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. 

The Great Cosmic Year

Our Milky Way Galaxy..
Creative Commons License photo credit: Sir Mervs

One of the biggest challenges in teaching astronomy to kids – or even to the general public – is that astronomy involves numbers so big as to be virtually meaningless. Consider the age of the universe, for example. Our best data indicate that the Big Bang, where space and time began, occurred about 13.7 billion years ago. As very few of us have seen 13.7 billion of anything before, how can we appreciate how long a time that is?

One way is to use a scale-model. Just as we use globes because the real Earth is too big to look at, we can ‘shrink’ the 13.7 billion year history of the universe into one year. Imagine a Great Cosmic Year, in which the Big Bang occurs at 12:00:00 am on January 1 and the present moment is 11:59:59.9999999 pm on December 31. On this time scale, each day represents (13.7 billion/365) years, or about 37.5 million years. Our best estimates for when the events listed below occurred are approximate; the dates listed may need to be adjusted slightly in the future.

Still, locating the events in the history of the universe, the Sun, and the Earth on this calendar can give us a better sense of how much time is involved.

January 1, midnight The Big Bang occurs.

January 13 The oldest known star in our galaxy (designated HE 1523-0901) forms.

‘HE’ here refers to the Hamburg/ESO (European Southern Observatory) survey, in which the star is catalogued. Being about 100 times too dim to be seen with the unaided eye, the star has no common name. It is in the constellation Libra.

Planet Earth (III)
Creative Commons License photo credit: Aaron Escobar

January 4-27 Re-ionization occurs.

We take for granted that the universe is transparent; that we can look through space and see galaxies, stars, and other planets. However, once hydrogen atoms formed in the early universe, this would have been impossible, as hydrogen atoms readily absorb photons (light particles). After the first billion years (corresponding to January 27 in the Great Cosmic Year), the hydrogen had been re-ionized. This happens when the electron in the hydrogen atom is too energetic too remain in orbit around the single proton which makes up the hydrogen nucleus. Newly formed stars and galaxies provided much of this energy.

April 14 First Sun-like stars (population I) appear.

Hydrogen and helium are so abundant in the universe that astronomers lump all other elements into a catch-all category called ‘metals.’ Astronomers divide stars into three categories based on their ‘metallicity,’ or how much stuff other than H or He they contain. This is important because those ‘metals’ ultimately make up solid things such as planet Earth, or you or me. Our Sun is only about 2% ‘metal.’

Stars of comparable metallicity are the youngest and are placed in population I. Some older stars in the distant halo of our galaxy are much less ‘metallic’ than our Sun, in some cases by a factor of 1,000 or 10,000; these are population II stars. Since all elements heavier than helium are formed in stars, astronomers speculate that the very first stars had virtually no metals, but such ‘population III’ stars have yet to be discovered.

It took about four billion years to make the first population I stars, bringing us to April 14 in our Great Cosmic Year.

Andromeda, again.
Creative Commons License photo credit: makelessnoise

May 23 The Milky Way’s galactic thin disc forms. This part of our galaxy includes our Sun.

August 31 Our solar system forms from a spinning cloud of dust.

The first population I stars to formed back on ‘April 14′ did not include our Sun. Astronomers recently discovered decay products of 60Fe, an isotope of iron that results from supernovae (exploding stars), in some meteorites. This suggests that a nearby supernova ejected this material into the dust cloud that became our solar system, making our sun at least a second generation population I star.

September 2 Earth begins to form.

Bad Moon Rising
Creative Commons License photo credit: makelessnoise

September 3 The Moon forms when a Mars-sized object called ‘Theia’ strikes Earth.

September 21 Earth begins to solidify.

This corresponds to the end of the Late Heavy Bombardment, a period of frequent impacts on all bodies in the inner solar system. Up to this point, consistent bombardment kept the Earth molten, with magma seas. With the end of the bombardment, Earth began to cool, solid rocks appeared, and Earth’s geologic history began.

September 29 Life begins on Earth.

October 12 The first continent (called Ur) appears on Earth.

November 2 Oxygen (O2) builds up in Earth’s atmosphere.

November 14 Eukaryotes (with distinct nuclei in the cell) exist on Earth.

November 27 Multicellular organisms exist.

December 5 The supercontinent Rodinia forms.

December 17 Cambrian explosion: earliest forms of most types (phyla) of animals appear.

December 20 First life on land

triceratops
Of course, the real dinosaurs were bigger,
and not made of paper.
Creative Commons License
photo credit: kekremsi

December 25-29 Age of the dinosaurs

December 30 (morning) Chicxulub meteor impact helps cause extinction of about 3/4 of all life, including the dinosaurs.

The following events all occur on December 31:

9:17 am Drake passage completes the isolation of Antarctica; the continent freezes over.

7:30 pm Human ancestors diverge from chimpanzees.

9:57 pm Lucy lives in east Africa.

11:52 pm Homo sapiens sapiens exists.

11:59:14 pm Last Glacial Maximum (most recent Ice Age)

11:59:45pm Uruk, in Sumer, is one of the first cities on Earth.

Our existence as a species, compared to the whole universe, is about eight minutes out of a year. All of human civilization amounts to about 15 seconds. Once, I presented this calendar and was told that the smallness of our existence was an attack on religious faith. Perhaps, however, this need not be so. After all, an important virtue in most religious traditions is humility. This is not the denial of our talents and value, but the realization that we, with our goals, hopes, and dreams, are but one element of a much larger whole. As you reflect back on 2008 this holiday season, I invite you to reflect on the Great Cosmic Year. I find that the resulting wonderment and awe deepens my appreciation of the universe, and reminds me why I studied science in the first place.

Happy New Year!

