The Inferior Planets
Having dealt with the theories of the Sun, the Moon, and the superior planets, we are left with the problem of describing the motions of Venus and Mercury. The reason why these planets are called inferior is that according to the usual order of the celestial bodies (see page 261) they are situated below the Sun, or, more precisely, between the orbits of the Sun and the Moon. Now in ancient astronomy the order of the planets is more or less a convention, without any sure foundation upon observable facts. Thus Ptolemy reminds us that there is no perceptible parallax in any of the planets, and that one has never observed a passage of Venus or Mercury before the disc of the Sun. He even maintains that such a passage has never occurred; but on the other hand this latter circumstance is no proof that there are no planets below the Sun since their orbits could be so inclined that a conjunction never results in a passage [IX, 1; Hei 2, 207]. It is clear, however, that it is too superficial to distinguish the inferior from the superior planets by their conventional and essentially arbitrary order.