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Is a zodiac constellation a star sign? If not, why not?

Sign - Constellation: They are very different things. The term ‘zodiac constellation’ applies to a group of stars that lies in the area of sky measured by the zodiac. Different cultures, and even the same cultures at different periods of history, have recognized varying numbers of constellations in this region of the sky, as adjustments were made to constellation boundaries. However, the zodiac itself, being a mathematical division of an astronomical circle, has only ever been divided into twelve equally spaced ‘zodiac signs’. The words ‘constellation’ and ‘sign’ depict different things.

A constellation can be visibly seen. It is a grouping of stars that appear together in the sky, as the word constellation suggests (it derives from the Latin con, ‘together’ + stellas, ‘stars’).

A sign communicates information. Zodiac signs, sometimes popularly called ‘star signs’, are determined by astronomical principles. The astronomers who originated the zodiac system also worked as astrologers and sought to attach and extract more than purely astronomical information from them; however, zodiac signs never have, and never will be visible - nor are they expected to make a close astronomical alignment with the constellations whose names they share. This would not be possible because constellations vary greatly in size whereas zodiac signs do not.

It is worth considering that when the Babylonians developed their zodiac they recognised seventeen visible constellations that lay in the area measured by the zodiac. The reduction of the number of their constellations was done over time to facilitate astronomical measurement, and in response to the increased importance of the zodiac. But note that the constellation boundaries were adjusted to ease association of constellations with zodiac signs, not vice versa. The twelve-fold design of the zodiac is never changed to reflect alterations of constellation boundaries.

Why does the BBC tell us “The astronomical zodiac actually contains thirteen star signs - the twelve that we're familiar with plus another one, called Ophiuchus”?
This information is incorrect and based on Dara O’Briain confusing constellations with star signs, and also failing to distinguish between the ecliptic and the zodiac. What would be correct to say is that the ecliptic passes through thirteen constellations. These do not constitute star signs.

Following the redefinition of constellation boundaries by the International Astronomical Union during the 1930s, the ecliptic currently passes through thirteen officially recognised constellations: the twelve constellations historically described as ‘zodiac constellations’ (because either the whole or greater part of them falls within the zodiac belt), plus the constellation Ophiuchus, which mainly lies outside it.

Serpent bearer Ophiuchus is not a new constellation: it was catalogued with many others in Ptolemy's 2nd-Century treatise, Almagest, (an astronomical text of huge historical standing), where it is referred to as Septentarius, ‘the serpent holder’. It was always realised that the ecliptic cuts through a small part of this constellation, but it is a very large constellation, the majority of which the zodiac fails to embrace, so it was not considered a ‘zodiac constellation’. The clarifying account by the editors of Encyclopaedia Britannica (2014) gives a very accurate and reliable explanation of what the zodiac is and why the Sun “regularly passes through one constellation (Ophiuchus) that is not considered a member of the zodiac”. [9]

Regardless of where the boundaries of Ophiuchus are placed, astrologers have always taken meaningful associations from the stars of this and other constellations. Astrological interest focuses on the brightest and most culturally significant stars in the sky, and the meaning attributed to them has been acquired over many centuries. This long-established meaning is not lost according to what division of the zodiac a star falls in, or if the boundaries of the constellations are adjusted, although this might give rise to new cultural associations that become meaningful for astrologers over time. [For the astrological meaning attributed to Ophiuchus and its stars, see: www.skyscript.co.uk]


Why is news of a thirteen-sign zodiac like an urban myth that keeps circulating, even though it is not true?

The BBC has a habit of raising this story to promote their astronomy programmes and in many ways has acted as the source of it. The astronomically incorrect description of Ophiuchus as an ‘astrological sign’ first drew prominent media attention in Britain on January 20, 1995, following the BBC's Nine O’Clock News announcement that “an extra sign of the zodiac has been announced by the Royal Astronomical Society”. The story made the headlines of the Times and Telegraph the following morning.

News of this supposedly thirteen-sign zodiac quickly travelled across the world, being featured (on the strength of the BBC's reputation) on NBC news in the US, and making countless sensational newspaper headlines throughout Europe, Canada, Brazil and Australia. Investigation into the source of the story revealed there had been no such announcement. [11] The BBC had merely misreported the International Astronomical Union's old decision, in the 1930s, to alter the officially recognised boundaries of constellations that cross the ecliptic. The Nine O’Clock News announcement was made as part of the BBC's promotion for a forthcoming programme on astronomy that featured the zodiac and explored the issue of ‘precession of the equinoxes’ (a technical theme that few members of the public would be likely to get excited about without some kind of public interest attachment). Despite complaints, the BBC made no attempt to set the record straight or to explain why their highly profiled news story was so factually flawed and misleading.

A similar situation occurred in January 2011 after international media blazed with reports that astronomer Parke Kunkle had claimed Ophiuchus was the zodiac's ‘thirteenth sign’. The original story, published in the Minneapolis Star-Tribune, went viral and suffered increasing misrepresentation as this sensational detail became repeated by countless news organisations. The internet swiftly gained a host of new articles showing how the ecliptic crosses Ophiuchus, and referring to it as the thirteenth sign of the zodiac.

In response to the unexpected avalanche of publicity, Kunkle issued a statement to say that he had not reported that the zodiac ought to include thirteen signs instead of twelve, but was only mentioning that there were thirteen constellations. [12] Again, the rather boring details of this clarifying statement failed to attract any significant media attention, although the BBC - at that time - did publish an article which stated that the issues do not affect our understanding of the zodiac system used in the West. This concluded: “For all those people that believe in astrology or set some kind of lesser emotional store on the signs of the zodiac, they can rest easy. The Tauruses are still Taurus and the Arieses are still Aries”. [10]

The zodiac is no longer a preferred system of astronomical measurement; so many individual astronomers who comment on this matter make no use of it, and have a poor understanding of its history, definition and purpose. Many have been influenced by these kinds of sensationalised media reports, and so do not realise how inappropriate it is, astronomically, to refer to a constellation as a ‘star sign’. Many more will now be influenced by the BBC's new BBCiWonder webpage, which was published in association with the BBC's Stargazing Live astronomy programme, and presents content that appears to be shaped by their own promotional interests rather than educational purposes.

http://www.astro.com/astrology/aa_article150701_e.htm

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