Astronomy

How many constellations in the Zodiac?

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?


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, 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.

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.


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


Astrology and science

Astrology consists of a number of belief systems that hold that there is a relationship between astronomical phenomena and events or descriptions of personality in the human world. Astrology has been rejected by the scientific community as having no explanatory power for describing the universe. Scientific testing has found no evidence to support the premises or purported effects outlined in astrological traditions. [1]

Where astrology has made falsifiable predictions, it has been falsified. [1] : 424 The most famous test was headed by Shawn Carlson and included a committee of scientists and a committee of astrologers. It led to the conclusion that natal astrology performed no better than chance. Astrologer and psychologist Michel Gauquelin claimed to have found statistical support for "the Mars effect" in the birth dates of athletes, but it could not be replicated in further studies. [2] : 213–214 The organisers of later studies claimed that Gauquelin had tried to influence their inclusion criteria for the study by suggesting specific individuals be removed. It has also been suggested, by Geoffrey Dean, that the reporting of birth times by parents (before the 1950s) may have caused the apparent effect.

Astrology has not demonstrated its effectiveness in controlled studies and has no scientific validity, [1] [3] : 85 and is thus regarded as pseudoscience. [4] [5] : 1350 There is no proposed mechanism of action by which the positions and motions of stars and planets could affect people and events on Earth in the way astrologers say they do that does not contradict well-understood, basic aspects of biology and physics. [6] : 249 [7]


The zodiac stretches about 8-9 degrees either side of the ecliptic, and is the region of the sky where we can find the Sun, Moon and planets (except for Pluto). The zodiac is narrow because most of the planets have orbits that are only slightly inclined to that of the Earth. The exception is Pluto, whose orbital inclination of 17 degrees takes it out of the zodiac during part of its orbit.

There are 13 constellations through which the ecliptic passes. Known as the Zodiacal Constellations, they are:

Aries Taurus Gemini Cancer Leo Virgo Libra
Scorpius Ophiuchus Sagittarius Capricornus Aquarius Pisces

The above explanation of how you are assigned your star sign is not quite correct these days. These star signs were developed over a thousand years ago and, due to the precession of the equinoxes, the Sun is no longer in the constellation originally attributed to each star sign. For example, if you were born an Aquarian 3,000 years ago, the Sun was in the constellation Aquarius. If you are born an Aquarian now, the Sun will actually be in the constellation Capricornus.

You will also note that under the constellation boundaries defined by the International Astronomical Union, the ecliptic passes through the constellation Ophiuchus – which is not included in the astrological signs of the zodiac, however which is considered an astronomical Zodiacal Constellation.

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The decans

Two other astronomical reference systems developed independently in early antiquity, the lunar mansions and the Egyptian decans. The decans are 36 star configurations circling the sky somewhat to the south of the ecliptic. They make their appearance in drawings and texts inside coffin lids of the 10th dynasty (about 2100 bce ) and are shown on the tomb ceilings of Seti I (1318–04 bce ) and of some of the Rameses in Thebes. The decans appear to have provided the basis for the division of the day into 24 hours.

Besides representing star configurations as decans, the Egyptians marked out about 25 constellations, such as crocodile, hippopotamus, lion, and a falcon-headed god. Their constellations can be divided into northern and southern groups, but the various representations are so discordant that only three constellations have been identified with certainty: Orion (depicted as Osiris), Sirius (a recumbent cow), and Ursa Major (foreleg or front part of a bull). The most famous Egyptian star map is a 1st-century- bce stone chart found in the temple at Dandarah and now in the Louvre. The Zodiac of Dandarah illustrates the Egyptian decans and constellations, but since it incorporates the Babylonian zodiac as well, many stars must be doubly represented, and the stone can hardly be considered an accurate mapping of the heavens.


How To Learn The Zodiac Signs Constellations!

