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We have mentioned, when speaking of the zodiac, that it is the plane of the Earth's orbit that is used as the fundamental reference plane for the ecliptic system of coordinates. We may specify the position of all objects as either above (north) or below (south) of this plane by a number of degrees of arc that range from 0° (the plane itself) to 90° above or below this plane -- the north and south poles of the ecliptic. We must also choose (and this is the most arbitrary factor) a point or direction in space (somewhere along the plane itself) from which to measure longitude of arc from 0° to 360°-- zodiac longitude. In the tropical ecliptic system used by most Western astrologers, this point is the zero-degrees Aries point or Vernal Equinox (to be explained later).
Be sure to get a feeling for what a system of coordinates is and how such a system is defined. All coordinates systems will have a center, a plane of reference (i.e. a north and south pole), and a point along the plane from which to measure the longitude factor. The latitude factor is measured above and below the reference plane.
There are many useful coordinate systems in astrological work besides the zodiac or ecliptic. The two outstanding other systems that must be understood for competent astrological considerations are the Equatorial System of right ascension and declination and the Horizon System of azimuth and altitude. Yet other systems include the Galactic System, Supergalactic System, and the Local System (the systems of near stars of which our Sun is one member). This is in addition to the various orbital planes of the planets other than Earth, each of which has its own "ecliptic," based on the inclination of its particular orbital plane. Still other systems exist, based on the equatorial inclination of the planets, the equator of the Sun, and the Invariable Plane of the solar system. It can get complex.
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