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I have run across several references to Aristotle's arguments for a spherical earth which claim that he noted that ships sink over horizon hull-first. For instance, Isaac Asimov writes in his essay The Relativity of Wrong:

There were reasons, to be sure, to find the flat-earth theory unsatisfactory and, about 350 B.C., the Greek philosopher Aristotle summarized them. First, certain stars disappeared beyond the Southern Hemisphere as one traveled north, and beyond the Northern Hemisphere as one traveled south. Second, the earth's shadow on the moon during a lunar eclipse was always the arc of a circle. Third, here on the earth itself, ships disappeared beyond the horizon hull-first in whatever direction they were traveling.

However, Aristotle's arguments for the sphericity of the earth appear in On the Heavens, Book II, Part 14, and I see no reference to ships sinking below the horizon.

Did Aristotle in fact make this argument somewhere else, and if not, who was the first person to take this as evidence of a spherical earth?

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    $\begingroup$ Same Query 125 is in journals.uchicago.edu/doi/abs/10.1086/349149?journalCode=isis $\endgroup$ – sand1 Jun 28 '18 at 20:19
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    $\begingroup$ My guess is that this is just a usual conflation, Aristotle's case for sphericity is best known so additional arguments derived from Pliny and Ptolemy get ascribed to him too. $\endgroup$ – Conifold Jun 28 '18 at 23:25
  • $\begingroup$ @sand1, Thanks! So I'm not totally crazy. This seems to more or less answer it. It sounds like it's often falsely attributed to Aristotle, but the argument seems to date to Pliny the Elder (or earlier). If you wrote this up as an answer, I'd accept it. $\endgroup$ – Endy Jun 29 '18 at 15:05
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I dug up some text from a translation of "on the heavens."

There are similar disputes about the shape of the earth. Some think it is spherical, others that it is flat and drum-shaped. For evidence they bring the fact that, as the sun rises and sets, the part concealed by the earth shows a straight and not a curved edge, whereas if the earth were spherical the line of section would have to be circular. In this they leave out of account the great distance of the sun from the earth and the great size of the circumference, which, seen from a distance on these apparently small circles appears straight. Such an appearance ought not to make them doubt the circular shape of the earth.

He goes on to discuss why the "whirling" does not require wind or loss of atmosphere. There does not appear to be any mention of observing ships.

Later, he invokes eclipses of the moon:

How else would eclipses of the moon show segments ments shaped as we see them? As it is, the shapes which the moon itself each month shows are of every kind straight, gibbous, and concave-but in eclipses the outline is always curved: and, since it is the interposition of the earth that makes the eclipse, the form of this line will be caused by the form of the earth's surface, which is therefore spherical

Followed by a discussion of the change in star positions, roughly as a function of latitude, to show the earth cannot be flat.

I'm far from a knowledgeable scholar of ancient history, so cannot state that there's no other document written by Aristotle on this topic.

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  • $\begingroup$ Thanks for your answer. In this case, though I knew the ships-sinking-on-the-horizon argument was not made in that section, but wanted to know if Aristotle made the argument somewhere else in his works, and if not, who originally did make this argument. $\endgroup$ – Endy Jul 3 '18 at 15:02
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From "Queries and Answers," Isis 41, no. 2, linked to by sand1 in the comments above, we see that the proof of the sphericity of the earth based on the fact that the masts of a distant ship are visible above the horizon when the body of the ship is no longer visible

...has been attributed to Aristotle in De Caelo, but it does not appear there. It does, however appear in De Revolutionibus of Copernicus in reference to a light on the tip of a mast. A Similar statement may be found in Ptolemy's Almagest where it is stated: "Whenever we sail towards mountains or any high places from whatever angle, in whatever direction, we see their bulk little by little increasing as if they were arising from the sea, whereas before they seemed submerged because of the curvature of the water's surface." [Ptolemy, C., Mathematical Composition, transl. by R. C. Taliaferro, mimeographed for St John's College, Baltimore, 1938, Book I, Section 4.].

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A century earlier than Ptolemy, Pliny the Elder, in his Natural History, Book 2, LXIV. 160, ff. (Loeb Library) included in his discussion of the earth's sphericity, "The same cause explains why the land is not visible from the deck of a ship when in sight from the mast-head; and why as a vessel passes far into the distance, if some shining object is tied to the top of the mast it appears slowly to sink and finally it is hidden from sight."

So it appears that while Aristotle had several arguments for the sphericity of earth, he did not in fact argue it based on ships sinking below the horizon. However, it is often falsely attributed to him. The earliest I can find of anyone making this specific argument is then Pliny the Elder, around 77 A.D.

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