Ophiuchus and the '13th Zodiac Sign': Constellations vs Zodiac Signs

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Every few years a post goes round saying that NASA has added a thirteenth zodiac sign, Ophiuchus, and that most people's signs have changed. Your Tropical sign has not changed. The story mixes up two things that happen to share names: zodiac signs, which are equal slices of a circle, and constellations, which are regions of sky with formal borders. This page separates the two, explains where Ophiuchus fits, and says what NASA did and did not say.

About this site. Astrologist.com uses the Tropical zodiac of twelve signs. The free birth chart and the compatibility tool do not calculate placements in the astronomical constellations, Sidereal positions or ayanamsas, and neither has an Ophiuchus sign or setting. The Astrologist chat is given your chart's Tropical placements by sign (and by house when the birth time is known), not by degree, and it is not given constellation positions. This page is education about astronomy terms that are often confused with signs. It does not add a sign, a constellation mode or any change to how the site calculates charts; the methodology page lists the site's settings.

Zodiac signs: twelve equal sectors

A zodiac sign, as this site uses the word, is a 30° stretch of the ecliptic, the Sun's apparent yearly path through the sky. There are twelve of them, all exactly the same size. In the Tropical zodiac they are counted from the March equinox point: 0° to 30° is Aries, 30° to 60° is Taurus, and so on round to Pisces. The signs are defined by that arithmetic, not by where any star happens to be. So the boundaries of the signs do not depend on the shapes of star patterns at all.

Constellations: unequal regions with borders

Astronomers use the word constellation differently. For the International Astronomical Union (IAU), a constellation is an area of the sky enclosed by a boundary, and the whole sky is divided into 88 of them. The IAU agreed the list of 88 in 1922. The boundaries came later: Eugène Delporte drew them along lines of right ascension and declination, the IAU approved them at its General Assembly in Leiden in 1928, and they were published in 1930 as Délimitation scientifique des constellations. When an astronomer says the Sun is "in" a constellation, it means the Sun lies within those lines, seen from Earth.

The regions are nothing like equal. Measured against the IAU boundaries for this page, the Sun spends about 44½ days a year within the borders of Virgo and only about 6½ days in Scorpius. A sign is 30° by definition; a constellation is whatever shape its borders give it.

Twelve constellations share their names with the twelve signs, which is where most of the confusion starts. The sign Aries and the constellation Aries are not the same stretch of sky. For example, at the March equinox, when the Sun enters Tropical Aries, it currently stands within the boundaries of the constellation Pisces.

Where Ophiuchus comes in

Ophiuchus, the Serpent Bearer, is one of the 88 IAU constellations, and the Sun's apparent path crosses its southern part, between Scorpius and Sagittarius. Through a year the Sun passes in front of thirteen constellations: the twelve with zodiac names and Ophiuchus.

For every year from 2000 to 2030, a calculation made for this page against the IAU boundaries puts the Sun within Ophiuchus from about 29 or 30 November to about 17 or 18 December (UTC), roughly 18 days. Throughout those days the Tropical Sun is in Sagittarius.

That is an astronomical fact about a constellation. It does not make Ophiuchus a sign. Tropical signs are not defined by constellation borders, so a constellation crossed by the Sun adds no sector and moves no boundary. Someone born on 5 December has the Sun in Sagittarius in a Tropical chart, whichever constellation stood behind the Sun that day.

Did NASA add a sign?

No. NASA did not add an astrological sign or change anyone's sign. When the claim spread in 2016, NASA published an explanation saying, in its own words, "We didn't change any zodiac signs, we just did the math", and repeated the point when the story came back in later years. NASA studies astronomy, not astrology. Its material separates the two ideas this page separates: the zodiac signs as equal divisions, and the constellations, which the Sun lines up with for different lengths of time because they differ in size.

NASA's piece also gives a brief historical aside about how the twelve signs were chosen. This page does not repeat that history, and nothing here depends on it. The key point needs no history: a sign and a constellation are different kinds of thing.

Why "new sign dates" lists mix two systems

Lists of thirteen "new" sign dates usually take the dates when the Sun is within each constellation's borders and present them as signs. That combines a constellation calendar with sign names and calls the result your zodiac sign. It is not the Tropical zodiac, which has twelve equal signs counted from the equinox. It is not a Sidereal zodiac either: Sidereal zodiacs also have twelve equal 30° signs, counted from a starting point set by the stars, as Tropical vs Sidereal zodiac explains.

Constellation dates also drift slowly across the calendar over the centuries, because of the slow wobble of Earth's axis. The Tropical zodiac stays tied to the equinox, so this drift does not move Tropical sign dates. The Sidereal page covers the wobble, called precession, and the offset it creates between the two zodiacs.

This site offers no thirteen-sign calendar and no Ophiuchus personality profile. Its method uses twelve Tropical signs, and placing a planet in Ophiuchus is not part of it.

Checking your own sign

• Your Tropical Sun sign is in the planet list of the free birth chart. For the usual date ranges, see zodiac sign dates.
• A birthday near a change of sign needs a birth time and place to settle it; born on the cusp shows how.
• A Vedic or Sidereal app with a different sign is using another zodiac, not a thirteen-sign scheme. Tropical vs Sidereal zodiac explains the difference.
• A post saying you are now an Ophiuchus describes a constellation, not a sign. Nothing in your Tropical chart has changed.

The Zodiac Signs library lists every guide, including the meaning of each of the twelve signs.

Related articles

• Tropical vs Sidereal Zodiac: Why Your Sign Can Change Between Systems
• Zodiac Sign Dates (All 12 Signs and Their Dates)
• Born on the Cusp? Why Zodiac Sign Dates Shift From Year to Year
• Which Zodiac Sign Is the Rarest?
• Zodiac Signs Explained (All 12 Signs and What They Mean)

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