Astronomy and Astrology

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Greg Dawes writes …

One often reads, at least in popular works, that pre-modern European thinkers made no distinction between astronomy and astrology. A Wikipedia article, for example, begins by claiming that “astronomy and astrology were archaically one and the same discipline (Latin: astrologia), and were only gradually recognized as separate in Western 17th century philosophy.” Another website claims that “astronomy and astrology cannot be separated in the Middle Ages and the early modern period,” while a third asserts that in the early university faculties, “astronomy and astrology were not separate,” but were “indivisible sciences.”

Galileo Galilei in 1636 (Portrait by Justus Sustermans)

But is this correct? It is true that in medieval and early modern thought astronomy and astrology were closely associated. Furthermore, many authors of this time regarded both as reliable sources of knowledge.  Indeed astrology continued to be taken seriously well into the seventeenth century.  As H. Darrel Rutkin has noted (Galilaeana (2 [2005]: 107–142), early seventeenth century mathematicians, even those employed by universities, were often called on to cast horoscopes. No less a figure than Galileo seems to have taken astrology seriously, casting horoscopes not only when he was requested to do so, but also, for example, to predict the characters of his two daughters.

Nonetheless, it is wrong to say that pre-modern European thinkers made no distinction between astronomy and astrology. There was, first of all, a long tradition of scepticism regarding what is known as “judicial” astronomy, with its claims to be able to predict the course of an individual’s life. This tradition dates from Cicero’s De divinatione and continues with the work of St Augustine. It finds a late medieval expression in the criticisms of Nicole Oresme (1320–82) and reaches its high point in the work of Pico della Mirandola (1463–94). These writers rejected judicial astrology, but there is no reason to think they were rejecting astronomy. They offered no arguments against a mathematical treatment of heavenly movements, the science which had found its fullest expression in Ptolemy’s Almagest.

A sixteenth-century edition of a thirteenth-century work on Ptolemaic astronomy. (Photograph by Wolfgang Sauber)

Even among those who approved of astrology, a distinction was made between astronomy and astrology. It can be found, for instance, at the very beginning of the work known as the Speculum astronomiae, the “mirror of astronomy,” generally attributed to Albert the Great (ca, 1200–80). While using the same word (astronomia) for both astronomy and astrology, it begins by making a clear distinction between them. “There are two great wisdoms,” it begins, which go by this name: the first has to do with the configuration and movements of the heavens, while the second has to do with “the judgements of the stars.”

Still more interesting is a second work, the De fato, which dates from the same period. This not only makes the distinction, but spells it out in terms of the differing kinds of knowledge that are involved. The passage in question is responding to the objection that astrological prediction is not a science: what demonstrable connection is there, the objector asks, between (for example) the position of the moon in Leo and the type of clothing one should wear? The author’s response is to note that

as Ptolemy says, in astronomy we must distinguish two parts: the first concerns the positions of the superior bodies, their measures and their passions; this part of astronomy can be reached through demonstration. The other part concerns the effects of the stars in inferior things, effects which are received differently in these mutable objects [qui in rebus mutabilibus mutabiliter recipiuntur]. Thus, the second part can only be reached through conjecture. … From conjectures, which derive from mutable data [cum sit ex signis mutabilibus], comes a mental attitude endowed with less certainty than science or opinion. (De fato 4.7, translation by Paola Zambelli)

As the reference to Ptolemy suggests, the author considers that he is doing nothing new in making distinctions of this kind. They are already ancient, going back to the work of the great second-century astronomer, who wrote one work, the Almagest, on what we would call “astronomy,” and another, the Tetrabiblos, on astrology. The latter begins by making just the same point.

Of the means of prediction through astronomy, … two are the most important and valid. One, which is first both in order and in effectiveness, is that whereby we apprehend the aspects of the movements of sun, moon, and stars in relation to each other and to the earth, as they occur from time to time; the second is that in which by means of the natural character of these aspects themselves we investigate the changes which they bring about in that which they surround. (Ptolemy, Tetrabiblos I.1, Loeb translation)

What about the early modern period? Was the same distinction made then? It seems that it was, at least by some thinkers. I have already noted that Galileo does not deny the possibility of astral and planetary influences on human affairs. Indeed, his practice of astrology assumes it. But he does deny that it can produce the kind of certain knowledge that he, like his predecessors, regarded as essential for a science. Again, H. Darrel Rutkin provides evidence of this, drawing our attention to a letter of 1633 in which Galileo comments on the views of Jean-Baptiste Morin (1583–1656). Galileo criticises Morin, not for believing in astrology itself, but for believing that astrology could attain to certainty and for placing it at the head of the sciences.

So do premodern European thinkers make a distinction between astronomy and astrology? Yes, they do. Is their distinction identical with ours? No, for the most part it is not. But it would be wrong to overlook the way in which late medieval and early modern thinkers distinguished between what they considered differing forms of knowledge and the degree of certainty to which each could aspire.

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About Greg Dawes

Greg gained his first graduate degree at the Pontifical Biblical Institute in Rome before returning to New Zealand to complete a PhD in Biblical Studies at the University of Otago. More recently, he completed a second PhD, in philosophy, while continuing to teach in both Religious Studies and Philosophy. Some of his other postings can be found on his blog, at

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