<p>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 <em>orbit</em> of the Moon as distinct from the ecliptic.</p>
<p>If I’m not mistaken, your quote is from the beginning of Book IV of the <em>Almagest</em>. 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 <em>not</em> considered to be the projection of the Moon’s monthly path in the sky.</p>
<p>I have translated the <em>Almagest</em> to French and present it statically in a PDF at <a href="https://ecliptiqc.ca/Almageste.pdf" rel="nofollow noreferrer">https://ecliptiqc.ca/Almageste.pdf</a> ; there is also an interactive version of this very diagram at <a href="https://ecliptiqc.ca/Almageste_Livre4.php#IV5" rel="nofollow noreferrer">https://ecliptiqc.ca/Almageste_Livre4.php#IV5</a> (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.</p>

CC BY-SA 4.0
2023-01-03T01:52:23.483Z
2023-01-03T04:59:33.870Z
2023-01-03T04:59:33.870Z
Pierre Paquette
<p>This is from a translation of Ptolemy's <em>Almagest</em>:</p>
<blockquote>
<p>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).</p>
</blockquote>
<p>I don't understand what is means when is refers to &quot;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?</p>

CC BY-SA 3.0
2018-01-12T18:53:19.517Z
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.
JohnHoltz
CC BY-SA 3.0
2018-01-12T20:04:23.553Z
The Almagest doesn't describe a current model of the solar system. You may get better answers at the "history of science" stack exchange.
James K
<the-moon><coordinate><ecliptic>
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"
Callum
# 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"

(Asked by: username_0 on 2018-01-12)

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?

**Tags:** the-moon, coordinate, ecliptic

> **Comments:**
> **username_1**: 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.  
> **username_2**: The Almagest doesn't describe a current model of the solar system. You may get better answers at the "history of science" stack exchange.  

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## 1 answers

### Answer by username_3 on 2023-01-03. Score: 4

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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