Calculating the End Time of Shabbos

 

Question:

Can the time of צאת שבת be established by adding a proportional amount of time of the שעה זמנית of the day to the זמן הדלקת נרות of that Shabbos?

I am trying to understand the following [all times presented below are from the Tzivos Hashem calendar for Crown Heights]:

During most of the year the difference between הדלקת נרות and צאת שבת times is around an hour (and sometimes less). However, there are two annual periods when the distance gets longer. The first is when the days are the shortest in the winter, when Shabbos starts at 4:11 and ends at 5:14. The second period is when the days are the longest, when Shabbos starts at 8:13 and ends at 9:21.

These two periods also include the few weeks before and after the solstices when the difference gradually changes from an hour to those longer discrepancies (63 minutes in the winter, and 68 minutes in the summer) and then back to an hour again.

The above cannot be caused by longer שעות זמניות in the summer, because then the difference should be 52 minutes in the winter due to shorter שעות זמניות. Also, if שעות זמניות was the reason, the entire year’s זמני שבת would reflect it in a constant gradual increase and decrease as Shabbos gets later and earlier during the two halves of the year. But a careful look at the trend line shows that not to be the case.

In light of the above, what is the actual reason for the extra minutes between the start and end of Shabbos during the weeks near the two solstices?

 

Answer:

No.

In essence, you’re asking why the timeframe between שקיעה and צאת is not associated with the שעה זמנית of a given date (something which is not exclusively connected to Shabbos or candle lighting times etc.).

The reason for this is because צאת is calculated when the sun is at a certain position in the sky (e.g., צאת שבת is when it is 8.5° below the horizon [see also here]). Therefore, the amount of time between שקיעה and צאת is not connected to the specific שעה זמנית of the day, but rather increases from equinox to solstice and decreases from solstice to equinox.

 

Sources:

וידוע שיש שפירשו ד׳ מילין בשעות זמניות שסברו שתלוי בשעות זמניות. והמנחת כהן בתוכם. וכ״מ דעת אדה”ז מעיקרא במכתב בגליון השו”ע סי׳ רסא. וכבר הקשו שהוא מנוגד למציאות. ואכ”מ.

 

 

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