noun
descending node
Astronomy term: the point where an orbit crosses a reference plane from north to south.
月の降交点は約18.6年周期で移動する。
The Moon's descending node moves with a period of about 18.6 years.
昇交点 is the ascending node, where the orbit crosses from south to north, the opposite of 降交点.
Compound of 降 (descending) + 交 (intersection) + 点 (point), a straightforward technical term.