Question

Hello Rabbi,
I’ve had a chance to learn a bit about the famous dispute between Magen Avraham and the Vilna Gaon regarding the times of day—whether they are measured from dawn until nightfall, or from sunrise until sunset. I wanted to ask whether you have a view on the matter after studying the topic from both a scientific and a halakhic perspective—which of the two seems more plausible to you?
Also, I saw that the Vilna Gaon in Orach Chayim 459 (s.v. “mil,” if I remember correctly) cites the view of the experts in astronomy, the astronomers, and tries to use them to argue against the view of Terumat HaDeshen that the day begins at dawn, because there are astronomical difficulties with that—see there. Do today’s astronomers also hold that way?
I’d be glad to hear the Rabbi’s opinion on the subject; in any case, it has a practical implication for me regarding how long I can sleep in the morning with respect to reciting Shema 🙂