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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?

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  • $\begingroup$ Not that I really care about the signs of the Zodiac. :-) $\endgroup$ – jvriesem Oct 3 '15 at 22:51
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    $\begingroup$ I'm voting to close this question as off-topic because it is primarily about the the astrological concept of "the zodiac" $\endgroup$ – James K Oct 4 '15 at 19:39
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    $\begingroup$ @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). $\endgroup$ – jvriesem Oct 4 '15 at 20:26
  • $\begingroup$ 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. $\endgroup$ – James K Oct 4 '15 at 20:31
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    $\begingroup$ @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. $\endgroup$ – jvriesem Oct 4 '15 at 20:37
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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

"Constellations ecliptic equirectangular plot" by Cmglee, Timwi, NASA - Own work, http://svs.gsfc.nasa.gov/vis/a000000/a003500/a003572. 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

Source: journeytothestars.wordpress.com

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.

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  • $\begingroup$ 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. $\endgroup$ – barrycarter Jan 5 '16 at 18:58
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The issue is the definition, as always. What is the Zodiac?

If you mean the zone where the sun stays in the sky during the year, a strip around the ecliptic, with current astronomical constellations, the solar path crosses 13 constellations. If you include the moon, Venus and the planets, the answer is 21 or 25, depending on the time scale.

Note that the definition is also dependent on the constellation patches and regions, so it is relative. Moreover, as the Universe moves, and stars move too, it could change in the past and it could change in the future. Millions or billions of years in the future, supposing Earth and the Sun are still alive.

Reference here http://www.ips-planetarium.org/?page=a_mosley1999b

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    $\begingroup$ Please read the tour You need to include your answer in the question. Link only answers may be deleted. $\endgroup$ – James K Sep 9 '17 at 19:41
  • $\begingroup$ It is done. Thanks! Indeed, I almost forgot that. Too much time without answering properly a question, sorry! $\endgroup$ – riemannium Sep 9 '17 at 19:53

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