daf bit: Eruvin 41

[cellio]

There is a limit to how far one may go "from one's place" on Shabbat. A mishna on today's daf teaches: one who was brought, by non-Jews or by an evil spirit, beyond this limit has no more than four cubits in which to move. If he was brought back, however, he is regarded as if he had never left and has the usual range of movement. If he was taken to another town or put in a cattle-pen, there is a disagreement about whether he can use the whole area (which has a boundary) or is limited to four cubits.

Once, this mishna says, people were on a ship on the eve of Shabbat and the ship did not enter the harbor until dusk. They asked R. Gamaliel if they could disembark. He said yes, because he had carefully observed where the ship was before Shabbat started and determined that they were within the limit. (41b)

I don't know if this means that estimation by sighting, without measuring, is always ok, or if R. Gamaliel had special status as a sage.

Comments

[loosecanon.livejournal.com]

those who have practice and experience at something often develop what looks like a mystical skill at it.
I know many people who can sight a perfect half-mile, many of them are farmers and contractors who are often outside in large open areas.
It would not surprise me at all if this were meant to be "trust the judgement of someone you know has good judgement."
Though I was told one could *not* disembark except to a dock, that disembarking to a rowboat was switching conveyances and forbidden, unless the original conveyance was starting a voyage away during the time of no-travel.

[http://users.livejournal.com/merle_/]

So if a non-Jew drugged you and dropped you off in the middle of a sufficiently large desert or other inhospitable place, you would be unable to leave? That sounds like a horrible loophole. Especially for the area of the world where the religion began (yeah, heresy, but...).