The Trouble with Ptolemy

Claudius Ptolemy (c. 100 – c. 170 CE) was regarded as one of the ancient world’s most preeminent astronomers. He was also an astrologer. His dedication to proving astrology right would be astrology's downfall. 

Note: I published this on my website back in 2021, removed it, and then lost the original publication date.

Claudius Ptolemy (c. 100 – c. 170 CE) was regarded as one of the ancient world’s most preeminent astronomers. His astronomical treatise the Almagest guided scientific thinking for hundreds of years. Additionally, Ptolemy’s work Tetrabiblos was (and still is) considered to be one of tropical astrology’s key foundational documents.

However, there are a number of competing statements in Ptolemy’s Almagest and Tetrabiblos that seem to confirm Robert Newton’s conclusion that "Ptolemy is not the greatest astronomer of antiquity, but he is something still more unusual: He is the most successful fraud in the history of science." (Newton, R. (1977). The Crimes of Claudius Ptolemy. Baltimore : Johns Hopkins University Press) And by extension, I would also propose that he is the man that eventually destroyed any respect for astrology in the Western world due to his determination to ignore the evidence.

My argument becomes clearer if we regard Ptolemy as both an astronomer and astrologer, rather an astronomer only. This wouldn’t have been unusual for the time, and astronomy was generally considered the sibling of astrology.

Before proceeding further, here’s a quick summary of Ptolemy’s model.

1. It’s a geocentric model. The Earth is the center of the universe.

2. Ptolemy uses epicycles to account for the motion and speed of the planets, along with the Sun and the Moon. That is, a planet is moving in its own circle, within a bigger circle. Epicycles were used before Ptolemy wrote the Almagest and were used for hundreds of years after its publication. To account for the seeming errors in Ptolemy's work, and as observations became better over time, more circles were added to other circles in ever increasing complexity.

3. Ptolemy knew about the precession of the equinoxes due to previous calculations done by Hipparchus. Ptolemy incorporated Hipparchus’ thinking into the Almagest. Strangely, Ptolemy didn’t feel that precession needed to be completely accounted for. Instead, he fixed the position of the equinox and increased the longitude of the fixed stars by the rate of precession that he’d calculated based on Hipparchus’ work (which was 1 degree per year – equating to the Egyptian year of exactly 365 days).

The major issue with Ptolemy's work raised by numerous astronomers throughout the centuries was the degree of variation in the accuracy of his calculations coupled with false statements. For example, Ptolemy claimed he observed an autumnal equinox in Alexandria, Egypt at 2PM on 25 September 132CE. Despite saying he observed it, the actual equinox occurred on the 24 September 132CE. On a more obvious note, if Ptolemy observed the actual equinox, he would have determined that the equinox occurred in Virgo, rather than Libra.

Ignoring the zodiac constellations where the equinoxes took place and insisting that the March equinox was anchored in Aries and the September equinox in Libra, seems like an odd choice for a man who was supposed to be a dedicated mathematician and astronomer.

However, it begins to make more sense if Ptolemy was primarily concerned with making some of the astronomy fit into his astrological theories. This would explain his insistence in both the Almagest and the Tetrabiblos that the Northern Hemisphere spring equinox always occurred in Aries.

Owen Gingerich in his paper, Was Ptolemy a Fraud? (Royal Astronomical Society, 1980, 21, (253-266) floats the idea that when Ptolemy was faced with choosing between difference sets of conflicting data, he probably corrected his observations to fit his theories. “Because Ptolemy’s parameters are generally pretty good, we must assume that he had a substantial data base beyond what is specifically preserved in his treatise. But a redundant data base with random observational errors must have yielded conflicting parameters that depended on the particular minimal combination used. Ptolemy realized this, and quite likely he noticed that in using different combinations, certain results appeared more frequently, giving him the basis for a preferred set of parameters. At the same time, he might have realized that most, if not all, of his observations contained accidental observational errors. Although my suspicions remain highly speculative, I suspect that Ptolemy, convinced in the intrinsic soundness of his theory, simply replaced the only partially trustworthy observations by what he perceived to be the ‘correct’ data.”

