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invariable planeThe unvarying reference plane through the center of mass of the Solar System and perpendicular to the direction of the angular momentum vector of the Solar System. At present the ecliptic is inclined by 1°.58 to the invariable plane.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006
invariable plane[in′ver·ē·ə·bəl ′plān]
A plane which is perpendicular to the angular momentum vector of a rotating rigid body not subject to external torque, and which is always tangent to its inertia ellipsoid.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.