Interpretation. Applied astrologically to an individual judgment as to the significance of a
configuration of birth planets, or of transiting or progressed aspects to a birth configuration.
Invariable Plane. The Solar System possesses two fixed planes: that of the Solar Equator;
and the Invariable plane - a central plane of the Solar System discovered by Laplace, which,
passing through its center of gravity at a mean inclination of about 1°35' to the Ecliptic, is
independent of the mutual perturbations of the planets. The inclination of the orbits of Venus
and Mercury to the Solar Equator, slightly more than 3 degrees, is less than that of the other
planets. Since these are the two closest planets to the Sun, the Solar Equator can be
considered as their reference plane. The Earth's inclination to this plane, of slightly more than
7", is exceeded only by that of Pluto. The Ecliptic, the Earth's plane of revolution around the
Sun, intersects the Solar Equator in the middle of Gemini and Sagittarius, at which points the
Earth has no heliographic Latitude North or South. However, the values of these inclinations
and the longitudes of the planetary Nodes along this plane are all variable due to motions of
the orbits themselves with respect to the other great reference plane of the Solar System: the
Invariable Plane - so-called because its position remains unaltered by any forces within the
system. In this plane the combined angular momenta of all the planets is a maximum. There
are two classes of disturbances in the normal undisturbed elliptical motion of the planets in
their orbits in space: periodic perturbations and secular perturbations. Periodic perturbations
are deviations due to the gravitational pulls of the planets on each other. However, after the
planets have revolved a considerable number of times, and thus have been in all possible
relations to each other, these periodic perturbations cancel each other. A famous example is
the long period inequality in the motions of Jupiter and Saturn, which shifts their positions by
a degree or so - one forward and the other backward - in a period of about 918 years. Actually
their recurrence cycle in the fixed Zodiac - 46 conjunctions distributed nearly evenly in all 12
Signs of that Zodiac - is about 913½ years, but the inequality lengthens this to 918 years.
Therefore the periodic perturbations are determined by the second order recurrence cycles of
the planets in the fixed Zodiac. However, at the end of such a cycle of fluctuation the
distorting effects have not been completely cancelled out, and small remaining residues show
up in what are termed the elements of the planets' orbits. As a result of this residue, the
nodical line at which the orbit plane of each planet intersects the invariable plane, is displaced
backward, in a precessional motion of the whole orbit plane. In addition, the inclination of the
orbit plane to the invariable plane is slightly decreased or increased. A third effect is a shift in
the position of the major axis - longest axis of the orbital ellipse. This shift may be either
forward or backward, but it is more likely to be forward for all of the planets except Venus.
Finally the shortest axis of the orbital ellipse - the minor axis which crosses the major axis at
the center of the ellipse - increases or decreases its length; i.e, the eccentricity of the ellipse
becomes greater or less.
Two great masters of celestial mechanics, Lagrange and Laplace, demonstrated that while