The standard geocentric model assumed that the planets were arranged out from the center, the earth, according to the period of revolution. On this assumption, the shorter the period, the closer the planet was to the stationary, central Earth. Claudius Ptolemy ensconced the order in his Syntaxis (commonly know as his Almagest) sometime in the second century CE (out from the earth): moon, Mercury, Venus, sun, Mars, Jupiter, Saturn. Until at least the sixteenth century, this was the accepted order of the planets. Any quick look through medieval astronomical or astrological manuscripts turns up diagrams illustrating this order of the planets. It became part of the university curriculum in Sacrobosco’s De sphaera.
As late as the seventeenth century printed copies of Sacrobosco’s De sphaera include a similar diagram.
So I was surprised to see a different order of planets in a sixteenth-century Greek manuscript. Here, among a collection of astronomical and astrological diagrams, is a diagram showing the earth at the center surrounded by the planets.
But in this diagram the order of the planets seems to be off. At the center we see the earth (γῆ), surrounded by spheres of the Moon (σελήν), Mercury (ἐρμῆς), Venus (ἀφροδίτη), Mars (ἄρης), the sun (perhaps?), Jupiter (ζεύς), and Saturn (κρόνος). Then, the signs of the zodiac encircle the entire diagram.
I have previously noticed this diagram and its unusual ordering. I suggested that there was nothing terribly interesting about it. Perhaps, I mused, what looks like Mars (the label “ἄρης”) in the sphere below the sun arose from the fact that the fifth sphere (typically the sphere for Mars) was colored in black. Consequently, the label had to be moved to the sphere below it (the fourth sphere, typically the sphere of the sun). I’m not sure that I am convinced. Sure it’s plausible, but plausible is only the most basic of explanatory requirements, a sort of useful but not binding sine qua non. Among other issues, that explanation still leaves the sun unlabeled. Moreover, I can imagine the reddish circle in the fourth sphere being that color to represent Mars, a reddish planet. Another possible explanation is simply that the scribe made a mistake. We often assume scribes were particularly inattentive or inept, introducing errors into the texts and diagrams they copied. But without any context, we are simply speculating. I have not found a source for this diagram, which might shed some light on the issue.
So at this point it’s just an interesting planetary diagram. I will certainly be keeping my eyes open for others.
Ok, after Ptolemy, the earth wasn’t really at the center. To get the model to work correctly, the earth was displaced slightly from the center. The distance it was off center depended on the particular planet being modeled. The details are too complicated for a footnote. Nonetheless, we call it a geocentric model. ↩
Bob Westman notes that Pico della Mirandola based part of his attack on astrology in the absence of any proof for this order. This aspect of Pico’s attack was crucially important for Westman’s larger argument about Copernicus’s motivations for developing a heliocentric system. See his massive The Copernican Question or his much more condensed Copernicus and the Astrologers. ↩