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Medieval astronomers drew a dragon across the sky and meant something exact by it. The Moon stands at ten degrees of Aquarius today, eighty two percent lit, climbing toward the point where her path crosses the Sun’s. She reaches it at 02:40 UTC Thursday morning, still in Aquarius. That crossing is the head of the dragon. The opposite one, half a circle away, is the tail.
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The geometry is simple and the consequence is not. Her orbit tilts about five degrees against the plane of the ecliptic, so she runs half of each month above it and half below, touching it twice. Those two touches are the nodes, and every eclipse in history depends on them. The lights can only be eclipsed when a new or full Moon falls near a crossing, which is why eclipses arrive in seasons instead of monthly. Al-Biruni, in eleventh century Persia, treats the nodes and the eclipse conditions as basic equipment rather than as a curiosity.
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This week puts the arithmetic where it can be checked. She crosses northward on Thursday. The full Moon follows on Saturday afternoon, some two and a half days later, far enough past the crossing that the Earth’s shadow does not reach her. The last time the alignment was tight enough was the partial lunar eclipse of the twenty seventh of August. That is the whole of the difference between an ordinary full Moon and an eclipsed one, and the nodes are where it is decided.
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