The Zodiac plays a big part in the world, especially in pop culture, astrology, and even mythology. But how much do you actually know about the Zodiac? Join us as we dive deep into it all!
13. What Is The Zodiac?
There are actually quite a few Zodiacs out there, including one referencing the Greeks (which is what we’ll be referring to here) and the Chinese Zodiac (for which they label a year after whatever animal is next in the Zodiac). But for the Greek one, what does it reference, what does it all mean?
To keep it very basic, the Zodiac is a series of 12 constellations that are in ecliptic, or the path of the sun, our sun. So thus these 12 are seen as special because the sun passes by them when it moves. Or at least, that’s how we perceive it here on Earth (as we’re technically the ones moving).
Now, as for where the word “Zodiac” comes from, that would be the Greek word “Zodiakos” which means “circle of animals”. Which is poetic because all of the constellations (save for one) represents living animals in our world today.
And because there are 12 constellations in the Zodiac, and 12 months of the year, we’ve assigned special dates to them based on the position of the sun when it passes through them and thus we get the ‘signs’. Which eventually led to horoscopes…and that’s another list entirely.
So, knowing all of that, shall we get started on the significance and details of each constellation?
12. Aries
Not to be confused with the Greek God Ares, Aries is a ram, and is passed by the sun between April 19th till May 13th. As for the stars that make it up, that would be Hamal, Sheratan, 41 Ari, Botein, HD 20644, ε Ari, 35 Ari, and γ2 Ari. Yeah, that’s a mouthful, isn’t it?
The constellation is quite a ways away, such as with its brightest star, Hamal, which is 66 light years away from where Earth it. Yet that proves how bright it is so that we can see it from Earth itself.
Aries the ram is depicted many different ways across many different mythologies, including ones in both western, Egyptian, and the Greek. In terms of Greek mythology, the ram known as Aries was one with a golden fleece, one that helped save a son from being sacrificed by his father after a conspiracy had been hatched to end them. The ram was later sacrificed to Zeus, and the fleece was given to him as a tribute, who later gave it to others, who then hung it in a cave guarded by a dragon.
Also though, for those who know their Greek tales, you’ll know that the Golden Fleece was the goal/reward of Jason and Argonauts, and they were able to steal it successfully.
11. Taurus
Is anyone feeling rather bullish today? You know…because is a bull…nevermind. In terms of the path of this constellation, it is crossed by the sun between May 14th till June 19th. The stars that make it up include Aldebaran, Elnath, Alcyone, ζ Tau, and θ2 Tau2. Try saying some of those names 5 times fast.
Taurus is actually quite far away from Earth, almost 400 light years away, so don’t expect to see it up close.
Not unlike Aries, there are many different depictions of Taurus the Bull in both mythologies aboard and Greek mythology itself. For example, there are many references to the bull being Zeus, who was known to shapeshift all the time to be close to his many mistresses. One story even states that he turned one of this mistresses to a bull to hide her from Hera (who was known for jealously and lashing out at said mistresses). Others though believe that it could be a reference to the Cretan Bull that Heracles had to deal with in one of his many labors.
Either way, the bull became not just a constellation, but a figure of many different mythologies, including ones worshiped to this day.
10. Gemini
Ok, how many twins in the house? No, really, I’m curious, and it’s poetic because Gemini is a term that is used in pop culture and culture the world over to talk about the “twins” of the world. Including a recent movie “Gemini Man”.
As for the stars, there are 85 stars that technically make up the constellation, but the main ones are Pollux, Alhena, Castor A, Tejat Posterior and Castor B…to name a few. Again, there’s 85 of them and that’s just the ones that we can see!
In terms of distance, the closest star of the constellation Tejat Posterier is 230 light years from Earth.
Going to the Greek Mythology texts for the origin of the twins…well let’s just say it’s a bit…messy. You see, while they were twins, each boy, Pollux and Castor, had a different father. I’m not saying a word…
Anyway, their mother was Leda, but Zeus was the father of one, and the Spartan King Tyndareus. Again, I’m not going there. The problem here was that because Pollux was a demigod, that made him immortal, but Castor was mortal thanks to his human father. Thus, when Castor died, Pollux implored Zeus to let him live forever.

#astrology #zodiacsigns #insanecuriosity #zodiacconstellations #constellations


Star Groupings and Asterisms


Some of the more familiar "constellations" are technically not constellations at all. For example, the grouping of stars known as the Big Dipper is probably familiar to most, but it is not actually a constellation. The Big Dipper is part of a larger grouping of stars called the Big Bear (Ursa Major) that is a constellation.

A well-known grouping of stars like the Big Dipper that is not officially recognized as a constellation is called an asterism.


How many constellations in the Zodiac? - Astronomy

How many constellations are there now?

There are 88 official constellations. But astronomers haven't made up new constellations for hundreds of years! When new stars are discovered, they are considered to be a part of whatever constellation they are closest to.

