TL;DR: Ophiuchus is not the 13th zodiac sign in astrology. Under the IAU constellation boundaries of 1930, the Sun's path crosses 13 constellations, including Ophiuchus, but Western astrology still uses 12 signs of 30 degrees measured from the vernal equinox, not the actual constellation boundaries. The Sun is in Ophiuchus from about 30 November to 17 December, dates astrology still counts as Sagittarius.
What is Ophiuchus and why is there a rumour about a 13th zodiac sign?
Ophiuchus is a large constellation that lies on the ecliptic, but it is not one of the 12 astrological zodiac signs. The rumour about a "13th zodiac sign" has resurfaced several times, most visibly in 2011 and 2016, when the press repeated astronomy explainers noting that, under the constellation boundaries of the International Astronomical Union (IAU), the Sun passes through 13 constellations rather than 12. However, Western astrology since the time of Ptolemy has kept the "sign" separate from the "constellation", and still keeps 12 signs of 30 degrees each.
How many constellations does the Sun pass through under the 1930 IAU boundaries?
The Sun passes through 13 constellations: the 12 that share the names of the signs, plus Ophiuchus. The ecliptic is the apparent path of the Sun across the celestial sphere over a year.
The boundaries of the 88 constellations were drawn by the Belgian astronomer Eugène Delporte along lines of right ascension and declination and published by the IAU in 1930; under those boundaries the ecliptic crosses the southern part of Ophiuchus, the constellation that sits between the two halves of Serpens. The Sun is in Ophiuchus from about 30 November to 17 December each year.
The constellations vary in width, so the Sun spends very different lengths of time in each: more than 40 days in Virgo, only about a week in Scorpius. Astrological signs are equal, 30 degrees each.
Why does astrology still keep 12 signs instead of 13?
Western astrology uses 12 signs of 30 degrees each, starting from the vernal equinox point, and does not rely on the actual constellation boundaries. In the Tetrabiblos, Ptolemy takes the vernal equinox as the starting point of the zodiac[1]. By that reckoning the first 30 degrees after the vernal equinox are the sign Aries, even though the stars of the constellation Aries now lie elsewhere in the sky.
Alan Leo, an early 20th-century English astrological author, wrote in Astrology for All that the signs of the zodiac must not be confused with the twelve constellations of the same names[2].
Does precession shift the zodiac signs away from the constellations?
Yes, precession makes the zodiac signs and the constellations of the same name drift further and further apart, but astrology keeps its old frame of reference. Precession is the slow wobble of the Earth's axis, which makes the vernal equinox point slide backwards along the ecliptic, against the Sun's yearly motion[3].
The point moves about 1 degree every 72 years and completes a circuit in about 25,800 years. Some two thousand years ago the vernal equinox lay at the edge of the constellation Aries; today it lies in Pisces.
Western astrology using the tropical zodiac does not change its reckoning because of this drift: a sign is defined by the vernal equinox, not by the stars.
Does Ophiuchus have any influence in astrology?
In mainstream Western astrology, Ophiuchus is not a sign and has no place in a chart. Bringing Ophiuchus into astrology is mainly a media and social media phenomenon, not an astrological tradition.
Comparison table of the 12 zodiac signs and the 13 IAU constellations
| Sign (tropical astrology) | Sun in the sign (approximate) | Constellation (IAU boundaries) | Sun in the constellation (approximate) |
|---|---|---|---|
| Aries | 21/3 - 19/4 | Aries | 19/4 - 13/5 |
| Taurus | 20/4 - 20/5 | Taurus | 14/5 - 21/6 |
| Gemini | 21/5 - 20/6 | Gemini | 22/6 - 20/7 |
| Cancer | 21/6 - 22/7 | Cancer | 21/7 - 10/8 |
| Leo | 23/7 - 22/8 | Leo | 11/8 - 16/9 |
| Virgo | 23/8 - 22/9 | Virgo | 17/9 - 30/10 |
| Libra | 23/9 - 22/10 | Libra | 31/10 - 22/11 |
| Scorpio | 23/10 - 21/11 | Scorpius | 23/11 - 29/11 |
| (none) | none | Ophiuchus | 30/11 - 17/12 |
| Sagittarius | 22/11 - 21/12 | Sagittarius | 18/12 - 19/1 |
| Capricorn | 22/12 - 19/1 | Capricornus | 20/1 - 15/2 |
| Aquarius | 20/1 - 18/2 | Aquarius | 16/2 - 11/3 |
| Pisces | 19/2 - 20/3 | Pisces | 12/3 - 18/4 |
*Dates shift by a day depending on the year. Constellation dates are for the present era and drift slowly with precession.*
Why does the rumour of a 13th zodiac sign spread?
The rumour spreads partly because the public does not clearly distinguish astronomy from astrology. In 2016 NASA had to state that it studies astronomy, not astrology, and had not changed any zodiac sign. The rumour still returns on social media in waves.
Conclusion: is Ophiuchus the 13th zodiac sign?
No. Ophiuchus is a constellation on the ecliptic under the 1930 IAU boundaries, but it is not a zodiac sign in astrology. The Sun passing through 13 constellations is an astronomical fact; Western astrology counts 12 equal signs of 30 degrees from the vernal equinox, so the number of constellations does not change the number of signs.
