Ophiuchus, the 13th zodiac sign

Why there are 13 constellations on the Sun's path

Through the year the Sun appears to move once around the sky against the background stars, along the line called the ecliptic. The zodiac is the band of constellations along that line. When the Babylonians set it up in the first millennium BC, they divided the path into twelve equal parts of 30°, one for each month of their year, and named each part after a constellation nearby. The constellations were patterns of stars, of all sizes, and they never fitted twelve equal boxes.

In 1928 the International Astronomical Union agreed official borders for all 88 constellations, drawn by the Belgian astronomer Eugène Delporte and published in 1930. Those borders follow lines of right ascension and declination, and they let the ecliptic cross the lower part of Ophiuchus, between Scorpius and Sagittarius. So by the official map the Sun passes through 13 constellations, and it spends far longer in front of Ophiuchus (18 days in 2026) than in front of Scorpius (6.5 days). The strip above shows how uneven the real constellations are, measured by this page's own calculation of the Sun's position.

Precession: why the dates moved

The second reason your birthday rarely matches its constellation is precession. The Earth's axis slowly swings round like a spinning top's, once every 25,800 years or so, which moves the March equinox backwards along the zodiac by about 1° every 72 years. Western astrology fixes Aries to the March equinox, so the signs travel with the seasons, while the constellations stay with the stars. Over the roughly 2,000 years since the signs and the constellations were last lined up, the gap has grown to about 24°, nearly a whole sign. Today the March equinox lies in the constellation Pisces; the astronomer Jean Meeus calculated that by the IAU borders it will cross into Aquarius in the year 2597, the real "dawning of the Age of Aquarius".

Put the two together and most birthdays fall in the constellation before their sign. Today, October 11, the Sun is in the sign Libra but in front of the stars of Virgo. Twelve of the 13 constellations share a name with a sign, but their dates run roughly a month later than the sign dates, as the two tables show.

The 2016 "NASA changed your sign" story

In 2016 NASA's education site for children, Space Place, published a short explainer on the zodiac: the Babylonians knew of 13 constellations on the Sun's path, picked 12 to match their calendar, and the sky has shifted since. Newspapers and social media turned it into "NASA adds a 13th sign" and "your star sign has changed". NASA responded that it had changed nothing; it had only done the astronomy. None of this was new: astronomers have pointed out Ophiuchus for decades, and an Ophiuchus scare made headlines in 2011 too.

Why astrologers keep 12 signs

For astrologers the signs were never meant to be the constellations. Western (tropical) astrology defines the signs by the seasons: Aries begins at the spring equinox, Cancer at the summer solstice, Libra at the autumn equinox and Capricorn at the winter solstice. The whole system of elements, modalities, rulerships and houses is built on twelve equal 30° divisions, and Ophiuchus has no place in it. Sidereal astrology, used in India, ties its twelve equal signs to the stars instead (about 24° behind the tropical ones) and still has no Ophiuchus. "13-sign" astrology, with Ophiuchus as a sign and unequal dates, was popularised in the 1990s by the British astrologer Walter Berg and has a small following. So two statements are both true: your astrological sign has not changed, and on your birthday the Sun may well have been in front of Ophiuchus.

Ophiuchus in the sky

Ophiuchus is the Serpent Bearer, a large, fairly faint constellation straddling the celestial equator, shown as a man holding a great snake: the constellation Serpens, split into a head and a tail on either side of him. The Greeks linked him with Asclepius, the god of medicine. Its brightest star is Rasalhague, at the figure's head. Inside its borders are Barnard's Star, the closest single star to the Sun and the fastest-moving star across our sky, and the site of Kepler's Supernova of 1604, the last supernova seen in our own galaxy with the naked eye.

You cannot see Ophiuchus around an Ophiuchus birthday, because the Sun is in front of it. Look for it on summer evenings instead, above the red star Antares in Scorpius: the sky view above is set to a moonless night at the start of the northern summer. Because the Moon and planets also travel near the ecliptic, they regularly pass through Ophiuchus too: the Moon is in front of it for a day or two every month.

Related: Big three calculator (with the real constellation behind your Sun and Moon) · Moon sign calculator · Rising sign calculator · Wheel of the Year · Planets tonight