When is Halachic nightfall?

“And there was evening and there was morning, one day.”

According to the Torah, the new day begins with nightfall. Ok, so when is that? Seems simple enough - when it gets dark outside. But how dark? And what if you are not able to observe yourself? Is there a fixed amount of time to rely on, or is it relative to the conditions of the observer? The Gemara explores this in a couple of places, but there seem to be contradictions. How do we reconcile the various statements of our Sages?

As Rabbi Paysach Yehuda Diner explained is his precise and lucid shiur, there are two main opinions: Rabbeinu Tam, and the Geonim with the Vilna Gaon. But how does that translate to the times used for nightfall in contemporary America- 42, 50, 60 minutes?

Listen to the highly relevant shiur that brought a packed house to the Kollel on everyone’s day off! Follow along with the sources here.