[Avodah] zman hadloko erev Shabbos and motzoei Shabbos

[Marty Bluke]

R' Zev Sero wrote:
<They might have thought that perhaps all of those stars were "kochavim
gedolim", and not the three "kochavim beinunim" that are required.

That is hard to imagine. For example, 50 minutes after shkia it is pitch
black outside and you can see countless stars.

<The truth is that something approximating "Laila DeRT" does match a real
phenomenon -- when the sun is 18 degrees below the horizon none of its light
is still in the sky, and it's as dark as it's going to get. This is what the
charts call "astronomical twilight".

However, this does not correspond to 72 minutes.

In NY it varies from approximately 90 - 120 minutes
In Yerushalayim it varies from approximately 80 -100 minutes.
(all times are from http://www.sunrisesunset.com/custom_srss_calendar.asp)

In cities such as Vilna, Moscow, Kovna, etc. in the summer there is no
astronomical twilight.

In any case what we see from here is to say that R"T is a fixed 72 minutes
is very very difficult.
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