The Primum Mobile in Astrology

Recently I had a conversation with an astrological colleague about Ptolemy’s geocentric model of the universe, which consists of a series of nested spheres whose motion depends upon the movement of an outermost sphere called the “primum mobile” or prime mover. Because this Ptolemaic geocentric model rests at the heart of Western astrology, I thought it would be worth investigating it a bit further in this blog.

Over the centuries following Ptolemy, his 2nd century CE theory of concentric celestial spheres was modified by subsequent authors on the basis of new scientific observations and differing philosophical and theological views. The medieval version of the Ptolemaic system defines the primum mobile as an outer sphere which moves around the earth every 24 hours, carrying the inner spheres with it. What initially sets this outermost sphere in motion is the will of a patriarchal God and his love for his creation, although He may send one of his angels from the Empyrean to look after it. A version of Ptolemy’s system was popularized by Dante in his Divine Comedy, in which he made the primum mobile the 9th of the ten heavens comprising his Paradiso.

A popular Renaissance model of Ptolemy’s scheme can be found in the 1539 diagram of Petri Apiani, reproduced here from Wikipedia.

The Nested Celestial Spheres of the Geocentric Universe

In Apiani’s version of Ptolemy’s scheme, the center of the Earth is at the center of the universe. Around the Earth, there appear the spheres of the seven visible planets in order of their relative speeds and distances from the Earth. Beyond the orb of Saturn, the furthest planet visible to the naked eye, lies the 8th sphere of the fixed stars and constellations.

Just outside the 8th sphere of fixed stars and their constellations lies a starless 9th sphere labelled “Nonu coelum cristallinum” or the 9th crystalline sky, presumably added to make the model consistent with the Old Testament account of creation in the Bible (Genesis 1:7) which posits the presence of “waters” above the firmament of fixed stars. The 8th orb of fixed stars and constellations and the 9th crystalline sphere each begin at 0 degrees Aries, suggesting that the zodiac depicted in the 9th sphere is the sidereal zodiac. The zodiac of the 10th and outermost sphere appears to be the tropical zodiac, offset from the sidereal zodiac of the 9th sphere by the factor of the precession of the equinoxes. Thus, God apparently established the tropical zodiac, with its twelve equally sized signs, as part of the 10th sphere when he created the primum mobile at the Big Bang. This idea of the 12 zodiacal signs being essential building blocks of the universe from the very beginning is consistent with Bonatti’s statement that “the signs are not corrupted but the elements are corrupted” by the motion of the stars, which causes the elements to combine in various proportions and thereby generate the multitude of entities in existence.

This card from the 1465 Mantenga tarot depicts the angel in charge of the outermost sphere, that is, the orb of the primum mobile or first moved mover of the system of nested celestial spheres.
Image from https://en.wikipedia.org/wiki/Primum_Mobile

Outside of the crystalline 9th sphere lies the 10th sphere, which is called “Decimum coelum primu mobile” or the 10th heaven, first mover. The metaphor here is that God is a watchmaker who has created an exquisite clock, which we know as our universe. This clock is powered by a spring, the primum mobile, which God has wound up with the force of his will. In his 2016 doctoral dissertation in Philosophy about the Primum Mobile, John G. Brungardt translates “primum mobile” as the first moved mover, meaning that God set the outermost sphere into motion to keep the entire universe running like clockwork. Brungardt also cites a cogent quote about the value of studying the symbolic significance of ancient theories, such as that of Ptolemy, even though they are currently at variance with the world view of modern science:

“From the fact that the experimental sciences go farther in the direction of concretion one cannot conclude that they can be substituted for the philosophy of nature of the ancients. To identify the philosophy of nature with the experimental sciences which are only its dialectical extension is to destroy it in its root, to deny the most certain part of our knowledge of nature, as well as its most noble natural subject. For that reason, the identification of the two misses in the most complete manner the point of the ancients and of wisdom.”
— Charles De Koninck, cited from “Are the Experimental Sciences Distinct from the Philosophy of Nature?“

Finally, Apiani’s diagram shows the Empyrean lying outside the ten celestial spheres. The Empyrean is the dwelling place of God (“Habitaculum Dei”) and the chosen ones who have made their way to heaven. The word ’empyrean’ comes via medieval Latin from Greek empurios, from en- ‘in’ + pur ‘fire’ and is the location of the pure Fire element, which is associated with the light and brightness of the divine spirit. The Empyrean lies outside the scheme of nested celestial spheres, just as the clockmaker resides outside the clock which he has constructed and set in motion.

God as Watchmaker, sitting in his Empyrean, builds an elaborate clock (our universe) and sets it in motion by adding a Prime Mover (primum mobile) to keep it going.
Image from https://tyrannyoftradition.files.wordpress.com/2011/05/watchmaker.jpg