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Re: shortest twilight problem...
From: George Huxtable
Date: 2010 Jun 29, 13:27 +0100
From: George Huxtable
Date: 2010 Jun 29, 13:27 +0100
Marcel wrote, about lengths of twilight- | Since the problem has an Arab origin: Could it be that it was | originally related to the prayer times? The duration of this twilight | corresponds to the duration between evening and night prayer. One | being done right after sunset and the other, depending on the | understanding, when the sun is 17 to 19 deg below the horizon. I think he could be on to something. That seems plausible. If there was a prayer-time at sunset and another, when the sky became completely black, that might explain why it was useful to be able to predict the interval between them, to time with (say) a water-clock, if the sky wasn't clear. And similarly in the morning. But for such purposes, it's the actual length of twilight that needs predicting. Joel's posting relates, not to that length, but to the date of its minimum, which appears to me to be a much less interesting (indeed, entirely trivial) matter. Other possibilities come to mind. Might it, I wonder, be related to the definition of the lunar month, in Jewish / Arabic calendar, in which the date of first viewing of a crescent Moon, shortly after sunset, becomes important? But that observation doesn't call for a very dark sky. Or could it go back to earlier calendars still, from Babylonian times, in which the "heliacal rising" of a star (often Sothis = Sirius) became important in establishing the season of the year? Heliacal rising meant that the star could be observed, for the first time that year, just before the brightening dawn sky blotted it out. But again, I wouldn't expect that to call for astronomical twilight, with the Sun at -18�. George. contact George Huxtable, at george@hux.me.uk or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.