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.

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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. 

Go Stargazing! May Edition

pleiades
 The Pleiades
Creative Commons License photo credit: Evilnick

Have you ever wondered when the ‘Dog Days’ begin?  The term is based on Sirius, the Dog Star.  In the time of the Ancient Egyptians, Sirius rose right before sunrise at the summer solstice, after having been invisible for about 2.5 months.  Egyptians began their calendar years with this event, and also considered Sirius a herald of summer.  Ancient Greeks and Romans believed that Sirius, as the brightest star in the night sky, helped cause the hot and oppressive weather of summer when it rose with the Sun.

Because Earth wobbles as it spins, we no longer see the constellations in quite the same places the ancients did (this is called precession of the equinoxes.)  As a result, Sirius now rises with the Sun in mid-August.  So that the term refers to roughly the same time of year, modern folklorists (such as the Farmer’s Almanac) take the Dog Days to end with the reappearance of Sirius and to begin when Sirius leaves the evening sky.  May is the month to watch Sirius slowly go away.  It’s easily visible in the southwest tonight and next week.  As May continues, notice Sirius gets lower and lower each night.  By Memorial Day, you’ll need perfect viewing and no trees or buildings to the southwest in order to see it.  How long can you follow it?  All the way to May 31? Once it’s gone, the Dog Days are upon us.

Saturn remains well placed in the evening sky this month.  Look for it in the south at dusk.  Mercury is briefly visible at dusk for the first week in May.  Mercury is bright enough to appear in twilight while most stars aren’t.  Look low in the west northwest at dusk, right over the point of sunset. A compact cluster of stars called the Pleiades is nearby. 

orion

 The Constellation Orion
Creative Commons License photo credit: Wisze

Jupiter, in the south-southeast at dawn, is the brightest thing in that part of the sky unless the Moon is nearby (as it is on May 17).  Venus is a dazzling morning star this month.  Look east right at day break for the brightest thing there except for the Moon.  Venus remains the ‘morning star’ for the rest of 2009.  Mars remains close to the horizon at dawn much of the spring, and takes longer to fully emerge into the morning sky.   

Look in the south at dusk for stars in the shape of a backwards question mark, with a right triangle to the left of that.  These are the stars in Leo the Lion.  Saturn is under the ‘right angle’ in that right triangle.  The Big Dipper is highest on spring evenings.  From the Big Dipper’s handle, you can arc to Arcturus.  Arcturus, in the east at dusk, is the fourth brightest star we ever see at night and will be the brightest one left once Sirius sets. Continuing the curve of the Big Dipper’s handle past Arcturus, you can “speed on to Spica“, a star low in the southeast at dusk.  Spica represents a stalk of wheat held by Virgo the Virgin, who is the harvest goddess.

Dazzling Orion leaves the evening sky this month; you can see him only right at dusk in early May.  His belt now points right to Aldebaran, the eye of Taurus the Bull, which sets with Orion in the west.  The Dog Stars Sirius and Procyon also set in the west, to Orion’s left.  Sirius is the brightest star we ever see at night.  Now above Orion are two stars of similar brightness less than five degrees apart.  These are Castor and Pollux, marking the heads of Gemini, the Twins.

Moon Phases in May 2009:

1st Quarter                         May 1, 3:44 pm
                                         May 30, 10:22 pm

Full                                    May 8, 11:01 pm

Last Quarter                       May 17, 2:27 am

New                                   May 24, 7:11 pm