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I read the following in C. S. Lewis, Miracles (page 77-8)

The immensity of the universe is not a recent discovery. More than seventeen hundred years ago Ptolemy taught that in relation to the distance of the fixed stars the whole Earth must be regarded as a point with no magnitude.

There was no reference given, but I assume this would be in the Almagest. However, I don't know where in that work (or elsewhere) Ptolemy discussed this. I want to read his explanation for that view.

Hence the question: Where did Ptolemy compare the Earth to the distance of fixed stars?

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2 Answers 2

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Almagest, Book 1, chap. 5 contains what you are looking for.

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  • $\begingroup$ This basically answered my question where in the Almagest I need to look. $\endgroup$
    – Frank Hubeny
    Apr 21, 2019 at 22:54
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    $\begingroup$ Could you please provide a relevant quote from the source? Otherwise, if the link goes dead, your answer is worthless. $\endgroup$
    – Thunderforge
    Apr 22, 2019 at 2:03
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    $\begingroup$ This passage appears as Book 1 Chapter 6 in Toomer's 1984 English translation cited by the Wikipedia "Almagest" article. My (no-doubt over terse) summary is: "we see no parallax." $\endgroup$
    – kimchi lover
    Apr 22, 2019 at 14:52
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    $\begingroup$ Please edit this answer to include the essential content of the source you refer to. $\endgroup$
    – Danu ♦
    Apr 25, 2019 at 14:38
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Ptolemy's argument can be simply explained in plain English. If the distance to stars was comparable to the size of the Earth, the stars would experience a diurnal parallax, that is the visible relative positions of the stars in the sky would change during one night. But this is not observed.

By the way, a similar argument will imply that if the Earth is rotating about the Sun (or moves in some other way with respect to stars), the distance to the stars must be much larger than the size of this motion. In particular the diameter of the orbit of the Earth around the Sun is negligible in comparison to the distance to the stars.

This was the most serious argument against the Heliocentric system, and it is also due to Ptolemy. Until the acceptance of Copernicus theory, people just could not believe that the distance to the stars can be so large. Why would God create such an enormous Universe only to populate a tiny piece of it with humans?

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  • $\begingroup$ Thank you for confirming that it was parallax and for connecting this to questions about the Heliocentric system. $\endgroup$
    – Frank Hubeny
    Apr 21, 2019 at 22:53

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