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The common מנהג in America is to light Shabbos candles 18 minutes before שקיעה. The source for this מנהג, however, is not entirely clear.
Rabbi Yonansan Shteiff suggests the following explanation for this מנהג. There is a disagreement between ראשונים as to when exactly בין השמשות begins and מלאכת שבת becomes forbidden. The most well-known opinions in this argument are those of the Vilna Gaon and Rabeinu Tam. The Vilna Gaon holds that בין השמשות begins immediately after sunset, while Rabeinu Tam maintains that it begins 3¼ mil (approximately 59 minutes) after sunset.
The Yerayim, however, has a third, lesser known, opinion on this subject. He maintains that בין השמשות begins ¾ mil before שקיעה. The number of minutes before שקיעה that marks the beginning of בין השמשות (according to the Yerayim) therefore depends on the exact length of a mil. Although the Shulchan Aruch rules that a mil is the equivalent of 18 minutes, the Rambam writes that a mil is actually 24 minutes long. That would mean that if one wishes to hold both according to the Rambam (mil =24 minutes), and the Yerayim (בין השמשות begins ¾ mil before שקיעה), then מלאכת שבת would actually be forbidden starting from 18 minutes (i.e., ¾ of a 24-minute mil) before שקיעה. Rabbi Shteiff explains that this is in fact the source for the מנהג to light candles – and thus accept Shabbos – 18 minutes before שקיעה. We want to be careful not to perform מלאכת שבת during a time that may be considered the onset of בין השמשות – albeit only according to the Yerayim.
Rabbi Moshe Feinstein cites the above explanation, but rejects it as improbable. He writes if that if the reason we light candles 18 minutes before שקיעה is in deference to the opinion of the Yerayim, who holds it is forbidden to perform מלאכה from that point on, then why do we find that even G-d fearing men seem to have no problem performing מלאכה after their wives have already lit candles? It seems clear that our מנהג is to consider the prohibition of מלאכת שבת as beginning with the actual שקיעה itself (in accordance with the opinion of the Vilna Gaon). Rav Moshe therefore explains that the reason we light Shabbos candles 18 minutes before שקיעה is in order to fulfill a different aspect of הלכות שבת.
There is a mitzvah of ,תוספת שבת which means to accept Shabbos while it is still daytime on Friday. By doing so, one is considered to be adding מחול על הקודש, from the mundane onto the holy. As a result, the Shulchan Aruch writes that one should light the Shabbos candles some time before שקיעה. The Shulchan Aruch does not specify, however, exactly how much time before שקיעה the lighting should take place. Rav Moshe explains that the reason that we light candles 18 minutes before שקיעה is in order to fulfill this מצוה. According to Rav Moshe, the fact that we light Shabbos candles exactly 18 minutes before שקיעה is not the result of any precise calculation, but rather it is the accepted amount of time that we are נוהג to add “from the mundane onto the holy”. Indeed, Rav Moshe himself writes that many people had the מנהג to fulfill this concept by lighting candles only 10 minutes, rather than 18 minutes, before שקיעה.
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© Yosef Brecher