The New Moon of Monday, September 29, is an important one to many people of the world. In the Hebrew Calendar, it marks Rosh Hashanah (literally, ‘head of the year’) which is the beginning of year 5769. On Tuesday night, September 30, Muslims across the world will see the first slender crescent of this lunar cycle. That will mark the end of Ramadan and the beginning of the next month, Shawwal. 1 Shawwal is ‘Eid ul Fitr, one of the greatest holidays in the Islamic calendar. This week, then, is a good time to think about the Moon, why it’s here, how it orbits, and how we have used it to measure time.

Moon and stars
Creative Commons License photo credit: joiseyshowaa

Unlike our months, Hebrew and Islamic months begin with the New Moon. Because twelve lunar months add to only 354 days, less than the 365.25 day solar year, an extra month is occasionally needed to keep the months roughly aligned with the seasons. In a 19 year cycle, years 3, 6, 8, 11, 14, 17, and 19 have the extra month. The year that is ending, 5678, is number 11 in its cycle and was a leap year.

Interestingly, the Jewish year has two ‘beginnings’. Tishrei (the month which begins now) is the first month of the civil calendar, and the month where 5678 becomes 5679. However, it is actually the seventh month of the religious calendar, which begins at Nisan (the month of Passover).

The Islamic calendar functions slightly differently. Its months begin with the first visible crescent low in the west at dusk, which is not with the actual New Moon. Keep in mind that at New Moon, the Moon is in line with the Earth and Sun, and the entire near side of the Moon has nighttime (and is therefore dark). The New Moon is visible, therefore, only if it blocks the Sun during an eclipse.

Since this New Moon occurs early Monday morning, the 29th, we expect it to be visible by Tuesday evening, the 30th. Observant Muslims, then, will continue to fast in daylight hours Monday and Tuesday. Upon seeing the Moon Tuesday night, they will know that Ramadan has become Shawwal, and they may break their fast on Wednesday.

Due to early controversy as to which years would have it, Muhammad outlawed the 13th month that kept Islamic months tied to the seasons. As a result, Ramadan (and each other month in that calendar) begins 11 days earlier each year according to our Gregorian calendar.

The Moon is Earth’s only natural satellite, orbiting our planet once every 27.3 days. However, a cycle of moon phases (say, from New Moon to the next New Moon), takes 29.54 days. This is because the Earth itself is moving during each 27.3 day Moon orbit. Since it is much easier to observe the Moon’s changing phase cycle than to observe the Sun directly, the 29.54 day phase cycle was the basis of many ancient calendars. Words for ‘moon’ and ‘month’ are related in English and are identical in many other languages. There is some evidence that our word ‘moon’ is ultimately related to an Indo-European word for ‘measure.’ Given how long we’ve measured time by the Moon, it is easy to take its presence for granted.

Released to Public: Jupiter Montage (NASA)
Creative Commons License photo credit: pingnews.com

However, our Moon is quite remarkable in several ways. Moons in our solar system are generally much smaller than the planet they orbit. Jupiter and Saturn, for example, are about 25 times bigger across than their biggest moons. Earth, though, is only 3.67 times the diameter of our Moon. Also, moons usually orbit in the same plane as their planet’s equator. Our Moon, though, orbits within about 5 degrees of Earth’s orbital plane, called the ecliptic, which is not the plane of the equator since Earth is tilted 23.5 degrees on its axis.

This leads most astronomers to believe that the Moon did not form with the Earth, but is the result of a collision with with an object roughly the size of Mars. According to this theory, the impactor (sometimes called ‘Theia’) struck a glancing blow on the Earth and was completely destroyed, and the Moon formed from the debris of Theia’s and Earth’s mantles.

This impact is what left Earth with a Moon much larger than what a planet Earth’s size would normally have, and left that Moon near Earth’s orbital plane (where the impact occured). Our relatively big moon has crucial effects not only on our tides, but also on the stability of Earth’s tilt.

Earth’s orbital tilt of about 23 and a half degrees as it goes around the Sun causes the seasons. The axis precesses, describing an aparent circle roughly every 26,000 years, but the amount of tilt (obliquity) stays nearly the same. Because the Moon acts a counterweight, the obliquity varies only between 22.1 degrees and 24.5 degrees over about 41,00 years (we are now at 23.44 degrees and decreasing). Even this orderly variation, called the Milankovitch cycle, is enough to influence our Ice Ages. Imagine the impact on Earth’s climate if there were no Moon, and the obliquity varied chaotically. This is exactly what happens at Mars, where the tiny moons Phobos and Deimos are not massive enough to influence Mars’ tilt.

Public Domain: Apollo 8 Looks at the Moon (NARA/NASA)
Creative Commons License photo credit: pingnews.com

One thing our Moon does have in common with most others is that it orbits the Earth and rotates on its own axis at the same rate. This is called ‘synchronous rotation’ and it occurs because the Moon is not exactly uniform in composition. From the time the Moon formed, the slightly heavier side was attracted to the Earth. Over time, this effect de-spun the Moon until it attained synchronous rotation. The Moon’s gravitational attraction also de-spins the Earth, although much more slowly as the Moon is less massive. As it does so, the Moon moves slightly farther from the Earth (just over 3 cm per year). The Moon is now 1.5 meters farther away that it was when Apollo astronauts went there. Don’t worry, though, by the time the Moon is far enough away to escape, the Sun will have become a red giant and swallowed both Earth and Moon anyway.

What is the shape of the Moon’s trajectory around the Sun? Perhaps not what you’d expect.

So, I encourage every one to watch for the reappearance of the Moon in the evening sky this week, even if you aren’t celebrating a New Year or an ‘Eid. The Earth’s companion gives all of us something to appreciate.