“Thus Ptolemy, like many of the brilliant theoreticians who followed him, was perfectly willing to believe that his theory represented Nature better than the error-marred individual observations of the day.”

In his paper, Gingerich casts Ptolemy as a genius that decided to trust his own calculations instead of the data he’d collected.

Which leads to the astrology issues. If we accept that Ptolemy massaged his data to fit his own theories, this may explain why he insisted that the Northern Hemisphere spring equinox was anchored to Aries and the autumnal equinox to Libra. It may not have been due to difficulties with the calculations but because he was a dedicated astrologer using previous astrologers’ work and he wanted to continue using Aries as the point where the Northern Hemisphere spring equinox occurs. Aries represents the start of spring, and this is needed to match Aristotle’s theories of the classical elements.

In the Tetrabiblos, we can see Ptolemy determinedly connecting Aries to the spring equinox, and the start of the spring season because (in torturous circular logic), Aries is the ram and therefore represents spring. This is from Chapter XII (The Annual Seasons) (translation by J. M. Ashmand,1822).

“The beginning of the whole zodiacal circle (which in its nature as a circle can have no beginning, nor end, capable of being determined), is therefore assumed to be the sign of Aries, which commences at the vernal equinox since the moisture of spring forms a primary beginning in the zodiac, analogous to the beginning of all animal life; which, in its first age of existence, abounds principally in moisture: the spring, too, like the first age of animal life is soft and tender; it is therefore suitably placed as the opening of the year, and it is followed by the other seasons in quick succession.”

The key information to remember is that a large number of astrological delineations and significations had been developed centuries before Ptolemy was born--when the equinox was actually in Aries. Astrologers/astronomers had already observed where the northern pole star was, and that the Sun traveled around the ecliptic, and that a narrow band of stars intersected with the ecliptic. Ptolemy would have been well aware of these works, including, presumably, works that did not survive antiquity. As already assumed for his astronomical treatise, other astrologer's work may have been appropriated by Ptolemy for Tetrabiblos and he didn’t see the point in mentioning them.

The more serious issue, as pointed out by Gingerich is that Ptolemy did not develop his theories based on how anyone working in the sciences today would approach a theory. Instead of Ptolemy asking himself if astrology still worked with the spring equinox in Pisces, Ptolemy seems to have decided that astrology only works if the equinox is in Aries, and then sets about proving it.

Which is a tenuous position to take because that position states that astrology can only operate within an extremely narrow set of parameters. Astrology, based on Ptolemy's assertations, must always anchor the vernal equinox to Aries because it's the only way the ancient association of zodiac signs to the seasons can still be applied.

This position would eventually lead to astrology’s downfall in the West once the Enlightenment gained traction. Up until the seventeenth century both Ptolemy’s astronomical and astrological works remained widely accepted. It seems that because Ptolemy’s Almagest was highly regarded, by extension Tetrabiblos was also correct.

However, things began to change when Copernicus established the heliocentric model and proved that the Sun was the center of the solar system in 1543. Johannes Kepler progressed this model by establishing that the orbit of the planets were elliptic instead of circular. Having said that, Kepler was still interested in astrology, and earned his living as a court astrologer. He still published works on astrology, trying to arrive at some middle ground. But by then there were also a number of voices dismissing astrology as utter rubbish. Kepler took the view that although he disdained much of astrology there might still be some gems to be found by a diligent scientifically grounded astrologer.

“No one should consider it unbelievable that out of astrological foolishness and godlessness a useful sense and holiness could not also be found, that in unclean slime could not also be scraped out a snail, mussel, oyster, or eel useful for eating, that a silk spinner could not be discovered in a big heap of caterpillar egg droppings, and finally, that a good granule from a busy hen or a peach or a gold nugget might be found in an evil-smelling dung heap.”

