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.

Small, rounded mass of lymphoid tissue contained in connective tissue. They occur all along lymphatic vessels, with clusters in certain areas (e.g., neck, groin, armpits). They filter bacteria and other foreign materials out of lymph and expose them to lymphocytes and macrophages that can engulf them; these cells multiply in response to accumulation of such materials, which is why lymph nodes swell during infections. The nodes also produce lymphocytes and antibodies, to be carried by lymph throughout the lymphatic system. In Hodgkin disease and other lymphomas, malignant lymph cells proliferate, causing lymph node enlargement. Other cancers often invade lymphatic vessels, which can carry cells from the tumour to lymph nodes, where they are trapped and grow into secondary tumours. Lymph nodes are therefore removed in cancer surgery to detect or prevent tumour spread.

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  • Nodal, a protein involved in TGF beta signaling.

See also

  • In other languages, the words for some meanings of "node" and "knot" are the same.

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