Astronomy for Astrologers
concerned, there is no difference between these centers
because of the infinite distance the zodiac is considered
to extend. In other words, the geocentric and
heliocentric zodiacs are the same, because the zodiac
is considered to extend an infinite distance from both.
The differences between geo and helio planetary
positions are due to differing perspectives—how the
Sun and planets are seen, whether they are seen from
the Earth or from the Sun—and 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 which passes through the center of its 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 ranging from 0° (the plane
itself) to 90° either 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°—the zodiac
longitude. In the tropical ecliptic system used by most
Western astrologers, this point is the zero-degrees Aries
point, or the Vernal Equinox (to be explained elsewhere).