The Astrologer’s Triumph

Ancient people thought the sky was a window into the fundamental nature of the universe. They were right.

Constellations as seen from Tenerife. Credit: Juan Carlos Casado


On a clear spring night in 762 AD, the great caliph Al-Mansur stood near a bend in the Tigris River, some fifty miles north of the ancient ruins of Babylon. He was flanked by a group of sages, all of them were looking up at the stars.

Though not yet 50 years old, Al-Mansur reigned over territory that stretched from North Africa to India, a landmass larger than the Roman Empire at its peak. He was here near the midpoint of his caliphate to found a new capital city: one that would solidify his rule over a domain that varied as much in culture as in geography.

Sky from southern Iran above a 3rd century AD site from the Sassanid empire. Credit: Babak A. Tafreshi

It was no accident that stargazers surrounded Al-Mansur this evening. These were influential men of learning — all astrologers — who had been summoned from Central Asia and the Middle East because of their status as intellectuals. According to their practice, the positions of the stars and planets overhead had important implications for events on the ground. Their task was to use their knowledge of the heavens to determine the most advantageous date for construction on the new city to begin.

Working together, they charted the sky and all agreed on a single day: July 31, 762. Sanctioned by the stars, this date would mark the founding of Al-Mansur’s new capital, a city that came to be known as Baghdad.

Within half a century, Baghdad would rank among the largest, most cosmopolitan, and intellectually vibrant cities on Earth. Possibly more than any single center in the Middle Ages, Al-Mansur’s capital on the Tigris would kindle the fires of scientific revolution that would burn brightest some eight centuries later in Europe. It was in Baghdad, at the famous House of Wisdom, where scholars would translate and confront the ancient Greek astronomical texts of Ptolemy and others, where mathematicians would invent algebra and develop the Hindu-Arabic number system, the decimal-based format that we all use today.

House of Wisdome. Maqamat of al-Hariri Illustration by Yahyá al-Wasiti, 1237 (Wikimedia commons)

One hundred years after this founding, the rulers of Baghdad would fund one of the greatest astronomical observatories in the world. Equipped with mechanical devices (telescopes were not yet invented), astronomers charted the sky with unsurpassed precision, and first called into question the notion that the planets and stars orbit the Earth on celestial spheres. This observatory and the star maps developed there played a crucial role in the rise of modern astronomy and the ensuing downfall of astrology as a science.

The astrologers that guided the founding of Baghdad had no idea they were contributing to the demise of their own profession.

Milky Way Above Easter Island. Image Credit & Copyright: Manel Soria

The Old Hypothesis

Every night, a canopy of fixed stars rotates in lockstep over our heads. Five lights are different than the others, however. These are the visible planets. They are brighter than the distant stars and change their positions against the background constellations from month to month in repeating cycles. Ancient people, including those that lived thousands of years before the founding of Baghdad, were enthralled with this regularity.

The night sky as viewed from one of the world’s oldest man-made structures, a stone-age sanctuary known as Göbekli Tepe. Located in southeastern Turkey, the ruins here predate the beginning of agriculture, reaching back some 11,500 years. Image credit: Tunç Tezel

Those stargazers noticed something remarkable: the nightly pattern varied in tune with the seasons. Changes in the positions of the stars and planets seemed to herald changes in the weather, migratory patters of game, the length of day and night. It is natural that some mistook this correlation for causation. Ancient astrologers came to believe that the movements of the stars dictated not just the weather but also events on Earth and the lives of people. This is not a bad hypothesis, really. It simply happened to be entirely wrong.

What the ancients didn’t know is that the Earth is moving. The stars do not orbit around us in a synchronized dance. They hover distantly, unconcerned, as we swivel in our seats every twenty-four hours and circle about the Sun every year. At night we face away from the Sun. The stars we see are just those that happen to lie in our field of view.

The motion of Mars relative to background stars over the course of a year. The backwards progression in the middle owes to the fact that the Earth is moving around the Sun. We are over-taking Mars as we both orbit a common center, like a runner on the inside track in a race who overtakes a rival in the outer track. Credit: Tunç Tezel.

If one night we could stop the Earth’s spin and motion relative to the Sun entirely, the stars above would lock into place and we would have an endless night. In this scenario the visible planets would continue to move in front of the distant field of stars as they race around the Sun over the course of months and years. Eventually, over thousands of years, the pattern of stars would begin to change. The stars, like the planets, are moving with respect to the Sun, but they are so far away that they must move great distances in order for us to tell.

