Siman - Eruvin Daf 58
- The source that the measuring rope is fifty amos.
The Mishnah at the bottom of Daf 57b states, אין מודדין אלא בחבל של נ' אמה לא פחות ולא יותר – We may not measure with anything but a fifty-amah rope, neither shorter or longer.
The Gemara seeks the source for this ruling, and Rav Yehudah in the name of Rav brings the passuk regarding the Mishkan, ארך החצר מאה באמה ורחב חמשים בחמשים – The length of the courtyard is one hundred amos, and its width is fifty by fifty. What does the passuk mean when it says, “fifty by fifty”? אמרה תורה בחבל של חמשים אמה מדוד- The Torah is instructing to measure with a fifty-amah rope. The Gemara objects since that passuk was brought on 23b to teach that one takes fifty amos and surrounds fifty amos to make a perfect square, which was the source that a karpaf must be square. The Gemara answers that the passuk could have just said, חמשים חמשים. From the fact it said, חמשים בחמשים we learn both these laws.
In terms of why the Mishnah said that the rope must be fifty amos, לא פחות ולא יותר – neither more or less, a Baraisa explains, לא פחות מפני שמרבה – no less than fifty amos because a short rope increases the measure since it stretches when pulled taut, ולא יותר מפני שממעט – and no more than fifty amos because a long rope decreases the measure since it sags in the middle .
- Measuring inclines
The Mishnah stated that when the one measuring the techum reaches a valley or mountain, he should measure by מבליע ואם לאו מקדרין – spanning or piercing, which enables him to measure in a straight line and not include the length of the incline in the measurement. Rav Yehudah said in the name of Rav that this only applies, שאין חוט המשקולת יורד כנגדו – when a plumb line would descend downward four amos from the top edge, אבל חוט המשקולת יורד כנגדו מודדו מדידה יפה – but if the plumb line would descend straight down, because the incline is so steep, he measures the valley properly along the floor and can ignore the distance of the incline. This is a leniency because it will lengthen the overall distance.
- Using an expert to measure the techum
The next Mishnah states, אין מודדין אלא מן המומחה – we do not measure the techum with anyone but an expert surveyor. ריבה למקום אחד ומיעט למקום אחר – If it was found that the expert extended the techum at one point and shortened it at another along the same side of the city, שומעין למקום שריבה - we accept the measurement in the place that he extended it. Rashi explains that we accept the larger measurement as accurate and explain the discrepancy by saying that the short measurement resulted from not pulling the rope taut enough.
