ or $l$):
$$t_\mathrm{tr} = \alpha - \theta_0 - l = \alpha - \theta.$$