The Mishnayot (5:4-5) set down some rules for measuring the two thousand amot of the techum (the boundary beyond which a person may not travel on Shabbat - approximately 1.1-1.2 kilometres). Rambam explains in his commentary to the Mishnah that every measurement has some degree of inaccuracy. The Chachamim enacted a number of detailed rules to ensure that the techum is measured in such a way so as to minimise this inherent inaccuracy.
There is an interesting tension between the Chachamim’s desire to make the measurement as accurate as possible and the various constraints and practicalities within which the Chachamim operate and which reduce the accuracy of the measurement. For example, the Shulchan Aruch (399:1) rules that the techum should only be measured with a rope made of linen. The Chachamim have a tradition to this effect which is supported by a reference in Yechezkel (40:3) to linen rope being used for measuring. The Gemara (Eiruvin 58a) quotes a Beraitah explaining that iron chains are actually the most suitable for measuring because they do not stretch. However, as the Beraitah explains, the Chachamim are constrained by the Torah’s preference for rope. Some commentators (including the Meiri) hold that metal chains can actually be used, although as stated above, this is not the view of the Shulchan Aruch.
Being restricted to linen rope, the Chachamim have sought to minimise the inaccuracy of the measurement by various rulings. For example, the size of the rope used to measure the Techum is restricted to 50 amot. Any longer than this and the rope will sag; any shorter and it will stretch. The Shulchan Aruch (399:3) rules that the surveyors must pull the rope with all their might when performing the measurement. The Mishnah Berurah explains that otherwise the rope will be weighed down in the middle and the measurement will therefore be inaccurate. Furthermore, the Mishnah Berurah (399:7) rules that the surveyors must be experts who are familiar with the laws concerning the measuring of the techum.
The Chachamim have also allowed a number of leniencies on the basis of practicalities. For example, the Ritva (5:4) notes that the most exact way to measure the techum is to lie the rope down on the ground. However this is physically difficult for the surveyors to do. Therefore the Chachamim allowed the surveyors to hold the rope at chest level, even though the accuracy of the measurement will thereby suffer. Designating a standard body part ensures that the rope is kept horizontal to the ground ensuring a more consistent measurement. As a practical measure, the Chachamim did not take into account differences in height of the surveyors and assumed that the surveyors will be of average height.
Further examples of leniencies allowed by the Chachamim are allowing surveyors to effectively ignore mountains (5:4) and believing a single witness, even if that witness is a slave, in matters relating to the location of the techum (5:5).
Previously (4:11) we saw that that by following the method prescribed by the Chachamim, the surveyors actually end up fifteen amot short of two-thousand (due to the length of rope that is held in their hands while measuring - see Rashi Eiruvin 52b). This is a built-in buffer to allow people who accidentally leave the Techum to return.
As we saw in the previous article, the laws relating to the two thousand amot techum are d’rabbanan. The Chachamim did not make them subject to all of the stringencies that apply to d’oraita laws and they enacted the laws with a provision that they be treated leniently (Rashi Eiruvin 58b). The Chachamim are still concerned that the two thousand amot be measured as accurately as possible but as we saw above they included certain leniencies where they deemed them to be appropriate.
Of course, in some cases it is not possible to accurately measure the two thousand techum border. For example, the Mishnah (4:4) discussed the case of a person who was in the middle of a journey when Shabbat was about to begin. This person’s techum is generally measured from their place at the onset of Shabbat. Obviously in these circumstances this person’s techum could not be measured accurately in advance before Shabbat. The Chachamim allowed such a person to estimate their techum on Shabbat on the basis that one medium stride is approximately equal to one amah. The Biur Halacha (399:1) explains that one must not measure out the full two thousand footsteps to allow for the fact that each footstep may actually be longer than one amah. This is another example of the tension between practicalities and measuring the techum accurately.
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