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. 

How Far are the Stars? (part 1)

Under the Milky Way
Creative Commons License photo credit: jurvetson

During a recent planetarium show, I discussed the stars of the Summer Triangle (up all night long in summer, they are still high in the west at dusk in autumn). I mentioned that Deneb, apparently dimmer than the Triangle’s other two stars Vega and Altair, is actually much larger and gives off much more light. It just seems dimmer because it’s about 100 times farther away. This prompted the question, “How can we tell how far away the stars are?”

This is a very perceptive question.  We obviously cannot directly measure the distance to a star like you might measure the height of a wall.  Neither can we use an odometer like you have in your car, since no one has been to the stars.  By doing a little trigonometry, however, we can get reliable distances to the stars nearest to us.


This animation is an example of
parallax – as the viewpoint moves
side to side, the objects closer to
the camera appear to move faster,
while the objects in the distance
appear to move slower.
Image by natejunk2004
Can’t see the Image? Click here.

This geometric way to measure distance is called parallax and you can illustrate it for yourself quite simply.  Hold your finger in front of your face.  Now close your left eye, leaving the right eye open.  Then close the right eye and open the left.  Repeat this sort of blinking several times and watch how you finger moves back and forth compared to things in the background.  Bring your finger close to your face, and repeat the experiment.  Now hold your finger at arm’s length, and repeat.  Notice how your finger seems to move farther when it is close to your face.  In fact, if you could measure how far your finger moves against the background objects, you could calculate how far it is from your face.

We can do the same thing with nearby stars. If we observe a star at a particular time of year (for example, in January) and then again six months later (in this case, in July), we can define an isosceles triangle where the base is the diameter of Earth’s orbit and the sides are the distance to the star.  The vertex angle of this triangle equals the apparent change in the star’s position due to the Earth’s yearly motion.  One half of this isosceles triangle is a right triangle where one leg is the known Earth-Sun distance (one AU, or astronomical unit), and the hypotenuse is the distance to the star. The angle opposite the one AU leg, which is one half the star’s apparent motion, is the parallax angle p.  Basic trigonometry then yields

sin p = 1 AU/ d,

where d is the distance to the star in question.  Since p is tiny for all stars, the small angle approximation sin p =p is valid.  We can define a standard distance by asking how far away a star would be if it had a parallax of one arcsecond (1/3600 degree).  Plugging d= 1 arcsecond into the equation gives us

d= 206265 AU,

where 206265 represents the conversion factor between radians and arcseconds, given that the approximation sin p =p holds only if the angle is in radians.  We have now defined the parsec, the distance at which a star has a parallax angle of one arcsecond.  It now becomes easy to determine stellar distances compared to this standard distance.  First, measure the parallax of a star in arcseconds.  Then take one over that value, and you have the distance to that star in parsecs.  By the way, although the general public prefers to think of distances to stars in light years, modern astronomers never quote them that way.  The parsec, directly related to a measurable quantity, is a much more preferable unit.  (One parsec is about 3.26 light years.)

This way of measuring distance has a limitation: most stars are too far away to have measurable parallaxes.  An imaginary sphere with a radius of one parsec centered on our Sun would contain precisely one star–the Sun.  The nearest star system to ours, that of Alpha Centauri, is 1.34 parsecs away, and therefore has a parallax of only about 0.75 arcseconds.  More distant stars have much smaller parallaxes, too small for most Earth based equipment to detect.

This began to change in 1989, however, when the European Space Agency (ESA) launched the High Precision Parallax Collecting Satellite, or Hipparcos.  The name was chosen in honor of the ancient Greek astronomer Hipparchus, who put together the first star catalog of the western world.  The first space experiment devoted to astrometry, Hipparcos catalogued 118,218 stars between 1989 and 1993.  The Hipparcos Catalogue was published in 1997.  Among its many scientific results, Hipparcos helped astronomers determine accurate proper motions (a star’s true motion through the galaxy) and was able to measure good parallaxes for stars up to about 1,000 parsecs away.

But, you may wonder, “What about stars more than a few thousand parsecs away from us? ”  Keep in mind that our Galaxy is about 100,000 light years, or just over 30,000 parcsecs across.  Most stars, to say nothing of distant galaxies, are so distant that not even Hipparcos can measure their infinitesimal parallaxes.  Fortunately, there are objects known as “standard candles”–celestial objects with a known intrinsic brightness.  Comparing their intrinsic brightness with their apparent brightness in our skies lets us figure out the distances to them.  In a later post, I’ll discuss how we identify and use “standard candles” to determine distances to much more distant stars and even to other galaxies.