Amos 7:7 Halajá: Gray Matter I

However, Rav Yitzchak Liebes, Rav J. David Bleich, Rav Hershel Schachter, Rav Feivel Cohen,³Rav Cohen believes that two sets of vertical poles must be constructed, one set that appears to be under the horizontal pole and one set that has been measured to be precisely under the horizontal pole (if the position determined by sight differs from the position determined by the plumb line). It is not clear, however, that Rav Moshe and Rav Soloveitchik invalidate an eruv that was measured by plumb line. It may be that they also recognize such an eruv but add that measuring by eyesight is also acceptable. Rav Shlomo Miller (in his letter of approbation for The Contemporary Eruv) presents an argument for why constructing an eruv with plumb line measurements suffices according to all authorities. and Rav Mordechai Willig (all through personal communication) rule that a plumb line (or other device for measuring verticality) is necessary to ensure that everything lines up appropriately. The Gemara (Eruvin 94b) requires constructing "halachic walls" (and presumably tzurot hapetach as well) in the same manner that people usually build walls (kede'avdei inshei).⁴The significance of kede'avdei inshei is particularly emphasized by Rav Shlomo Kluger (Teshuvot Ha'elef Lecha Shlomo 156, 157, 161, 170, 173, 174). Builders and carpenters have used plumb lines for thousands of years; they appear in Amos (7:7-8) and the Mishnah (for example, Kil'ayim 6:9 and Keilim 29:3). Accordingly, a plumb line must be used in constructing a tzurat hapetach. Rav Shlomo Zalman Auerbach told this author that, while it is best to use a plumb line, one may rely on eyesight alone if it is "impossible" to construct the eruv otherwise.⁵Of course, the definition of "impossible" is debatable. Rav Hershel Schachter (in response to what this author quoted from Rav Shlomo Zalman) insisted that it is never impossible to measure precisely, especially with the invention of devices such as laser pointers. Similar to Rav Shlomo Zalman's ruling, the Rama (O.C. 456:3) permits estimating the measurement for separating challah when measuring precisely is not feasible (although he addresses a halachic impediment, rather than practical difficulty).