On the Orthodox Union web site, accessing zemanim provides a variety of halakhic information including two alternative times for the end of Shabbat at locations around the world.  In the New York area, the first alternative, approximately 45 minutes after what is commonly referred to as sunset, is presented as the opinion of the geonim – Shabbat ends the time it takes to walk ¾ of a mil after the beginning of bein hashemashot.  The second alternative, 72 minutes, is presented as the opinion of Rabbeinu Tam – Shabbat ends the time it takes to walk four mil after sunset.  Of course, this is immediately problematic.  If the time to walk four mil is 72 minutes, then the time to walk one mil is 18 minutes and the time to walk ¾ of a mil equals 13.5 minutes, not 45.  Part (but certainly not all) of this discrepancy can be attributed to the fact that the opinion of the geonim is adjusted based on (location) latitude and further adjusted based on season of the year.  Those two adjustments alone applied to 13.5 minutes do not yield a range of approximately 40 to 50 minutes in the New York area.  Other factors including:
                 1.      the extension of the end of Shabbat from medium to small stars,
2.      the impact of doubt as applied to a prohibition as serious as the observance of Shabbat,
3.      the impact of tosefet Shabbat,
4.      the length of the time to walk a mil,
5.      the definition of sunset and the beginning of bein hashemashot, 
may all play a role, as well.  The opinion of Rabbeinu Tam, however, is fixed at a constant 72 minutes, at any latitude and during all seasons of the year.

Halakhic principles, logical consistency and theory notwithstanding, the vast majority of current practice follows one of these two opinions.  Unquestionably, common practice adjusts the length of bein hashemashot based on latitude (and even on occasion season) when following the opinion of the geonim, while leaving the length of bein hashemashot constant when following the opinion of Rabbeinu Tam.  Moreover, this is not surprising.   R. Elijah of Vilna, the (Vilna) Gaon, and R. Shneur Zalman of Liadi, the first Lubavitcher Rebbe,[1] both explicitly require these adjustments in applying the approach of the geonim.  They were two of the most prominent authorities that battled to reinstate the practice of the geonim against what had become the practice of an overwhelming majority of European Jewry who had adopted the opinion of Rabbeinu Tam.  That majority included many if not most subsequent rishonim, both R. Yosef Caro and Rama, and most early commentators on the Shulchan Aruch.  For those who followed the opinion of Rabbeinu Tam, there is no conclusive evidence[2] in either halakhic literature or practice, that Rabbeinu Tam or anyone until after the time of the Shulchan Aruch ever adjusted the start or the end of bein hashemashot based on latitude or season.[3]  Like the time to walk 4 mil, it was assumed invariant.

