Aristotle's geographical speculations anticipated by almost two thousand years the rationale behind Columbus' voyage. Ferdinand Columbus suggested that his father was familiar with the following passage: "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. . . . All of which goes to show not only that the earth is circular in shape, but that it is a sphere of no great size: for otherwise the effect of so slight a change of place would not be so quickly apparent. Hence, one should not be too sure of the incredibility of the view of those who conceive that there is a continuity between the parts about the pillars of Hercules [the strait of Gibraltar] and the parts about India, and that in this way the ocean is one."