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A mishna on Eiruvin 41b tells of a ship that was coming into harbor at dusk coming into Shabbat. Since Shabbat had already begun when the ship docked, those on board asked R. Gamaliel if they could disembark. He said yes, because he had observed that the ship was close enough before Shabbat started.

Obviously R. Gamaliel wasn't in a position to actually measure the ship's distance from the dock; he estimated. This leads me to wonder on what basis R. Gamaliel could make that ruling:

  • Perhaps the ship was obviously close enough. In that case I would have expected the answer he gave to say something about "yes you can move from a vessel to land", rather than bringing up the distance.

  • Perhaps R. Gamaliel, being a sage, had an ability to make these kinds of judgments that we can't assume anybody else has -- he could make this determination, but a regular person could not.

  • Perhaps R. Gamaliel had some relevant background that increased his ability to measure distances, sort of like how archers become good at estimating ranges. Under this reasoning someone with relevant skills could make this judgement but a regular person couldn't.

I'm curious about both R. Gamaliel's ruling and whether there is application beyond him (can people in general rely on estimation?).

Here is the passage:

פעם אחת לא נכנסו לנמל עד שחשיכה אמרו לו לרבן גמליאל מה אנו לירד אמר להם מותרים אתם שכבר הייתי מסתכל והיינו בתוך התחום עד שלא חשיכה

And from the Soncino English translation:

Once [on a Sabbath eve] they did not enter the harbour until dusk. "May we disembark?" they asked R. gamaliel. "You may", he told them, "for I have carefully observed [the distance from the shore and have ascertained] that before dusk we were already within the Sabbath limit".

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  • Distances or any other dimensions of shiurim-measurements? – Double AA♦ Apr 18 '13 at 16:08
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    Ha, I thought we had a relevant tag, but I was looking for "distance" and didn't think about "measurement". I'm asking about distances, but if the answer turns out to be that all measurements are handled the same, that would be good to bring up. – Monica Cellio Apr 18 '13 at 16:13
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    re 1: perhaps it was obviously within the distance (say, 1km) but the others on the ship didn't know if the requisite distance was 1 km, 100 m, 4 amot, or touching? – Double AA♦ Apr 18 '13 at 16:57
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    R' Rakeffet likes to remind his classes that using precise measuring devices for Halachic measurements is a relatively recent phenomenon. When he was a kid, no one measured out how much matza to eat at the seder using charts - they just ate the amount (by visual estimation) they had seen their forebears eating. In other words (Though that opens a different can of worms.) – Isaac Moses♦ Apr 18 '13 at 17:14
  • @IsaacMoses Pesachim 109a? אמר רבי יצחק קסתא דמוריסא דהוה בציפורי היא הות כמין לוגא דמקדשא ובה משערין רביעית של פסח אמר רבי יוחנן תמנייתא קדמייתא דהוה בטבריא הות יתירה על דא ריבעא ובה משערין רביעית של פסח – Double AA♦ Apr 18 '13 at 17:30
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The Gemara in Eruvin 43b discusses how R. Gamliel was able to measure the distance. The Gemara says that R. Gamliel had a hollow tube with which he was able to look and sight a distance of 2000 Amos. Per Rashi a long hollow tube narrows and shortens the viewer's range. The length of the tube can be calibrated to yield a maximum sighting distance of 2000 Amos. (thanks to Artscroll for the translation).

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  • Ah, the challenges of asking questions arising from daf yomi -- I should have just waited a couple days. :-) (Though the Soncino translation I'm working from doesn't include Rashi's range explanation, which is helpful. It talks about using shadows and geometry, but I'm not sure that works in this case.) – Monica Cellio Apr 18 '13 at 17:30
  • How does a tube shorten one's range of site? – Double AA♦ Apr 18 '13 at 17:31
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    Perhaps it was an Astrolabe – Double AA♦ Apr 18 '13 at 17:44
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    Discussion of the mechanism here and in more apparent depth, but behind a paywall here. – Isaac Moses♦ Apr 18 '13 at 18:25
  • @IsaacMoses The latter article focuses more on what trigonometry was used than how the scope worked. – Double AA♦ Apr 19 '13 at 14:33
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A normal person can see about three miles. This is about three times the length of the tchum. So his tube was meant to shorten his length of sight. The gemoro in Bchoros where it talks about a shepherd guarding his flock says he can see sixteen mil, a lot more than three miles. How does one reconcile this with the facts. One could say the gemoro means that it is the maximum a shepherd can see even if standing at a great height. But that would mean that every field contains such a 'height'. The MB says that when making an eruv and standing in the middle, if imagining there are no houses or other obstacles in the way one can see the walls. Which would mean an eruv cannot be larger than 6 miles in lenght or breadth.

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  • It sounds like you're saying the answer to my question is option 3, except instead of using a particular skill he used a particular tool, right? (So, in principle, anybody with a properly-calibrated tube would be qualified to judge whether the ship was close enough.) – Monica Cellio Apr 19 '13 at 14:08
  • The artscroll gives both the 'options' in option 3. – meir Apr 22 '13 at 18:27

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