What’s Your Sign? OR How The Zodiac Came To Be

On January 13, 2011, Minneapolis Community and Technical College astronomy instructor Parke Kunkle caused a stir by announcing that horoscopes are wrong because the zodiac has shifted. Not only do most people ‘belong’ to the sign immediately before the one they are traditionally assigned to, but there is a 13th ‘sign,’ Ophiuchus.

This then, is an ideal time to tell the story of what the zodiac is and how it came to be.

The Story of the Solar System
The Earth orbits the sun once a year.  This orbit defines a plane in space.  That plane, projected against the background stars, is a line in our sky which astronomers call the ecliptic.  The plane of Earth’s orbit contains the sun, so the Sun always appears on the ecliptic in our sky.

The solar system itself formed from a spinning disk of dust that flattened out as it spun.  As a result, the solar system today is so flat that all planets orbit almost (although not exactly) in the same plane.  The planet with the greatest inclination (deviation from the plane of Earth’s orbit) is Mercury, and it’s off by just seven degrees.  All planets, therefore, always appear near the ecliptic in our sky.

The best theory for the moon’s formation posits that shortly after Earth had formed, a Mars-sized body dubbed Theia crashed into Earth, throwing off debris which formed the moon.  Theia, like most everything else in the solar system, had been orbiting near Earth’s orbital plane.  As a result, our moon orbits within about five degrees of Earth’s orbital plane.  In our sky, then, the moon always appears within about five degrees of the ecliptic.

Can’t see the video? Click here.

With our sun, moon, and all planets near the same plane, only a small set of stars–those aligned with the ecliptic–can ever appear near them in the sky.

The First Astrologers
Patterns formed from these stars were therefore of great importance to observers of antiquity.   Those that we use today go back to Mesopotamia, particularly Babylonia, in about 1370 BCE.  It was about that time that Babylonians created a text called MUL.APIN, which lists all of their constellations as well as the times of year when each constellation rose with the sun. (MUL.APIN, meaning ‘The Plough,’ is the name of the first constellation listed.)

Tablet 1 of MUL.APIN also includes a list of all constellations near the path of the moon in the sky–a forerunner of our zodiac.  The 18 (or 17) star patterns on that path are:

1)  The Star Cluster                                      (The Pleiades)

2) The Bull of Heaven                                 (Taurus)

3) The Loyal Shepherd of Heaven        (Orion)

4) The Old Man                                             (Perseus)

5) The Scimitar                                             (Auriga)

6) The Great Twins                                      (Gemini)

7) The Crayfish                                             (Cancer)

8) The Lion                                                     (Leo)

9) The Seed Furrow                                    (Virgo)

10) The Scales of Heaven                         (Libra)

11) The Scorpion                                         (Scorpius)

12) Pabilsag (a Babylonian god)           (Sagittarius)

13) The Goat-Fish                                       (Capricornus)

14) The Great One                                       (Aquarius)

15) The Tails                                                 (Pisces–one of its fish)

16) The Great Swallow                              (part of Andromeda and Pisces–the other fish)

17) Anutitum (a goddess)                        (part of Andromeda)

18) The Hired Man                                       (Aries)

There is some disagreement as to whether patterns 15 and 16 represent one or two constellations, hence the uncertainty as to whether the list has 17 or 18 members.

The Ancient Greeks, by about the sixth century BCE, had modified that list and produced a zodiac more like the one we use today.  They did so by leaving out stars in Orion, Perseus, Auriga, and Andromeda, which are a bit off the ecliptic itself (although the moon, which deviates by up to 5 degrees, can pass through them).

The Greeks also treated the Pleiades and Taurus as one constellation.  Virgo, the Virgin, is nearly always depicted with a stalk of wheat in her left hand, revealing her association with agriculture, like the furrow.  Babylonians had depicted Pabilsag as a composite creature armed with a bow and arrow; the Greek centaur shooting an arrow which we call Sagittarius is a simplification of this.

Babylonians often associated the Hired Man with Dumuzi, a legendary shepherd.  This may have influenced the change from ‘Hired Man’ into Aries, the Ram. The Ancient Greeks made the Babylonian ‘Scales’ constellation into the claws of Scorpius, the Scorpion, but the Romans reintroduced the Scales, putting the zodiac in its current form.


Of all the objects to appear only in the zodiac, by far the most important was the sun.  By noting which zodiacal constellations rose just before the sun and set just after the sun, early observers could use the changing position of the Sun against the background stars as a guide to the seasons.

