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The Zodiac or Ecliptic Coordinate System

Let's examine what the zodiac or ecliptic coordinate system is and how we use it in our work. All reference coordinate systems (like the ecliptic) must have a center and here that will be either the Earth or the Sun. We use the Earth as a center for traditional geocentric (Earth-centered) astrology and the Sun as the center for heliocentric (Sun-centered) astrology. As far as the zodiac is concerned, there is no difference between these centers in terms of the infinite distance at which the Zodiac is considered to be. In other words, the geocentric and heliocentric zodiacs are the same because the zodiac is considered to be at an infinite distance from both. The differences between geo and helio planetary positions are due to the differing perspectives of the Sun and planets as seen from either the Earth or the Sun, and are not due to differing zodiacs. Since every center for a coordinate system is surrounded by 360 equal degrees of space in any direction, we must have a plane to which all objects, stars, planets, etc. may be referred — a reference plane. Every coordinate system must have a plane of reference that passes through the center of the system and which divides all space into two equal halves or hemispheres.

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.