Comporting with practice and consistent with the opinion of the major authorities whose opinion those practices appear to follow, would not normally warrant further analysis.  However, as I will quickly illustrate, almost every aspect of zemanim involves a challenging mosaic of major and minor halakhic issues that have to be resolved and then applied consistent with observable astronomical facts.
Consider two other contemporary opinions on this topic, Rabbi M. Feinstein,[4] and Rabbi M. Willig.[5]    R. Feinstein cites two alternatives to be followed in the New York metropolitan area: 72 minutes and approximately 50 minutes.  Clearly, the OU web site’s recommended practice is rather consistent.  R. Feinstein’s logic however, is radically different.  First, R. Feinstein argues that latitude is commonly accepted as a determinant in the length of bein hashemashot and is thus to be applied to the opinion of Rabbeinu Tam as well.  Second, R. Feinstein observes that while he waited 72 minutes in Lithuania, in accordance with Rabbeinu Tam, the sky is equivalently dark and starry in New York after only 50 minutes.  Therefore, R. Feinstein adjusts Rabbeinu Tam’s opinion based on latitude and concludes that there is an adequate halakhic basis for those living in the New York area and following Rabbeinu Tam to wait only 50 minutes.[6]  This opinion is similar to one given in applying Rabbeinu Tam’s opinion by Rabbi Pimential, in Minkhat Kohen.[7]  R.  Pimential suggested that the Jews living in the low altitude[8] of the Netherlands need only wait 48 minutes after sunset (in the spring) to end Shabbat according to Rabbeinu Tam.  In reality however, the length of time from sunset to darkness (however defined) increases as one moves further away from the equator traversing the latitudes of  the Middle East (~30 degrees), the Northeast United States (~40 degrees), the Netherlands (~50 degrees) and Lithuania (~55 degrees).   Assuming the application of adjustments based on latitude, the end of Shabbat is later after sunset in the New York area or the Netherlands than in Israel.  Problematically, R. Feinstein’s end to Shabbat is earlier in the in the New York area (similar to the Netherlands in the opinion of R. Pimential) than in either Lithuania or the Middle East.  Despite the widespread acceptance and application of this opinion of R. Feinstein in many popular and scholarly[9] contexts, analysis is often muted or absent entirely.
While the OU website aligns with R. Feinstein’s psak[10]  and common practice, it differs entirely as to rationale.[11]  When one waits approximately 50 minutes, the OU assumes one is following the geonim, while R. Feinstein assumes that one is still following Rabbeinu Tam.  Unlike R. Feinstein’s ruling, the OU web site does not adjust Rabbeinu Tam’s opinion for latitude or season.  Of course, one could argue that Rabbeinu Tam’s approach applies to France and not the Middle East.  This would imply that the sugyot that Rabbeinu Tam is analyzing defined zemanim for Europe and not the Middle East – hardly a plausible position.  Both historical practice and Rabbeinu Tam’s likely position made no explicit latitude adjustments; one can only conjecture that Rabbeinu Tam assumed his zemanim applied uniformly to both the Middle East and France.   In that regard, the OU website is consistent with both Rabbeinu Tam’s viewpoint and how it has been practiced.[12]

In his sefer Am Mordechai, R. Willig, like R. Feinstein, takes as a certainty that latitude must be a determinant in applying the approach of Rabbeinu Tam as well.  But R. Willig follows that to its logical conclusion:
v        The 72 minute practice in Lithuania was not, as R. Feinstein (and probably the vast majority of Orthodox Jewry) assumed, the correct practice of Rabbeinu Tam, but rather just the practice of the geonim applied at a location/latitude above 50 degrees from the equator.
In R. Willig’s approach, the logic of Rabbeinu Tam’s opinion, regardless of the practice actually followed by Rabbeinu Tam or those who adhered to his position, must be coupled with the laws of astronomy as currently understood.  Thus, the time to walk four mil that Rabbeinu Tam rules as the end of Shabbat, only applies around the spring and fall equinox and only at the latitude of Israel and Babylonia (approximately 30 degrees north latitude.)  Rabbeinu Tam’s locale in France, New York and Lithuania would require a significantly later end to Shabbat.  In fact, R. Willig provides both a slightly shorter (for those following the geonim) and significantly longer (for those following Rabbeinu Tam) end-time to Shabbat consistent with the opinion and (private) practice of his mentor Rabbi Joseph Soloveitchik.[13]  While their logic and application of basic astronomy to zemanim is impeccable, it hardly comports with practice.  Interestingly, both R. Soloveitchik’s personal stringency and R. Feinstein’s ruling for the entire community follow Rabbeinu Tam.  In addition, both agree that adjustments for latitude must be made.  However, their conclusions are radically different.  R. Feinstein ends up with a psak that comports with tradition, albeit, employing logic that is, at least to this author, puzzling.  R. Soloveitchik, on the other hand, followed (and refined) a family (Brisker) practice that is almost unprecedented within tradition, coupling a precise understanding of astronomy with a halakhic formulation rooted in Rabbeinu Tam’s approach.  As a result, R. Soloveitchik ended up waiting considerably longer for the end of Shabbat than either Rabbeinu Tam or the vast majority of his prior adherents.[14]