A Lunar Node

The North Node is the ascending node of the Moon, the point where the Moon's orbit crosses the ecliptic as it moves northward. Because eclipses occur only when the Sun, Moon and a node align closely, these points have been tracked since antiquity by observers working out eclipse cycles centuries before the mechanics of the Moon's orbit were understood in modern terms. The node itself is not fixed but regresses steadily backward around the zodiac, completing a full circuit in about eighteen and a half years, a period recognised in principle by ancient and medieval eclipse-prediction schemes even without a modern description of orbital mechanics.

Medieval Arabic astronomers folded the two nodes into the figure of a dragon, al-Tinnin, imagined as swallowing the Sun or Moon during an eclipse, with the dragon's head marking the ascending node and its tail the descending one, a scheme that passed into Latin and Sanskrit astrological traditions alike. The underlying eclipse cycle itself was known far earlier still: Babylonian astronomers had identified the roughly eighteen-year Saros cycle of 223 lunar months, the pattern by which eclipses recur when Sun, Moon and node realign, by around the seventh or eighth century BCE, working from centuries of clay-tablet observational records kept for the purpose of forecasting eclipses well before Greek astronomy adopted and geometrised the same cycle.