node, in astronomy, point at which the orbit of a body crosses a reference plane. One reference plane that is often used is the plane of the earth's orbit around the sun (ecliptic). Since the moon's orbit has an inclination of 5°9′ to the plane of the ecliptic, there are two nodes in the moon's orbit around the earth; the point where the moon in its orbit crosses from south of the ecliptic plane to north of it is called the ascending node, and the point where it crosses from north to south is called the descending node. A line connecting two nodes is called a line of nodes. The lunar nodes are the points where the moon's line of nodes, when extended, strike the celestial sphere. The lunar nodes regress (move westward along the ecliptic) due to perturbations from the other bodies in the solar system, e.g., the sun and planets. Another reference plane that can be used to define nodes is the plane of the earth's equator, which is also the plane of the celestial equator (see equatorial coordinate system). There are two nodes in the sun's apparent orbit around the earth. The ascending node, when the sun appears to cross the celestial equator from south to north, is the vernal equinox; the descending node is the autumnal equinox. Perturbations like those that cause regression of the lunar nodes cause the precession of the equinoxes.

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