Early lists of Babylonian patterns listed in MUL.APIN, possibly reflecting incipient stages in its formation, typically include the Bull, possibly indicating plowing season, the Lion, perhaps a symbol of the oppressive summer sun, since the sun rose with these stars in summer, the Scorpion, an emblem of death representing autumn, and the Water Bearer, representing the rains of winter.  Also often appearing on these early partial lists are the farrow and the goat/fish.  The former could represent the harvest season which follows the oppressive heat represented by the lion.  The latter is likely to represent Ea, Babylonian god of the waters, as the goat and the fish are animals associated with him.

When astrologers began using the positions of the planets, sun, and moon to describe people’s personalities, they focused on the sun.  The zodiac sign behind the sun (and thus not visible at night) on someone’s birthday was supposed to be most influential in determining that person’s character and destiny.  Although no evidence has established any connection between the apparent position of the sun and personality, belief in ‘sun signs’ continues to this day.

However, the stars’ positions in the 21st century are not the same as in antiquity.
As Earth orbits the sun, it wobbles.  After all, Earth could spin without wobbling only if no other forces whatsoever were acting on it, which is not the case.  However, Earth’s wobble is not as chaotic as it might be because we have a Moon relatively large for a planet as small as Earth.

With the Moon as a ‘counterweight,’ the Earth’s wobble becomes a more orderly precession in which the Earth’s axis describes an apparent circle on the sky once every 26,000 years.  This same precession causes the position of the sun on a given date to shift slightly–by about one degree every 72 years.  Since millennia have passed since Babylonians created the zodiac (about 1370 BCE) and since Romans finalized it (about 1 CE), the sun no longer aligns with the same patterns during the same seasons.

This brings us back to Kunkle’s announcement a few weeks ago.
It turns out that the dates traditionally associated with the ‘sun Signs’ are valid only for about the year 1 CE.  In general, the constellation actually behind the sun on your birthday is the one immediately before your traditional ‘sign.’  For example, astrologers would call me a Gemini, but the sun was in fact aligned with the stars of Taurus, the Bull, on my birthday.  You can compare the traditional dates and the actual constellations here. (The table is towards the bottom of the page).

Under the Milky Way
Creative Commons License photo credit: jurvetson

This is not a new discovery.
The ancient Greek astronomer Hipparchus noted that the stars Spica and Regulus were in slightly different positions in his time than on his predecessors’ star maps.  From this, he was able to deduce in the second century BCE that precession was occurring.  Astronomers have thus known of this effect for over two millennia.

So have astrologers, who maintain that they can still cast horoscopes because their ‘signs’ refer to fixed sectors of the sky and not to constellations.  As it happens, the traditional dates do roughly reflect when the sun would have aligned with the constellations about 2000 years ago.  Astrologers fail to explain why the constellations’ positions of 2,000 years ago might be magically relevant, however.

In 1930, astronomer Eugène Delporte helped fix the official constellation boundaries used by the International Astronomical Union.  These boundaries place a sizable chunk of the ecliptic in the constellation of Ophiuchus, a legendary healer who holds a large snake (Serpens) and stands on top of Scorpius, the Scorpion.  His stars are not on the Path of the Moon in MUL.APIN, although stars at some distance from the ecliptic, such as those in Orion or Perseus, are.  However, Ptolemy included this pattern in his list of 48 constellation in the Almagest.  Traditional skymaps of antiquity usually show the ecliptic passing through the Scorpion’s upper claw and legs, with Ophiuchus superimposed on Scorpius and standing on the ecliptic as if balancing on a high wire.  This is what may have influenced Delporte to assign most of that section of the ecliptic to Ophiuchus.

The idea of Ophiuchus as the ’13th sign’ is not new either.
Astronomers have been using Ophiuchus to point out the arbitrariness of astrology for at least 40 years.  Ophiuchus has been standing on the ecliptic for millennia, his right foot much closer to the planets than Scorpius’ stinger.  If the band of the ecliptic has powers over us, why doesn’t Ophiuchus partake of that power?  Several other constellations come near (but are not on) the ecliptic, including Cetus the Whale and Sextans, the Sextant.  The Moon and planets, which deviate by a few degrees from the ecliptic, can appear in them.  Should we factor them in as well?