This page updated on June 27, 2015

About the Author

Kate Becker

With more than a decade of experience as a science writer, Kate Becker has written on a wide variety of science and science policy subjects for web, print, radio, and television, with an emphasis on astronomy and physics. As a researcher for NOVA and NOVA scienceNOW, the nation's premiere science documentary series, Kate investigated everything from human hibernation to invisibility cloaks. She studied physics at Oberlin College and astronomy at Cornell University, and she's had the good fortune to observe with the Arecibo Observatory in Puerto Rico and the Very Large Array in New Mexico, two of the very best places on this pale blue dot of a planet.


The Zodiac Constellations

Most of us know the names of the 12 zodiac constellations that represent our horoscopes, but did you know that the International Astronomical Union recognizes 88 constellations covering the entire northern and southern sky? Those are a lot of constellations. However, the Zodiac constellations are special and unique for a reason that we will soon reveal to you. First, let’s find out when our ancestors started naming constellations.

History of Constellations

Just as our closest star the Sun moves across our sky, so do the stars. At different times of the year, different star patterns can be seen in our night sky. People long ago discovered that these changing patterns in their starry nights could be used to mark the change of seasons and to guide them at night. Like a real-life map that you can use any time at night when you look up to the sky. So who were the first people to name these star patterns that we now call constellations?

Most of the stars in our night sky have names originating from the ancient Greek or Roman language, however, people had been mapping the sky a long time before that. The Greeks adopted this system from the Babylonians who took it from the Sumerians 3,000 years prior. And archeologists have found carvings on a cave wall in France from over 17,000 years ago that clearly indicate drawings that map out the star clusters of Pleiades and Hyades.

Nowadays astrologers recognize 88 constellations, but actually these are more like clusters of stars that astronomers use to be able to communicate with each other about different regions of the sky. A lot of people confuse these with constellations, such as in the case of the Ursa Major, when it is actually recognized by astronomers as an asterism. But what makes the Zodiac constellations special?

Constellation Names and Zodiac Signs

As the Earth rotates, the sun, the moon, and planets travel on a set path through the sky known as the ecliptic. The list of 13 constellations they pass through are known as the stars of the zodiac. Yes, that’s right – there are 13, not 12 zodiac constellations. Their names and order in which the sun, moon, and planets travel through them are:

  • Capricorn
  • Aquarius
  • Pisces
  • Aries
  • Taurus
  • Gemini
  • Cancer
  • Leo
  • Virgo
  • Libra
  • Scorpio
  • Ophiucus
  • Saggitarius

Astrology uses 12 of these constellations as signs of the zodiac, omitting Ophiucus, to make predictions about our lives, compatibilities, and future. But did you know that if you are born under a particular sign, the constellation it is named for is not visible at night? Instead, the sun is passing through it around that time of year, making it a daytime constellation that can’t be seen. For example, now we are at the end of October and although we have just entered the Scorpio zodiac sign period, the constellations that are visible at night are Aquarius, Capricorn and Saggitarius. But the cool thing about adopting a star from one of the Zodiac Constellations is that you can easily find it whenever the constellation is visible in our night sky. Do you know somebody that is into horoscopes? Check out our Zodiac gift pack at Cosmonova. We guarantee your star will brighten up your loved one’s day and not to mention, you will immortalize their name in the sky forever.


8. References

The rotation of the Earth around its axis, which causes the alternation of day and night on its surface, is similar to that of a spinning top. The rotation axis of a spinning top itself rotates around the vertical direction, and in a similar way the rotation axis of the Earth changes its orientation, but only very slowly. The top of a spinning top traces out small circles in the air as the rotation axis changes direction, and the rotation axis of the Earth traces out a curve in the sky that is almost a circle, too. It takes about 26,000 years for the rotation axis to complete one cycle.

Because of the change in the rotation axis, the point in the sky among the stars where the Sun is at the beginning of spring (the vernal equinox) and the other seasons changes around, too. This is called the precession of the equinoxes, and it changes the relation between the stars and the seasons, i.e., which stars you can see in what season, and their times of rise and set.

The most common coordinate systems in the sky (for measuring the position of stars and planets and such e.g., the equatorial and ecliptic coordinate systems, but not the galactic or horizontal systems) are linked to the position of the vernal equinox, so even if a particular celestial object does not move at all its coordinates in those coordinate systems slowly change with time. This means that it is important to specify the time for which the used coordinate system is valid. This time is called the equinox, for instance "the equinox of 2000.0".

