calendars

[cellio]

Idle question the first: what is the origin of Shabbat (or Yom Tov) beginning 18 minutes before sunset? That is, the idea of starting early is solidly talmudic, but what is the significance of 18 minutes in particular? It is worth noting, as well, that many time differentials are in proportional hours (a day always has 12 hours; sometimes they're longer and sometimes they're shorter), but "18 minutes" is a constant. (Online search in the Soncino talmud isn't doing it for me, though I may be doing something wrong in specifying the search, given the lack of documentation.)

Idle question the second: the issues of Shabbat (etc) times for astronauts (e.g. on the shuttle or space station) are well-understood, but what will we do when we colonize other planets? If you don't go with the local times, things get rapidly wonky, with Shabbat perhaps starting mid-day one week and at dawn the next and so on. If you do go with local times, then you quickly have calendar drift with respect to Earth -- and you may need to change the number or length of months, if you want a physical year to equal a liturgical year. If you don't do that, your holidays move around all over the place like on the Muslim calendar.

Comments

[estherchaya.livejournal.com]

As far as I know, 18 minutes before sunset is dusk. We light at dusk, which is what makes shabbos 25 hours... it's dusk to twilight.

But I could be wrong.

[cellio]

The length of dusk varies somewhat by latitude. So perhaps 18 minutes was the length of dusk in a reference location? (Obvious candidates are Jerusalem and "where the guy who recorded this lived".)

[mishtaneh.livejournal.com]

This (http://www.hebrewcalendar.net/htdocs/zmanimhelp.en.html) references the Vilna Gaon about the "18 minutes", but does so unclearly so it may not actually relate to candle lighting time:

Regarding this the Vilna Gaon had an opinion that 18 minutes, and 42 should be used, normalized according to equinox at the latitude of Jerusalem.

It also fails to say why 18 minutes, or to give a more exact reference. (I seem to see a lot of that... sigh.)

[cellio]

Thanks. It sounds like they're saying that 18 minutes corresponds to a certain angle (would have to be above the horizon in that case) at a certain time in a certain place, so I could see that. I'm too lazy to do the astronomical math to determine whether that's actually correct. :-)

[mishtaneh.livejournal.com]

I understood it as "candle lighting 18 minutes before, tzeit 42 minutes after". It turns out that KaLuach's (it's available both from www.kaluach.com and rebadged from various other places including Aish.com) help includes a fairly complete discussion of various halachic issues of zmanim, although for this one it simply cites the "18 minutes" without an explanation. :/

I might try digging it out of the online Soncino Talmud translation if I have a few hours to sit digging at it....