The sky — astronomy

Ophiuchus: The Constellation That Isn’t a Zodiac Sign

Ophiuchus is one of the 88 constellations recognized by the International Astronomical Union (IAU), and the Sun really does pass through it every year. It is not a “13th zodiac sign” because the zodiac of astrological tradition is made of 12 equal 30° divisions of the ecliptic, not of constellation boundaries. Astronomy and astrological tradition are describing two different things, so they do not contradict each other.

The constellation and its myth

Ophiuchus (pronounced “off-ee-YOO-kus”, the Latinized form of a Greek name meaning “serpent-bearer”) is a large constellation straddling the celestial equator. From the Northern Hemisphere it is best seen on summer evenings, in the southern part of the sky. It is one of the 88 official constellations of the International Astronomical Union (IAU). Its figure is a man holding a snake, and the snake is its own constellation, Serpens, split into two parts on either side of Ophiuchus.

In the Greek tradition the figure is usually linked with Asclepius, the legendary healer, which is why the serpent is a symbol of medicine. The constellation was already described in antiquity and appears among the 48 constellations in Ptolemy’s Almagest. Its brightest star is Rasalhague (Alpha Ophiuchi). The constellation also holds Barnard’s Star, one of the Sun’s nearest stellar neighbours, and the remnant of the supernova of 1604, known as Kepler’s Supernova.

For the full list, see the 88 constellations.

The Sun passes through Ophiuchus

Over a year the Sun appears to travel against the background stars along a line called the ecliptic (the Sun’s apparent yearly path, a reflection of Earth orbiting the Sun; see the ecliptic). With the IAU’s official boundaries, that line crosses 13 constellations: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Ophiuchus, Sagittarius, Capricornus, Aquarius and Pisces.

The constellation boundaries were fixed by the IAU in the 1920s and published in 1930. They follow lines of celestial coordinates and are very unequal in size. As a result the Sun spends roughly from late November to mid-December in Ophiuchus, about two and a half weeks, while it spends only about a week in Scorpius. Exact dates shift slightly from year to year and depend on the coordinate epoch used.

Why astrological tradition has 12 signs

A zodiac sign in astrology is not a constellation but a segment of the Sun’s path. In the tradition the ecliptic is divided into 12 equal parts of 30° each (12 × 30° = 360°). Each part carries the name of a constellation, yet it is a mathematical division, not the outline of a star pattern. The scheme has roots in Babylonian astronomy and was developed further in the Greek and Roman world. Twelve divisions also echo the roughly twelve lunar months in a year.

In the tradition the circle is measured from the point of the March equinox (the so-called tropical zodiac), not from particular stars. That is why signs and constellations of the same name no longer line up by date today. The reason is precession, the slow wobble of Earth’s axis that completes one cycle in about 26,000 years and gradually shifts the equinox against the stars (see precession and tropical and sidereal zodiac).

So nothing was “left out” of the tradition: it was never built on constellation borders. Ophiuchus is simply not one of the 12 divisions, and that does not make it any less real as a feature of the sky.

Where the “new zodiac sign” headlines come from

From time to time headlines claim that “NASA added a 13th sign” or that “the zodiac has changed”. They usually stem from a misunderstanding: someone points out that the Sun passes through Ophiuchus, and it is presented as news. But astronomy has not changed astrology, and NASA does not define zodiac signs. That Ophiuchus lies along the ecliptic has been known since antiquity and has been part of the IAU’s official boundaries since 1930.

A more accurate way to put it: the sky has 13 constellations along the Sun’s path, while the traditional zodiac has 12 equal divisions. Two different systems, each answering its own question. If you would like to see Ophiuchus yourself, look for it on late-summer evenings between Scorpius and Hercules, away from city lights.

See also: zodiac constellations, is there a 13th zodiac sign and why zodiac signs don’t match the constellations.

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