The official boundaries of the constellations were defined for a particular, fixed equinox, I think the one of 1875. If you want to know what constellation a particular direction in the sky belongs to, you first have to translate its coordinates to the equinox of 1875, i.e., to the coordinate system that was valid in 1875. This means that the precession of the equinoxes cannot be the reason why a star moves from one constellation to another one.


What Are Constellations?

Constellations are patterns in the night sky as observed from Earth. We know that the stars within constellations share no definite relationship with one another, other than they can be seen at certain times in the same area of the sky. The stars in one constellation are not necessarily even near one another in space constellations are merely the product of perspective.

But constellations also change. Everything in space is in motion, so things shift over time. According to Discovery.com, "Stargazers of the future will look into a different night sky. That's because the stars are constantly moving relative to each other."

Whether or not you believe in astrology, you can use star charts to see the constellation each sign is based on in the night sky at the appropriate time of year. The constellation related to your astrological sign, however, won't necessarily be visible in the sky on your birthday. SPACE.com explains: "Ironically, if you are born under a particular sign, that constellation it is named for is not visible at night. Instead, the sun is passing through it around that time of year, making it a daytime constellation that can't be seen."


How many constellations in the Zodiac? - Astronomy

Can you please tell me why it is that star constellations have now 'shifted' into the next constellation, as such everyone's star signs are wrong. I know it's to do with the 'wobble' of Earth but find it hard to explain to my friend.

We have to start from the fact that Earth is not a perfect sphere but is a bit fatter at the equator and flattened at the poles. If we make a model of the Solar system with planetary orbits pictured as horizontal, Earth would look like a flattened top spinning on an axis that is somewhat tilted relative to the vertical direction. All the other bodies (the Sun, the Moon, the planets) are in this orbital plane and their gravity tries to "straighten up" Earth and bring the fattest part in the same plane with them. This is analogous to how gravity wants to topple a top spinning on a table. In both cases, the spin (technically, angular momentum) of the rotating body prevents it from falling or straightening up, but the outside force still has to produce some effect. For reasons that are non-obvious and have to do with vectors etc. the result of all this is wobble (technically "precession"). This means that while the tilt between Earth's axis and orbit stays the same, the direction of the axes execute a circular motion with a period of 26,000 years.

Now, the seasons are caused by Earth orbiting the Sun on an inclined axis. So when Earth is in the part of orbit where the Northern hemisphere tilts towards the Sun we have the summer here (and you guys have winter down there) and vice versa. The star signs were defined by the constellation that was behind the Sun when one was born. Obviously, the part of the Zodiac (the ring of constellations in the plane of Earth's orbit) towards which the Northern hemisphere leans contains the star sign that applies to June and so on for the whole year. But if the direction of Earth's axis changes, the constellation that appears behind the Sun during a particular season will also change. As Earth's axis wobbles, the star signs move forward through the year, making a full circle once in 26,000 years, which is close to a shift by one sign every 2000 years. The present Zodiac was introduced at Roman times, at the very beginning of Common Era. Since then about 2000 years have passed and the signs moved forward among the dates by about one sign. So if one's "official" sign is Libra, chances are that the Sun was in constellation Virgo at the time of his/her birth. The same applies to all other signs.

Now, this was astronomy. Astrologers who still use the old signs make distinction between "signs" and "constellations", claiming that the former don't have to have anything to do with the latter. I don't believe in astrology so my opinion cannot be objective, but I must say that I think this distinction is quite silly.

This page was last updated July 18, 2015.

About the Author

Matija Cuk

Matija works on the orbital dynamics of the lesser moons of Jupiter and Saturn. He graduated with his PhD from Cornell in November 2004 and is now working at the University of British Columbia in Canada.


These constellations are more familiar to us as the astrological ‘star signs’ (actually ‘sun signs’) under which a person is born. The star sign is determined by the position of the Sun ‘in’ a particular constellation at the time of birth. Put another way, as the Earth moves around the Sun in its orbit, each of the Zodiacal constellations will be hidden ‘behind’ the Sun at some point during the year. Whatever constellation is hidden behind the Sun at the time of your birth is known as your star sign. This is demonstrated in the diagram on the right.