The lunar nodes are the points where the orbit of the Moon crosses the ecliptic (which is the apparent path of the Sun across the heavens against the background stars). The North (or Ascending) Node is where the Moon crosses to the North of the ecliptic; the South (or Descending) Node where it crosses to the South.
Solar eclipses occur when the passage of the Moon through a node coincides with the New Moon; lunar eclipses occur when it coincides with the Full Moon. The lunar nodes precess rather quickly around the ecliptic, completing a revolution (called a draconitic or nodical period) in 6793.5 d or 18.5996 a.
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