And when, by the singular sharpness of his wit, he had satisfied himself that the body of heaven was finite, he wanted to find out, in the next place, of what form it was and how it was limited by the superficies that compassed it round.
[Hayy contemplates Sun, Moon, and Stars.]
First of all he contemplated the sun, moon, and stars, and saw that they all rose in the East, and set in the West; and those lights which went right over his head described a greater circle, whilst those at a greater distance from the vertical point towards the North or South described the lesser circle. So that the least circles which were described by any of the stars were those two which went round the two poles, the one North, the other South, the last of which is the circle of the star Suhail (that is Canopus) and the circle Al-farkadani, which was next the northern.
And, since he lived in an island situated under the equinoctial line, all those circles cut the horizon at right angles and had alike reference to North and South, seeing both the poles appeared to him at once. He observed that when a star arose in a larger circle and another in a lesser, yet they both arose together and set at the same time, and he noticed it to be the case with all stars at all times.
[Hayy concludes that the Heaven is of a spherical Figure.]
Therefore, it was evident to him that the heaven was of a spherical figure.
In this he was further confirmed by observing the return of the sun, moon, and the other stars to the East after their setting; and also because they always appeared to him of the same proportion of magnitude when they arose, when they were in the midst of heaven, and when they set; for if their motions had not been circular, they must have been nearer to sight at some time than at others; and then their dimensions would have appeared greater or lesser when they were nearer to him or further off.
But since there was no such appearance, he felt assured that the figure of heaven was spherical. Then he considered the motion of the moon, and saw that it was carried from the East to the West as the other planets were. So that at length a great part of astronomy became known to him.
It appeared to him, further, that the motions of the planets were in different spheres, all of which were comprehended in another that was above them all, and which turned about all the rest in the space of a day and a night. But it would be too tedious to set down, to explain in particular, how he advanced in this science; and what we have already said is quite sufficient for our present purpose.
Now, when he had attained to this degree of astronomical science, he found that the whole of the heavens and whatever it contained was one thing composed of parts mutually joined together; and that all those bodies which he had before considered—as earth, water, air, plants, animals—were all of them contained in it, so that none of them went beyond its bounds. He found also that the whole body was like one animal in which the stars answered to the senses; the spheres joined together answered to the limbs; and all those bodies therein, which were liable to generation and corruption, resembled those things which are contained in the belly of an animal.