However, the legitimacy of astrology begun to wane and by 1650, it was already fading in popularity. If Ptolemy’s theories and calculations of astronomy were incorrect, how could anything stated in the Tetrabiblos be valid?

On top of the complete rethink in regards to Earth's place in the solar system and universe, the fallout from the Thirty Years War (1618 to 1648) brought additional tensions. The social order of Europe was upended as various countries and religious institutions fought each other and their own citizens. After so much bloodshed, chaos, violence and death brought on by warring religious factions, the only way out seemed to be an agreement by all nations that everyone should leave everyone else alone. More importantly, men and women of reason should be allowed to get on with their job of contemplating the world. An explosion of ideas followed that spanned philosophy, literature, politics, religion, science, economics, and law, and included people such as Immanuel Kant, Mary Wollenstone Craft, Renee Descartes, Isaac Newton, Adam Smith, and Voltaire.

Ptolemy’s ideas were consigned to the dustbin of history, and by extension so was astrology.

The revival of astrology in the West in the late 19th century resurfaced Ptolemy's ideas, much to the amusement of astronomers. Tropical astrologers insisted, much like Ptolemy, that the Northern Hemisphere spring equinox occurred in Aries. Tropical astrologers pointed out that Ptolemy's system connects the signs to the seasons, not to the actual zodiac constellations. However, this argument failed to realize that, as stated above, that Ptolemy knew about the precession of the equinoxes, and chose to ignore it.

There’s also one more wrinkle in this argument. Chis Brennan notes in his book Hellenistic Astrology: The Study of Fate and Fortune that the Roman Empire and India were trading around the first century BCE. Groups of Greeks and Romans settled in India, and it was assumed the sometime around the second century CE, Hellenistic astrology texts were translated into Sanskrit, and this became the Yavanajātaka of Sphujidhvaja.

However, what is notable about the Indian tradition of astrology is that the equinox is not locked to Aries. Instead, the sidereal system is used, where the reference for the Sun’s movement and return to a specific point through the ecliptic is to a fixed star, not to the vernal equinox. In this case, a season starts when the Sun returns to the fixed star point, rather than the intersection of 0 degrees of the ecliptic with 0 hours of the celestial equator. In Vedic astrology, the precession of the equinoxes is accounted for, and if you ask a Vedic astrologer to cast a birth chart, and you were born around the 20th/21st of March, then the Sun is in Pisces. Vedic astrology, by extension, remains hemispherically neutral. The chart works whether you were born in the Northern or Southern hemisphere.

This seems to confirm that Indian astrologers were well aware of the precession of the equinoxes before Hellenistic astrology turned up, and they felt no need to incorporate Ptolemy’s views nor keep Aries anchored to the March equinox or Libra to the September equinox. Vedic astrologers have been highly regarded for thousands of years and continue to be well regarded in India for their accurate charts. They’ve also felt no need to incorporate any unobservable objects such as Neptune, Uranus or Pluto into their readings.

Adhering to the rules laid out by Ptolemy puts tropical astrologers into a bind. There is no such thing as precession of the equinoxes, and the astrology delineations are dependent on the seasonal significations of the Northern Hemisphere, along with Aristotle’s classic element theory. Tropical astrologers incorporate bodies such as Pluto, along with numerous asteroids and Kuiper Belt Objects into their readings, even though Ptolemy’s system of astronomy wouldn’t have allowed for these objects. Pluto's orbit is too eccentric to fit into a model where the heavens are fixed and everything moves in perfect circles.

Now, please be aware that I am not bagging tropical astrologers. I run an astrology blog for goodness sakes. However, by reading the available astronomical literature and papers, it seems that astrologer's should perhaps regard Ptolemy's astrological conclusions with skepticism. Maybe, like Kepler, we should be more scientifically grounded.