For all those, who like myself, can’t actually speak or read ancient Greek the title of this post is a phrase well known in the history of astronomy ‘saving the phenomena’, also sometimes rendered as ‘saving the appearances’. This post is in response to a request that I received from a reader asking me to explain what exactly this expressions means.

The phrase saving the phenomena was first introduced into the history of astronomy discourse by the late nineteenth-century and early twentieth-century French physicist and historian of science, Pierre Duhem. Duhem used the expression in the title of his work on physical theory *Sauver les Phénomènes. Essai sur la Notion de Théorie Physique de Platon à Galilée*, (1908), which was translated into English in 1969, as *To Save the Phenomena, an Essay on the Idea of Physical Theory from Plato to Galileo*. In this work Duhem argued that all mathematical astronomy from Plato up to Copernicus consisted of mathematical models designed to save the phenomena and were not considered to represent reality. The phenomena that needed to be saved were the so-called Platonic axioms, i.e. that the seven planets (Mercury, Venus, Moon, Sun, Mars, Jupiter and Saturn) move in circles at a constant speed. It is fairly obvious that the planets do not move in circles or at a constant speed thus posing a difficult problem for the mathematical astronomers, in order to save the phenomena they have to present a mathematical model, which can account for the apparent irregularity of planetary motions in the form of a more fundamental real regularity.

Duhem’s thesis suffers from several historical problems. He bases his argument on a quote from Simplicius’ *On Aristotle, On the Heavens*, which dates from the sixth century CE. According to Simplicius Plato challenged the astronomers to solve the following problem:

*“…**by hypothesizing what uniform and ordered motions is it possible to save the phenomena relating to planetary motions.” [1]*

Simplicius goes on to say:

*“In the true account the planets do not stop or retrogress nor is there any increase or decrease in their speeds, even if they appear to move in such ways … the heavenly motions are shown to be simple and circular and uniform and ordered from the evidence of their own substance.”*

Simplicius attribution of the concept of saving the phenomena to Plato is made more than nine hundred years after Plato lived. In fact there is no mention in the work of Plato of the principle of uniform circular motion, the earliest known example being in Aristotle. The earliest example of the phrase ‘saving the phenomena’ occurs in Plutarch’s *On the Face in the Orb of the Moon*, from the first century CE and does not refer to planetary motions but to Aristarchus’ attempt to explain the revolution of the sphere of the fixed stars and the movement of the Sun through heliocentricity.

We find some support for the view of Simplicius in the introduction to astronomy of Geminus of Rhodes in the first century BCE, although he doesn’t use the explicit phrase to save or saving the phenomena, he writes:

*“For the hypothesis, which underlies (hupokeitai) the whole of astronomy, is that the Sun, the Moon, and the five planets move circularly and at constant speed (isotachôs) in the direction opposite to that of the cosmos. The Pythagoreans, who first approached such investigations, hypothesized that the movements of the Sun, Moon, and the five wandering stars are circular and uniform … For this reason, they put forward the question: how would the phenomena be accounted for (apodotheiê) by means of uniform (homalôn) and circular motions.”*

As we can see Geminus attributes the concept of uniform circular motion to the Pythagoreans and not Plato. It should be pointed out that neither Simplicius nor Geminus was a mathematical astronomer.

Duhem also claimed that the most significant of all Greek astronomers, Ptolemaeus, adhered to the principle of saving the phenomena in his *Syntaxis Mathematiké*, the only substantial work of Greek mathematical astronomy to survive. However a careful reading of Ptolemaeus clearly shows that he regarded his models as representing reality and not just as saving the phenomena.

The most famous case of saving the phenomena can be found in Andreas Osiander’s *Ad lectorum* (to the reader) appended to the front of Copernicus’ *De revolutionibus.* In this infamous piece Osiander, who had seen the book through the press writes:

*For it is the duty of an astronomer to compose the history of the celestial motions through careful and expert study. Then he must conceive and devise the causes of these motions or hypotheses about them. Since he cannot in any way attain the true causes, he will adopt whatever suppositions enable the motions to be computed correctly from the principles of geometry for the future as well as the past. The present author has preformed both these duties excellently. For these hypotheses need not to be true nor even probable. On the contrary, if they provide a calculus consistent with the observations that is enough*. [2]

As can be clearly seen here Osiander is suggesting to the reader that Copernicus’ work is just a mathematical hypothesis and thus need not be regarded as mirroring reality. It is clear from the rest of his text that Osiander is trying to defuse any objections, religious or otherwise, that Copernicus’ heliocentricity might provoke. Of course his claims stand in contradiction to Copernicus’ text where it is obvious that Copernicus believes his system to reflect reality. Because Osiander’s *Ad lectorum* was published anonymously, it was assumed by many people that it was written by Copernicus himself a confusion that was only cleared up at the beginning of the seventeenth century.

It is not clear whether Osiander was appealing to a two thousand year old tradition of saving the phenomena, as Duhem would have us believe, or whether he, and possibly Petreius the publisher, had devised a strategy to avoid censure of the book and Copernicus’ radical idea.

Although many people continue to quote it as a historical fact it is highly doubtful that Duhem’s thesis of the saving of the phenomena ruling mathematical astronomy for the two thousand years from Plato to Galileo is true and it is fairly certain that most if not all mathematical astronomers, like Ptolemaeus, believed the models that they devised to be true representations of reality.

[1] This and all other quote from the Greek are taken from Mark Schiefsky, *“To save the phenomena” and curve fitting”* (pdf)

[2] *On The Revolutions*, translation and commentary by Edward Rosen, The Johns Hopkins University Press, Baltimore and London, pb., 1992, p. XX