The Midpoint Method: An Ecliptic-Based Boundary System for Zodiacal Constellations

The constellations of the zodiac, as drawn from observable stellar configurations, lack the natural ecliptic boundaries required for positional astrology. Existing schemes either divide the zodiac into twelve equal thirty-degree segments without reference to constellation extent, or apply the International Astronomical Union (IAU) constellation boundaries, which were defined in equatorial coordinates for cataloging purposes rather than for tracking bodies along the ecliptic. This paper presents the Midpoint Method, a boundary system in which the transition between two adjacent zodiacal constellations is defined as the midpoint, in ecliptic longitude, between the last line star of the preceding constellation and the first line star of the following constellation. Line stars are drawn from both the IAU (Modern) and Indian Vedic constellation traditions, with selection rules to handle cases of disagreement and constellation overlap. Applied to the thirteen constellations through which the Sun passes (including Ophiuchus), the method yields a complete, reproducible set of ecliptic boundaries derived from primary stellar catalog data. All input star positions are referred to the Hipparcos-2 catalog (van Leeuwen 2007) and reduced to the J2000.0 epoch in the barycentric mean ecliptic frame. The resulting boundary table is presented in full, along with the methodological choices and their limitations.