# How does Rav Moshe calculate the "50 minutes" between שקיעה and צה"כ?

For New York, Rabbi Moshe Feinstein maintained that the time between שקיעה and צה"כ is 50 mins (corresponding to walking 4 mil), unlike in Europe where it is 72 mins (see here):

... the calculation of the 4 mil between sunset and tzeis. His essential thesis is based on personal empirical observation. Rav Moshe recalled that when he was in Europe, they waited 72 minutes in following the shita of the R”T. Rav Moshe then claims that when he observed the sky in New York, New Jersey, and pashtus, the Catskills, the night sky resembled the 72 minute European night sky after only 50 minutes. Rav Moshe then deduced that the 4 mil of the gm in pesachim are actually 50 minutes in duration.

I have a few questions on this:

1) how does Rav Moshe get to the 50 minutes between שקיעה and צה"כ, is it an average of all times across the year?

2) in reality, there will be occasions where stars come out early or later than 50 minutes. He obviously weighed up the problem of making a blanket 50 minute rule. What are the implications of this?

3) at what latitude can the fluctuation of שקיעה to צה"כ not be averaged in this way?

4) is this method of calculating used anywhere else in the world?

• its generally understood that he thought 50 was the longest possible (though he may just not have understood how it fluctuates with the seasons, an issue shared by many even nowadays; the entire teshuva is astoundingly difficult to reconcile with reality or any classical sources, וצע"ג.) May 30, 2019 at 11:48
• It's Igrot Moshe 4:62 you can read for yourself hebrewbooks.org/pdfpager.aspx?req=920&st=&pgnum=97 May 30, 2019 at 12:02
• @DoubleAA thanks for the source material. Yes, difficult to understand. May 30, 2019 at 12:29
• Are there any English summaries or translations of this teshuva?
– user17319
Dec 1, 2020 at 14:44
• Is this more of a question on Reb Moshe than Rabeinu Tam? Or anyone else who gave a fixed time between sunset and full night? Aug 18, 2023 at 10:58