Sympathy for Ptolemaic Epicyclers

Histories of science tend to cast “old theory holdouts” as uncouth, backward, prejudiced, and stinky nitwits. If you don’t immediately jump on the shiny new science bandwagon you’re on the wrong side of history and doomed to a legacy of ridicule and disdain. Mind you, these just so histories fail to mention that most new theories are dead-ass wrong and that you need solid reasons for abandoning ideas that more or less work. The classic “old theory people are morons” tale appropriately revolves around astronomy’s famous Earth-centered versus Sun-centered debate. The standard tale casts Copernicus as a brave man of science (even though he didn’t fully publish while alive to avoid censure) waging righteous war with hidebound doctrinaire church-sanctioned Ptolemaic astronomers and their idiotic notion that the sky spins around the Earth. How stupid is that?

Actually, it wasn’t stupid. The Ptolemaic system was complicated, lacked a physical basis, and required constant tweaking and adjustment, (sounds like modern software), but at its core, the system was deeply empirical. It was based on watching the damn sky and here’s the kicker; it kind of worked. The epicylers predicted the planets’ locations, the moon’s phases, when stars rose and set, and even some lunar and solar eclipses. Ptolemaic calculations lacked modern precision but they were good enough for naked-eye astronomers. When astronomers switched to telescopes it quickly became apparent to anyone that cared to look that Ptolemy’s system was off and needed fixing. Kepler, Newton, and others provided the fix and the new celestial regime was demonstrably better than Ptolemy’s, and, as a bonus, Newtonian gravity finally offered deep reasons for why orbits took observed shapes.

This all happened centuries after Copernicus, and because his heliocentric model was essentially correct, people have forgotten that Copernicus Version 1.0 made crappy predictions. The first iterations of the heliocentric model did not match the accuracy of the hidebound doctrinaire old fart Ptolemaic system; this is what fueled resistance to the new model. Far from being morons, the Ptolemians were just being good skeptical philosophers. Sun-centered models were not new even in the 15th century. Aristarchus speculated about them around 200 BCE, and I’d bet big bucks the idea was old even then. Sun-centered models did not immediately catch on because theories must pay their way. A nice idea that doesn’t feed you, (I’m looking at you Multiverse, and you too String Theory), will be tossed aside for ones that do! Ptolemaic astronomy endured because it paid the bills and fed the hungry. Copernicus Version 1.0 did not.

All of this was driven home by my recent development of a J script riseset for calculating when officially named International Astronomical Union stars (IAU) culminate in my backyard. The script is available on GitHub and can be installed and run on current J systems with:

load 'pacman'
NB. files from
install 'github:bakerjd99/jackshacks'

load '~addons/jacks/riseset.ijs'
location_yellowstone 0
NB. IAU stars rising/setting over Old Faithful
fmt_today iau_today 0

Most of the algorithms in riseset derive from Jean Meeus’s book Astronomical Algorithms with supplementary material from Jay Tanner and NASA’s eclipse site. If you peruse the code or browse the source books you will quickly see that modern astronomical calculations are complicated and require constant tweaking — just like the old Ptolemaic system. The algorithms in Meeus will need updating every century or so and it is the update process that distinguishes modern calculations from the Ptolemaic system. Modern calculations are based on solving n-body celestial mechanics problems. The resulting, mostly series solutions, form the basis of working algorithms like Tanner’s nutation programs. N-body problems are grounded in gravitational theories and there was nothing like them in Ptolemy, but from the perspective of a backyard amateur astronomer rolling his own code, the differences between modern and Ptolemaic calculations seem moot. While working out riseset I acquired a new respect and sympathy for the ancient epicyclers. They carried out decent approximations, without computers, or even place value arithmetic, and came up with answers that almost matched what they saw. Not exactly the work of morons.

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