The Moon’s orbit is tilted about five degrees to the Sun’s path, so twice a month she crosses it, once heading north and once heading south. The two crossing points are the nodes: places on the zodiac rather than bodies, with nothing at all to be seen there, and they drift backward through the signs about a degree every nineteen days, taking eighteen years and seven months to go all the way around. The North Node is where the Moon crosses heading north; the South Node is exactly opposite.
The nodes matter to astronomy because eclipses happen at them, and to astrology because of what the tradition has read into that. The old astronomers called them the Dragon’s Head and the Dragon’s Tail, the mouth that swallows the light and the tail that lets it go, and the names survive in the glyphs. In a birth chart the nodes are read as an axis of direction: the South Node as what comes easily and can be over-relied on, the North Node as what is unfamiliar and asks to be grown into. In the sky, where Moondark shows them, the axis names what the whole eighteen-month season is leaning toward and away from.
Moondark shows the mean node, the smooth average position. There is also a true node, which wobbles a degree or so on either side of the mean as the Sun tugs at the Moon’s orbit, and the two can differ in sign for a few days around an ingress. Most astrological software offers both; Moondark chose the mean because it moves steadily backward and because the difference matters only to specialists. The Nodes page says which is shown.
The nodes change signs about every eighteen months, and the change is an event: the whole season’s question turns. Moondark names the day, gives the axis reading for the new orientation, and shows when the Moon herself next crosses a node, which she does twice a month. The Nodes page lives under the Moon tab, because the nodes are hers.