Astrology vs. Astronomy
The ‘new’ dates for the zodiac signs and the ’13th sign’ Ophiuchus serve to underscore the difference between astrology and astronomy.

Astronomy is a science.  Astronomers study real planets, stars, and galaxies to learn about the real universe around us.

Astrology is myth-making.  The real positions of the stars do not matter to astrologers because astrology has more to do with mankind’s psychological needs. These include the need to see patterns and impose meaning and order onto the world and the need to feel in control of our surroundings.  Astrology thus offers the comfort of feeling that apparently random events might be predictable and controllable.

But since the astrologer’s predictions are ‘..not in our stars, but in ourselves,” as Shakespeare might say, astrology offers none of the wonder and excitement that comes from seeing the celestial bodies as they actually are, apart from our needs and desires for influence.  For that, I recommend astronomy. 

Eight is Enough?

Creative Commons License photo credit: CommandZed

Two years ago this month, the International Astronomical Union adopted a new definition of ‘planet’ which excludes Pluto. Not only do I, as Planetarium Astronomer, continue to get questions about Pluto’s ‘demotion’, but scientists themselves continue to debate it. Right now (August 14-16, 2008), a conference called “The Great Planet Debate:Science as Process” is underway at the John’s Hopkins University Applied Physics Laboratory in Laurel, Maryland. The saga of Pluto and of the definition of ‘planet’ offers some insight into our solar system and into how science works.

northern tier sky
Creative Commons License photo credit: truello

The definition of ‘planet’ has changed before. Ancients looked at the sky and saw that certain ‘stars’ in the sky changed position, while most stars seemed to form the same patterns all of the time. The Ancient Greeks called the moving stars ‘planetes‘, or wanderers–this is the origin of the word. The Moon, too, appears near different stars each night. The Sun’s apparent motion is less obvious, since we don’t see the Sun and stars at the same time. Careful observers, however, can see that different stars rise and set with the Sun at different times of year. The full list of ‘planetes’, then, included the Sun, the Moon, Mercury, Venus, Mars, Jupiter, and Saturn. (Astrologers still use this archaic definition of planet).

Thanks to Copernicus and Galileo, people began to realize that the Sun, not the Earth, was the center of the solar system. The definition of ‘planet’ changed from ‘object which moves against the background stars’ to ‘object in orbit around the Sun’. The Sun and Moon, which had been planets, no longer were.

The position of Uranus, discovered in 1781, seemed to fit a pattern described by astronomers Johann Titius and Johann Bode. That same ‘Titius-Bode rule’ also predicted a planet between Mars and Jupiter, so when Giuseppe Piazza discovered Ceres at just the right distance in 1801, it was considered a planet. By 1807, four new ‘planets’ had been found between Mars and Jupiter (Ceres, Pallas, Juno, and Vesta). By the middle of that century, however, dozens of these new objects were being discovered; up to 100 had been found by 1868. It thus became clear that astronomers had in fact found a new category of solar system object. Astronomers adopted the term ‘asteroid‘, which William Herschel had recommended in 1802; ‘planet’ was redefined to exclude very small objects that occur in bunches. This is how science works; we must constantly revise even long standing definitions as we learn more about the universe around us.

In the late 19th century, astronomers noticed that Uranus and Neptune seemed to deviate ever so slightly from their predicted positions, suggesting that another planet was perturbing them. in 1906, Percival Lowell started a project to find the culprit, which he called ‘Planet X’. In 1930, Clyde W. Tombaugh located Pluto in sky photographs he took at Lowell Observatory in Arizona. It soon became apparent, however, that Pluto was not massive enough to influence the orbits of Uranus or Neptune. Throughout the mid 20th century, astronomers continued to revise Pluto’s estimated size downwards. From 1985 to 1990, Pluto’s equator was edge on to us, such that we saw its moon Charon pass directly in front of and behind Pluto’s disk. This allowed scientists to measure Pluto’s diameter more precisely, proving that it had not been the Planet X that Percival Lowell sought. Pluto’s diameter is just under 2400 km, a little less than the distance from the Rio Grande to the US/Canadian border. Pluto’s discovery, it turns out, was an accident.

