Constellation Boundaries and their Precessional Skew

Constellation boundaries are a recent development. In 1922, the IAU decided to demarcate constellations (88 of them) simply as a means to create areas in the celestial sphere with standardized names and abbreviations. This effort was done with the aid of American astronomer Henry Norris Russell. A proposal to define constellation boundaries was made at IAU’s second General Assembly in 1925 by Belgian astronomer Eugene Joseph Delporte. This effort was undertaken by the IAU with Delporte directing the effort. Delporte referenced his boundaries to Earth's celestial equator, not to the ecliptic plane. They were composed of "vertical and horizontal lines" of right ascension and declination based on 1875 epoch data. He used this epoch because American astronomer Benjamin Gould used that date to create southern constellation boundaries in 1875. Delporte and team also modified Gould’s boundaries to match the (Ra Dec) scheme used for the northern constellations. The entire effort was completed and the constellation boundaries and names were formalized in 1930.

This means that although the boundaries were vertical and horizontal relative to the celestial equator in 1875, they no longer are due to Earth's precession, and they will continue to skew throughout the precessional cycle as shown in the video below. The boundaries were drawn in rectangular, stair-step shape merely to encompass the various constellations, not to delineate any sort of energetic shift between constellations, nor is there an energetic shift where those boundaries cross the ecliptic.

You can pause the video and drag the progress bar to view any section of the video.

Sky Charts
Boundaries Ecliptic View
Boundaries Equatorial View
Boundary Skew Video