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We are continuing in siman 1. We left off with the Chayei Adam writing that one should minimally accept Shabbos 30 minutes before nightfall, because the combination of bein hashmashos, tosefes Shabbos, and the impercision time equal to about 30 minutes. In the Piskei Mishnah Berurah, in number 3, they bring that the Mishnah Berurah writes similarly that it is a good idea to stop performing melacha 30 minutes, or at least 20 minutes, before sunset.
We need to clarify that when Chazal discuss time, they would measure large amounts of time based on the measurements of the sun. For example, a third of the day (shlish hayom) or two hours in the day (shtai shaos bayom). They could do this by looking at the movement of the sun. However, when they needed to measure small amounts of time, they would measure it differently. The Gemara says that the average person can walk 40 mil in a day. If they are only walking 1 mil, they may be able to walk more quickly, but when walking 40 mil, they would walk at a more consistent pace. If so, we can use the amount of time it takes to walk one mil (i.e., dividing by 40) as a measure of time. The Gemara says that the time period of bein hashmashos is equivalent to the amount of time it takes to walk three quarters of one mil.
It is a machlokes in the Gemara how long it takes to walk one mil. The most famous opinion is that it takes 18 minutes. The other two opinions are 22.5 minutes and 24 minutes. If we understand bein hashmashos to be 24 minutes, three quarters of a mil will be 18 minutes. This is the number that the Chayei Adam is using in his calculations, which is why he suggests accepting Shabbos 30 minutes before nightfall. However, if it takes only 18 minutes to walk a mil, three quarters of a mil will be 13.5 minutes. If we add a few minutes for variables, and a few minutes for tosefes Shabbos, we would reach approximately 20 minutes, which is the minimal suggestion of the Mishnah Berurah.
Thus, a person should try to add at least 20 minutes, in accordance with the opinion that bein hashmashos is 13.5 minutes, but preferrably should add 30 minutes, in accordance with the opinion of the Chayei Adam and the larger shiur of bein hashmashos.
Summary
- Minimally, the Chayei Adam holds one should accept Shabbos 20 or 30 minutes before nightfall. This includes enough time for bein hashmashos, two minutes to avoid a safeik bein hashmashos, and a few minutes for tosefes Shabbos.
