Zodiacal Constellations
It is important to distinguish the zodiacal signs from the constellations associated with them, not only because of their drifting apart due to the precession of equinoxes but also because the physical constellations by nature of their varying shapes and forms take up varying widths of the ecliptic. Thus, Virgo takes up fully five times as much ecliptic longitude as Scorpius. The zodiacal signs, on the other hand, are an abstraction from the physical constellations designed to represent exactly one twelfth of the full circle each, or the longitude traversed by the Sun in about 30.4 days.
There have always been a number of "parazodiacal" constellations that are also touched by the paths of the planets. The MUL.APIN lists Orion, Perseus, Auriga, and Andromeda. Furthermore, there are a number of constellations mythologically associated with the zodiacal ones : Piscis Austrinus, The Southern Fish, is attached to Aquarius. In classical maps, it swallows the stream poured out of Aquarius' pitcher, but perhaps it formerly just swam in it. Aquila, The Eagle, was possibly associated with the zodiac by virtue of its main star, Altair. Hydra in the Early Bronze Age marked the celestial equator and was associated with Leo, which is shown standing on the serpent on the Dendera zodiac. Corvus is the Crow or Raven mysteriously perched on the tail of Hydra.
Due to the constellation boundaries being redefined in 1930 by the International Astronomical Union, the path of the ecliptic now officially passes through thirteen constellations: the twelve traditional 'zodiac constellations' plus Ophiuchus, the bottom part of which interjects between Scorpio and Sagittarius. Ophiuchus is an anciently recognized constellation, catalogued along with many others in Ptolemy's Almagest, but not historically referred to as a zodiac constellation.
The technically inaccurate description of Ophiuchus as a sign of the zodiac dates to the 1970s. This drew prominent media attention on 20 January 1985, following the BBC's opening 'Nine o'clock News' announcement that "an extra sign of the zodiac has been announced by the Royal Astronomical Society". Investigation into the source of the story revealed there had been no such announcement, and that the report had merely sensationalized the 67-year-old 'news' of the IAU's decision to alter the number of designated ecliptic constellations. This was done for the purposes of promoting a forthcoming BBC astronomy program, presented by the RAS's Public Relation Officer, which touched upon the topic of precession. The false assumption that Ophiuchus constitutes an astrological sign periodically resurfaces in the media, due to public misconception and failure to appreciate that the irregular astronomical demarcation of visible constellations does not relate to the separate frame of reference provided by the equally spaced twelve-fold longitude division of the ecliptic into zodiacal signs.
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