The Real Story of the Zodiac's 13th Constellation

Over a year the Sun appears to travel once around the sky along a line astronomers call the ecliptic. Two very different things sit along that line, and the whole Ophiuchus story comes from mixing them up.

The first is the constellations: patterns of stars, of all shapes and sizes, that cultures have named for thousands of years. Virgo is huge. Scorpius is compact. Their modern borders are a twentieth century convention: the astronomer Eugène Delporte drew boundaries for all 88 constellations, adopted by the International Astronomical Union and published in 1930.

The second is the signs. Around the end of the fifth century BC, and by modern estimates between 409 and 398 BC, Babylonian astronomers divided the ecliptic into twelve equal parts of 30 degrees each. They borrowed names from constellations near each part, drawing on older star lists such as the MUL.APIN catalogue of around 1000 BC, but the signs themselves are a ruler, not a picture. Think of the hours on a clock face: they are equal by design, whatever is painted on the dial.

Ophiuchus is a large constellation of a man holding a snake, which is the constellation Serpens. Its feet reach down to the ecliptic between Scorpius and Sagittarius. When Delporte drew the borders, a stretch of the ecliptic fell inside Ophiuchus rather than Scorpius. So by the 1930 borders the Sun passes through thirteen constellations, not twelve.

In 2027 the Sun enters Scorpius on November 23 or 24, depending on time zone, and leaves it six and a half days later. It then spends about 18 days in Ophiuchus, from November 30 to December 18, before reaching Sagittarius. Virgo, by contrast, holds the Sun for 44 and a half days.

Every few years a headline announces that NASA has added a thirteenth sign. It has not. When the story went viral in 2016 and again in 2020, NASA explained the arithmetic of the constellations and said it had not changed the zodiac. It added that the Babylonians had picked twelve of the constellations and given each an equal share of the year.

The figure is old even if its borders are not. Greek myth saw in it the god Apollo wrestling the great snake that guarded the oracle at Delphi; later writers of the Roman era named him Asclepius, the healer, struck down by Jupiter's lightning to stop humans becoming immortal and then set among the stars. The second-century astronomer Ptolemy listed Ophiuchus among his 48 constellations. That is worth sitting with: the same Ptolemy who described the zodiac as starting at the spring equinox knew the Serpent Bearer stood on the Sun's path and still counted twelve signs, because to him signs and constellations were different tools.

The constellation has its own claims to fame. A new star blazed up near the star Theta Ophiuchi in October 1604 and was followed by Johannes Kepler; astronomers now know it as Kepler's Supernova, SN 1604. Barnard's Star, one of the Sun's nearest neighbours, lies inside its borders too.

There is a second, slower reason the signs and the star patterns disagree. Earth's axis wobbles like a spinning top, tracing a full circle in about 26,000 years. The Greek astronomer Hipparchus is usually credited with discovering this in the second century BC. Astronomers call it precession.

Because of it, the point where the Sun crosses the equator in March, the spring equinox, slides backward along the ecliptic by about 50.3 arcseconds a year. That is one degree every 72 years or so. When the signs were named, around 400 BC, the March equinox sat among the stars of Aries. Today it sits in Pisces, edging toward Aquarius.

The two zodiacs of modern astrology part company over exactly this point.

Tropical astrology, the kind most Western astrologers practise, starts the zodiac at the March equinox: 0 degrees of Aries is the moment of the equinox, every year. Ptolemy explained in the second century that Greek astronomers began the zodiac at the spring equinox, and the convention is older than him. On this view the signs describe the cycle of the seasons, the lengthening and shortening of light, so precession does not move them and the constellations behind them are beside the point. Ophiuchus is a real star pattern, but the tropical zodiac was never a map of star patterns.

Sidereal astrology, the zodiac of Vedic astrology or Jyotish, ties the signs to the fixed stars instead. It keeps twelve equal signs but corrects for precession with an ayanamsa, the measured gap between the two zodiacs. Our free Kundali uses the Lahiri ayanamsa, which is 24 degrees 14 minutes at the start of 2027 and grows by about a degree every 72 years. By that formula the two zodiacs lined up in the late 200s AD. A sidereal astrologer answers the precession argument fully, and still has no thirteenth sign, because the signs remain equal slices.

A small number of astrologers do build systems on the real constellation borders, including some with thirteen or fourteen signs. They remain outside both main traditions.

Neither main tradition is "wrong" about the sky. They measure from different starting points, and both have done so on purpose for many centuries. Our guide to Vedic and Western astrology compares them in full, and the 27 lunar mansions of the sidereal zodiac are in the nakshatras list.

If you were born between about November 30 and December 18, the Sun was in front of the stars of Ophiuchus. Your tropical sign is still Sagittarius, and your sidereal sign is Scorpio for most of that window. All three statements are true at once; they answer different questions.

For a Western astrologer your sign is a season. For a Vedic astrologer it is a stretch of sky measured from the stars. For an astronomer the Sun's position is a point among 88 constellations with borders drawn in 1930. To see both of the astrological answers for your own birth moment, cast your Western birth chart and your Vedic Kundali side by side.