What does Ptolemy mean by "a line drawn from the centre of the earth through the centre of the moon to a point on the ecliptic"

Question

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?

Answer 1

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.