Ophiuchus has sometimes been used in sidereal astrology as a thirteenth astrological sign in addition to the twelve signs of the tropical zodiac. The constellation Ophiuchus, as defined by the 1930 International Astronomical Union's constellation boundaries, is situated behind the sun from November 29 to December 18. The idea appears to have originated in 1970 with Stephen Schmidt's suggestion of a 14-sign zodiac, also including Cetus as a sign. A 13-sign zodiac has been promulgated by Walter Berg and by Mark Yazaki in 1995, a suggestion that achieved some popularity in Japan, where Ophiuchus is known as. In sidereal and tropical astrology, a 12-sign zodiac is used based on dividing the ecliptic into 12 equal parts rather than the IAU constellation boundaries. That is, astrological signs do not correspond to the constellations which are their namesakes, particularly not in the case of the tropical system where the divisions are fixed relative to the equinox, moving relative to the constellations.
History
Ophiuchus and some of the fixed stars in it were sometimes used by some astrologers in antiquity as extra-zodiacal indicators. The constellation is described in the astrological poem of Marcus Manilius as the one that winds in loops: "But, bending its supple neck, the serpent looks back and returns: and the other's hands slide over the loosened coils. The struggle will last forever, since they wage it on level terms with equal powers". Later in his poem, he describes the astrological influence of Ophiuchus, when the constellation is in its rising phase, as one which offers affinity with snakes and protection from poisons, saying "he renders the forms of snakes innocuous to those born under him. They will receive snakes into the folds of their flowing robes, and will exchange kisses with these poisonous monsters and suffer no harm". A later 4th century astrologer, known as Anonymous of 379, associated "the bright star of Ophiuchus", Ras Alhague, with doctors, healers or physicians, which may have been because of the association between poisons and medicines. Based on the 1930 IAU constellation boundaries, suggestions that "there are really 13 astrological signs" because "the Sun is in the sign of Ophiuchus" between November 30 and December 18 have been published since at least the 1970s. In 1970, Stephen Schmidt in his Astrology 14 advocated a 14-sign zodiac, introducing Ophiuchus and Cetus as new signs. Within 20th-century sidereal astrology, the idea was taken up by Walter Berg in the form of his book, The 13 Signs of the Zodiac. In January 2011, a statement by Parke Kunkle of the Minnesota Planetarium Society repeating the idea of "the 13th zodiac sign Ophiuchus" made some headlines in the popular press.
