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.

[schulman.livejournal.com]

On the subject of Jews In Space, you have read William Tenn's "On Venus, Have We Got a Rabbi", haven't you? It's in the Wandering Stars anthology, and if you ever have the chance to hear him read it aloud, do not pass it up.

[aliza250.livejournal.com]

I was just in Australia. South of the equator, the seasons are flipped, and the first Jews in Oz had a discussion about how that would affect things - not so much the idea of, for example, celebrating Pesach in the fall, but little things like when to pray for dew vs. rain (tal vs. geshem.)

The answer was, there as everywhere else, was to follow Eretz Yisrael. Thus, I'd assume that Martian Jews would follow the holiday dates of Earth, and let the seasons float around the calendar.

There are bigger issues with *day* length - for example, how do you count the Omer if you miss one day out of every 25? (Mars days are slightly longer than Earth days, but not so much longer that people don't think our bodies could adapt to local daylight hours.)

[ichur72.livejournal.com]

Wish I could remember the exact source for this, but I remember being told that candle-lighting is 18 minutes before Shabbos b/c 18 is the gematria for ches-yud -- i.e., "chai" -- and these 18 minutes can mean the difference between life and death since violation of Shabbos is punishable (under certain circumstances) by death.

Meanwhile, here's what torah.org (halachah-yomi list) has to say about it:

"It is obviously impossible for us to determine the exact moment that Friday ends and Shabbos begins. According to most authorities, once the sun sets ('Shkiah'), we enter a period of time in which we are uncertain as to whether or not it is 'day' or 'night.' This period is called 'Bein Hashemashos,' and it is forbidden during this time of uncertainty to perform Melacha (Torah-prohibited labor) as well as most Rabbinically prohibited acts (certain Rabbinically prohibited acts may be performed in pressing circumstances or for the sake of a Mitzvah). However, some authorities rule that we don't enter the period of uncertainty until sometime after the sun has set, and its clear that, in this Halacha, the Kitzur Shulchan Aruch concurs with these authorities, and therefore sets candle-lighting time at a half an hour before nightfall, rather than a similar period before sunset. According to all authorities, the appearance of three medium-sized stars signals the beginning of the 'night.'

"According to the authorities who rule that the period of uncertainty begins at sunset, it should be permissible to light Shabbos candles until a moment before sunset. However, the custom in most communities is to light-candles and begin Shabbos about 18 minutes before sunset; there are two possible reasons for this practice. Firstly, most authorities rule that it is a Mitzvah (derived from the Torah) to begin Shabbos while it is still definitely 'daytime,' i.e. before sunset. This concept is called 'Mosif Mechol Al HaKodesh' (lit: 'adding part of the non-holy to the holy,' in other words, adding part of the weekday to Shabbos ). Secondly, even those authorities who rule that it is not an actual Mitzvah to begin Shabbos early, still rule that everyone should refrain from performing prohibited acts a short time before sunset, because if one got into the habit of waiting until a moment before sunset to begin Shabbos, one could easily end up performing a prohibited act after sunset (See Shulchan Aruch 261 and commentaries)."