The leading astrologers of days past were not irrational people, they were just working with the limited information they had at the time. Modern astrologers have no such excuse.

Constellations of the Zodiac seen with respect to the Earth’s progression. Credit: Teach Astronomy

The night sky changes throughout the year because the Earth is both moving and spinning. Its axis points in different directions as we circle the Sun. If our hemisphere is pointed towards the Sun then not only do we experience summer, but the stars we can see at night are different than we see when it’s pointed away from the Sun in the wintertime. The visible constellations change in response to the seasonal cycle, not the other way around.

Some people seem to prefer the idea that the stars pre-ordain our lives. The truth — that the stars go about their business oblivious to our existence — seems to make some people uncomfortable to the point that they don’t want to believe it. But the real story about stars is possibly even more compelling, more spectacular, than even the greatest myths of old: the stars are engines of creation.

Image Credit: NASA

When we look up and see stars we are seeing distant Suns making atoms that have never before existed. The Big Bang created only the simplest elements like hydrogen and helium, none of which are capable of interesting chemistry. It was only after the first stars were born and died that bigger, more interesting elements came into being. Every atom of oxygen that we breathe was made in stars. The iron in our blood and the carbon in our DNA? All forged in stars.

The stars responsible for making the atoms in our bodies have long since blown up or faded out. But that doesn’t mean the stars that still exist are uninteresting or irrelevant. Those stars are building new atoms, atoms that in a few billion years might find themselves on some future planet and possibly incorporated into a foreign form of life. Some of these atoms may even enable a new form of intelligent life to evolve. Imagine the possibility: a few billion years from now on the surface of some distant world, alien sages notice the enthralling regularity of the stars and begin to make sense of the Universe, just as we have done.

Ancient Babylonian tablet recording Halley’s comet (public domain, Wikipedia)

Ultimately the legacy of ancient astrology is one of triumph: precise star charts eventually revealed much about the deeper workings of the universe. Revered by kings and caliphs, astrologers were employed and empowered to keep accurate records of celestial events. Eventually, these data became so precise that the motion of the Earth became an undeniable reality and the fallacy of astrological causation was exposed. Studies of the night sky eventually confirmed Newton’s mathematically-based predictions that gravity operated in the heavens just as it does on Earth. This Newtonian realization transformed our understanding of how the universe works in the most fundamental way imaginable. Modern physics, modern science itself, was effectively born of this triumph.

The deepest image of the Universe ever taken. The Hubble Ultra Deep Field reveals galaxies that existed more than 12 billion years ago. This not only predates formation of the Earth but the formation of many of the atoms your body.
Image Credit: NASA, ESA, H.Teplitz and M.Rafelski (IPAC/Caltech),
A. Koekemoer (STScI), R. Windhorst(ASU), Z. Levay (STScI)

Our ancestors believed that the night sky offered a window into the fundamental nature of the universe. They were right.

Because of our initial fascination with the night sky we now have good evidence that everything in our Universe obeys a single set of physical laws, laws that in many cases can be used to predict the future with a precision that would have been unfathomable to even the most devoted astrologer of old (e.g., mechanical trajectories, circuit design, and our ability to regularly predict the weather would have all amazed our ancestors).

We now know that even the best ancient star charts contained only a tiny fraction of the stars that exist. Fewer than one thousanth of one percent of the stars in our Galaxy are visible by eye; there are hundreds of billions of other galaxies in the Universe. These are things we’ve learned not in spite of the efforts of ancient sages but because of them.

Stars in nearby Large Magellanic Cloud. Credit: NASA/Hubble/APOD

Two thousand years from now, future scientists will likely look back at the frontier of scientific cosmology today with some amusement at our naiveté. While we certainly know a lot about the cosmos, some of our untested hypotheses about the Universe— its fundamental unseen constituents, its birth, the overarching principles that govern its structure—are likely wrong, at least in detail if not in broad brush. Nevertheless, we can hope that those future thinkers will at least appreciate our motivations and our bold attempts invent and test ideas. After all, our motives are pure. Slowly, and sometimes erratically, we are unlocking the Universe, just as the ancients had foreseen.


This article was written by James Bullock, professor of Physics and Astronomy at U.C. Irvine. Follow him on Twitter at @jbprime.

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