A Lunar Node

The South Node is the descending node of the Moon, where its orbit crosses the ecliptic moving southward, always exactly opposite the North Node in the sky. It too has been followed since antiquity for its link to eclipses, since an eclipse can occur only when a node, rather than falling anywhere along the ecliptic, lies close to the Sun or Moon at the time of alignment. Because the nodes regress around the zodiac rather than holding still, ancient and medieval astronomers who wished to predict eclipses had to track this backward drift across the full circuit of the zodiac, a cycle of roughly eighteen and a half years, well before the underlying orbital geometry was described in modern astronomical language.

In the medieval dragon framework shared across Arabic, Latin and Sanskrit sources, the South Node was consistently treated as the tail of the eclipse-causing dragon, paired with but distinct from the head at the opposite point. That eighteen-and-a-half-year regression is the same cycle underlying the Saros, the roughly eighteen-year eclipse-recurrence pattern of 223 lunar months first identified by Babylonian observers working from long series of clay-tablet eclipse records centuries before Greek astronomers gave the geometry of the nodes a formal description.