Aristotle's globe arguments

In his work On the Heavens, Aristotle made several arguments for a spherical earth.

Argument from gravity

Aristotle called gravity a “centripetal impulse”, i.e. movement that converges toward a central point rather than in parallel lines.

He explicitly reasoned that this would mean earth is a sphere:

… Heavy bodies moving towards the earth do not parallel but so as to make equal angles, and thus to a single centre, that of the earth.

…

Its shape must necessarily be spherical. For every portion of earth has weight until it reaches the centre, and the jostling of parts greater and smaller would bring about not a waved surface, but rather compression and convergence of part and part until the centre is reached. … For if an equal amount is added on every side the extremity of the mass will be everywhere equidistant from its centre, i.e. the figure will be spherical. … The greater quantity must prevail until the body’s centre occupies the centre. For that is the goal of its impulse. Now it makes no difference whether we apply this to a clod or common fragment of earth or to the earth as a whole. The fact indicated does not depend upon degrees of size but applies universally to everything that has the centripetal impulse.

…

But the spherical shape, necessitated by this argument, follows also from the fact that the motions of heavy bodies always make equal angles, and are not parallel. This would be the natural form of movement towards what is naturally spherical.

Argument from lunar eclipses

But the spherical shape, necessitated by this argument, follows also from the fact that the motions of heavy bodies always make equal angles, and are not parallel.

…

The evidence of the senses further corroborates this. How else would eclipses of the moon show segments 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.

Argument from celestial latitude

Again, our observations of the stars make it evident, not only that the earth is circular, but also that it is a circle of no great size. For quite a small change of position to south or north causes a manifest alteration of the horizon. There is much change, I mean, in the stars which are overhead, and the stars seen are different, as one moves northward or southward. Indeed there are some stars seen in Egypt and in the neighbourhood of Cyprus which are not seen in the northerly regions; and stars, which in the north are never beyond the range of observation, in those regions rise and set. All of which goes to show not only that the earth is circular in shape, but also that it is a sphere of no great size: for otherwise the effect of so slight a change of place would not be quickly apparent.

Misattributed Pliny the Elder quote

There is an often-repeated misconception that one of Aristotle’s arguments was ships on the horizon. In reality this argument was given by Pliny the Elder:

The conformation of the waters also rises in a curve. … The same cause explains why the land is not visible from the deck of a ship when in sight from the masthead; 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.

This is immediately followed by an argument that parallels Aristotle, but applied to why oceans don’t fall off the globe:

Lastly, what other conformation could have caused the ocean, which we acknowledge to be at the extreme outside, to cohere and not fall away, if there is no boundary beyond to enclose it? The very question as to how, although the sea is globular in shape, its edge does not fall away, itself ranks with the marvellous. On the other side the Greek investigators, greatly to their delight and to their glory, prove by subtle mathematical reasoning that it cannot possibly be the case that the seas are really flat and have the shape that they appear to have. For, they argue, while it is the case that water travels downward from an elevation, and this is its admitted nature, and nobody doubts that the water on any coast has reached the farthest point allowed by the slope of the earth, it is manifest beyond doubt that the lower an object is the nearer it is to the centre of the earth, and that all the lines drawn from the centre to the nearest bodies of water are shorter than those drawn from the edge of these waters to the farthest point in the sea: it therefore follows that all the water from every direction converges towards the centre, this pressure inward being the cause of its not falling off.

See Also

• Flat earth taught in schools