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This is from a translation of Ptolemy's Almagest:

Hence it necessarily follows that the straight line drawn from the centre of the earth (which is the centre of the ecliptic) through the centre of the moon- to a point on the ecliptic, which determines the true position ([as it does] for all bodies).

I don't understand what is means when is refers to "a line drawn from the centre of the earth through the centre of the moon to a point on the ecliptic. If the moon is on the ecliptic why must this line go through it. If it is not on the ecliptic how can a straight line go through its centre to a point on the ecliptic?

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    $\begingroup$ We may need more context, but my interpretation is drawing 2 lines: one from the Earth to the Moon, and one from the Moon to the ecliptic. Essentially, forming a plane that is probably perpendicular to the ecliptic. This gives the ecliptic longitude, and the height of the Moon off the ecliptic gives the ecliptic latitude. $\endgroup$
    – JohnHoltz
    Jan 12, 2018 at 18:53
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    $\begingroup$ The Almagest doesn't describe a current model of the solar system. You may get better answers at the "history of science" stack exchange. $\endgroup$
    – James K
    Jan 12, 2018 at 20:04

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In his model, Ptolemy considered the ecliptic to be the path of the Sun—hence, a distinct circle than the path of the Moon. In modern terms, one could say that Ptolemy thus (rightly) considered the orbit of the Moon as distinct from the ecliptic.

If I’m not mistaken, your quote is from the beginning of Book IV of the Almagest. Further in the same Book, in Chapter 5, Ptolemy draws a diagram where “the circle concentric with the ecliptic be ABG on center D and diameter AD, and let the epicycle [of the Moon] be EZ on center G.” [Toomer’s translation, p. 181, with diagram on p. 188.] The diagram makes it clear that the ecliptic is not considered to be the projection of the Moon’s monthly path in the sky.

I have translated the Almagest to French and present it statically in a PDF at https://ecliptiqc.ca/Almageste.pdf ; there is also an interactive version of this very diagram at https://ecliptiqc.ca/Almageste_Livre4.php#IV5 (scroll a little down). While it’s all in French, it’s rather easy to understand even if you don’t read French, and there are free translation tools online.

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