How many constellations in the Zodiac?

  • The astronomical zodiac contains a bunch of constellations along the ecliptic.

    Some sources say there are 13 constellations in the astronomical zodiac. Other sources claim there are 12.

    According to NASA (source):

    If we count all the traditional constellations, there are 13 in the
    zodiac, not 12. So here are the dates (give or take one day each year)
    when the Sun is between Earth and each of these 13 constellations.)

    There seems to be some confusion out there between zodiac constellations (in astronomy) and zodiac signs (in astrology). From the research I've done so far (including the quote above), it seems as though one of them (Ophiuchus) is a "recent addition", possibly due to the Earth's precession since ancient times (source).

    What (officially?) are the constellations of the astronomical zodiac?
    Why do some sources list 12 and others list 13?

    Not that I really care about the signs of the Zodiac. :-)

    I'm voting to close this question as off-topic because it is primarily about the the astrological concept of "the zodiac"

    @JamesKilfiger: The Zodiac is an *astronomical term* used in modern astronomy, which is why I believe it to be on-topic. My question primarily asks how many constellations are in the *astronomical*--rather than the *astrological*--Zodiac (and why).

    If nobody did astrology then there would be no astronomy. However, "the zodiac" is not used in astronomy. Constellations are used as a convenient way of naming regions of the sky. But there is absolutely no astronomical significance to whether there are 12 or 13 constellations crossed by the path of the sun in the course of a year. The zodiac is a purely astrological concept. vote to close.

    @JamesKilfiger: Just because the ancients decided to attach metaphysical significance to the sky via astrology doesn't mean that astrology had to happen before astronomy. (But that's a side discussion.) Of course there is no astronomical "consequence" to the zodiac; that doesn't stop it from being taught in astronomy classes. It's an astronomical term borrowed from astrology.

    @JamesKilfiger: If you say that any post that mentions the zodiac is off-topic, why not all those discussing constellations, the naming of astronomical objects in ancient times, and so on?--theses also have their roots in astrology. The history tag is for the history of astronomy, which obviously includes astrology. I am editing the question to clarify my intent. Let me know how I could improve it.

    Close vote retracted, after some thinking. The zodiac is still related to astronomy.

  • aeroalias

    aeroalias Correct answer

    7 years ago

    There are thirteen modern constellations in the Zodiac. In modern astronomy, a constellation is a specific area of the celestial sphere as defined by the International Astronomical Union. In total, there are 88 constellations.

    Astronomy and Astrology are not the same thing. Astronomy is a science while Astrology is not. As such, I'll restrict myself to the historical and modern constellations of the Zodiac.

    According to Encylopedia Brittanica

    Zodiac (is) a belt around the heavens extending 9° on either side of the ecliptic, the plane of the earth’s orbit and of the sun’s apparent annual path.

    In historical astronomy, the zodiac is a circle of twelve 30° divisions of celestial longitude that are centered upon the ecliptic, the apparent path of the Sun across the celestial sphere over the course of the year. Historically, each of these divisions were called signs and named after a constellation: Sagittarius, Capricornus, Aquarius, Pisces, Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, and Scorpius.

    In 1930, the International Astronomical Union defined the boundaries between the various constellations, under Eugène Delporte , who,

    ... drew his boundaries along vertical lines of right ascension and horizontal parallels of declination. One governing principle was that all variable stars with an established designation would remain in that constellation, as requested by the IAU’s Variable Stars committee.


    "Constellations ecliptic equirectangular plot" by Cmglee, Timwi, NASA - Own work, Licensed under Public Domain via Commons.

    As a result, the path of the ecliptic now officially passes through thirteen constellations: the twelve traditional 'zodiac constellations' and Ophiuchus (which was one of the 48 constellations listed by the 2nd-century astronomer Ptolemy), the bottom part of which interjects between Scorpio and Sagittarius.

    Zodiacal Constellations


    So, the 13 constellations of the Zodiac are Capricornus, Aquarius, Pisces, Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Sagittarius and Ophiuchus.

    As seen from the (first) figure, the ecliptic also touches the edge of the constellation Cetus, though it in not usually included in the Zodiacal constellations.

    Note about Precession: Because the Earth in inclined (by $23.45^{\circ}$), it rotates like a top. This is called precession, which results in a shift in the position of the constellations relative to us on Earth. The result is that the 'Signs of the Zodiac' are off by about one month.

    Question: since the Zodiac is 9 degrees wide (presumably to account for Venus' inclination) according to Encyclopedia Brittanica, does it cross into other constellations as well? According to printed versions of the Old Farmer's Almanac, the moon sometimes crosses into Orion and a few other non-zodiac constellations.

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Content dated before 7/24/2021 11:53 AM