In addition to small size, Pluto has an unusual orbit. Planetary orbits are ellipses rather than perfect circles. The eccentricity of an ellipse indicates how ‘out-of-round’ it is on a scale from 0 (perfect circle) to 1 (parabola–far end at infinity). Pluto’s orbit has an eccentricity of about 0.25, much greater than that of planets such as Earth (0.01) or Venus (0.007). The planets orbit nearly (but not exactly) in the same plane; Mercury‘s orbit, inclined by 7 degrees, is the most ‘out of line’. Pluto’s orbit, however, is inclined by 17 degrees.

Released to Public: Solar System Montage (NASA)

Behold: a pluto-less solar system.
Creative Commons License photo credit: pingnews.com

We divide the planets of our solar system into two categories: the inner planets (Mercury, Venus, Earth, and Mars) which are made mostly of rock, and the outer planets (Jupiter, Saturn, Uranus, and Neptune) which are gas giants with no solid surface. Pluto, however, fits in neither of these categories, as it is made of ice and rock (by some estimates, it’s 70% rock and 30% ice; by others, it’s about 50/50).

With its small size and abnormal orbit and composition, Pluto was always a misfit. Textbooks noted how Pluto fit in with neither the rocky inner planets nor the gas giants in the outer solar system. Still, Pluto remained a ‘planet’ because we knew of nothing else like it. There was simply no good term for what Pluto is.

That began to change in 1992, when astronomers began finding Kuiper Belt objects. The Kuiper Belt is a group of small bodies similar to the asteroid belt. Kuiper Belt objects (KBOs), however, orbit beyond Neptune’s orbit. Also, the Kuiper Belt occupies more space and contains more mass than does the asteroid belt. Finally, while asteroids are made mostly of rock, KBOs are largely composed of ice, including frozen ammonia and methane as well as water–just like Pluto. In addition to the Kuiper Belt proper, there is a scattered disc of objects thought to have been perturbed by Neptune and placed in highly eccentric orbits. Objects in the Kuiper Belt, scattered disc, and the much more distant Oort Cloud are together called Trans-Neptunian Objects (TNOs)

With the discovery of more and more KBOs, astronomers began to wonder if Pluto might fit better in this new category. Not only was the composition similar, but there is even a group of KBOs called plutinos, with orbits similar to Pluto’s. In the Kuiper Belt and the scattered disc, astronomers began to find objects approaching Pluto’s size, including Makemake, Quaoar, and Sedna.

Pluto can't get no respect
Pluto takes advantage of the wildly (?)
popular LOLcats to plead its case
with mankind.
Creative Commons License photo credit: the mad LOLscientist

To call Pluto a planet, but not these others, seemed arbitrary.

Finally, in 2005, a team of astronomers located Eris, which is slightly bigger than Pluto. Clearly, Eris and Pluto are the same kind of thing; either both are planets or both are not. If they both are planets, however, then should we include Quaoar et al., above? We have only just begun to explore and understand the Kuiper Belt and the scattered disc. Might we eventually find dozens of new ‘planets’ like Eris? Hundreds? Thousands?

This is what led the International Astronomical Union to reconsider the definition of ‘planet’ two Augusts ago. The IAU decided it was simpler to limit the number of planets to eight (Mercury through Neptune) and classify Pluto (and Eris, Quaoar, et al.) among the Trans-Neptunian objects. A new term, “dwarf planet,” includes the biggest asteroids and TNOs–those big enough to have assumed a spheroid shape. Still, other astronomers remain dissatisfied, hence the discussion going on in Maryland now.

There are two things we must keep in mind if we’re wondering when the Pluto question will be ‘resolved.’ First, decisions and conclusions of scientists are not holy edicts to be obeyed and never questioned. Quite the contrary, all such conclusions are provisional, pending new discoveries and better information. Any new decision reached this weekend is likely to be revised when the IAU meets again in 2009, and again in 2015 when the New Horizons mission arrives at Pluto. If it were any other way, science could not function.

Secondly, all categories which help us organize and understand things in our minds (including ‘planet’) are pure human inventions that only roughly correspond to nature. Although we need to categorize the things we see, nature does not; no matter how we classify objects, nature presents us with borderline cases that challenge us. Pluto is the same thing today as it was in 2005 or even before it was discovered in 1930. We need to distinguish our need for neat categories from our need to explore and describe nature.

Proud to be a space cadet? Learn more about astronomy:
Dust off your telescope – or visit the George Observatory – to see what’s in the night sky this month.
Ten billion trillion trillion carats – the universe has great taste in diamonds
If it blew a hole in your roof, you’re on the right track – how do you tell a rock might be a meteorite?