A Source Book in Greek Science

Author: Claudius Ptolemy

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Hypotheses of Ptolemaic Astronomy

Ptolemy Almagest I. 2–31

On the Order of the Theorems

2. The work which we have projected commences with a consideration of the general relation between the earth as a whole and the heavens as a whole.2 Of the special treatments that follow, the first part3 deals with the position of the ecliptic, the places inhabited by the human race, and the differences among the successive places, in each separate horizon, along the curvature of the earth’s surface. The preliminary study of these relations makes easier the examination of the subsequent questions. The second part4 gives an account of the motion of the sun and the moon and of the phenomena that depend on these motions. For without the previous understanding of these matters it would be impossible to set forth a complete theory of the stars. Since the theory of the stars is contained, in accordance with the general plan, in the concluding portion of this essay,5 the investigation of the sphere of the so-called fixed stars would properly find its place there, and the material on the five so-called planets would follow. We shall try to set forth all this material using as the basic foundations for knowledge the manifest phenomena themselves and those recorded observations of the ancients and the moderns about which there is no dispute; and we shall seek to fit the propositions together by geometrical proofs.

With respect to the general portion of the treatise the following preliminary assumptions6 are to be made: (1) that the heaven is spherical in form and rotates as a sphere; (2) that the earth, too, viewed as a complete whole, is spherical in form; (3) that it is situated in the middle of the whole heaven, like a center; (4) that by reason of its size and its distance from the sphere of fixed stars the earth bears to this sphere the relation of a point;7 (5) that the earth does nor participate in any locomotion. We shall say a few words by way of commentary on each of these propositions.

1 On Ptolemy see p. 162, below. For a discussion of his astronomy see p. 128.

2 Book I. 3–11.

3 Book I. 12 to the end of Book II.

4 Books III–VI.

5 Books VII–XIII.

6 These are not arbitrary assumptions, for without seeking to prove them absolutely, the author tries to make them seem plausible (ch. 4–7).

7 Cf. p. 108, above.


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Chicago: Claudius Ptolemy, "On the Order of the Theorems," A Source Book in Greek Science in A Source Book in Greek Science, ed. Morris R. Cohen and I. E. Drabkin (Cambridge: Harvard University Press, 1948), 122. Original Sources, accessed June 9, 2023, http://originalsources.com/Document.aspx?DocID=IP6JS9DZFE8B2F7.

MLA: Ptolemy, Claudius. "On the Order of the Theorems." A Source Book in Greek Science, Vol. I, in A Source Book in Greek Science, edited by Morris R. Cohen and I. E. Drabkin, Cambridge, Harvard University Press, 1948, page 122. Original Sources. 9 Jun. 2023. http://originalsources.com/Document.aspx?DocID=IP6JS9DZFE8B2F7.

Harvard: Ptolemy, C, 'On the Order of the Theorems' in A Source Book in Greek Science. cited in 1948, A Source Book in Greek Science, ed. , Harvard University Press, Cambridge, pp.122. Original Sources, retrieved 9 June 2023, from http://originalsources.com/Document.aspx?DocID=IP6JS9DZFE8B2F7.