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I have restated JHunterJ's version to use well-defined astronomical terms. JHunterJ's version was unclear because
 There is no official definition of ""zodiacal longitude"".
 If we take ""zodiacal longitude"" to mean celestial longitude, then every celestial object and constellation has a (range of) celestial longitude, so the statement becomes meaningless.
 As far as I can tell, modern astronomers no longer offer precise definitions of ""sign of the zodiac"", and various media use the term in many different ways. Thus, the term is not well enough defined to decide of it is a mistake to describe Ophiuchus as the 13th sign of the zodiac.   "