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From: R. Halevi
Subject: Phase boundaries
Date: 16 Cheshvan 5783
Devorah's question touches on something the specification assumes but does not state.
The Shulchan Aruch, Orach Chaim 261, discusses the transition from day to twilight. It gives indicators: sun on treetops, roosters and ravens on beams. But it also notes that the duration of twilight is disputed — the Geonim say one thing, Rabbeinu Tam another.
The specification adopts the phase categories but not the boundaries. We say "twilight exists" without saying when it begins or ends. This is deliberate. But I wonder whether it is sustainable.
—Dov Halevi
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