------------------------------------------------------------------------ [160] _Conv._ II. xiv. 244-217. “It is noble and lofty because of its noble and lofty subject, which is the movement of the heavens; it is lofty and noble because of its certainty, which is without flaw.” ------------------------------------------------------------------------ This quotation from Ptolemy is from his book on “judicial astronomy,”[161] and it may be that Dante was partly thinking of astrological predictions, as he almost certainly was in the passage that follows, where he adds that astronomy takes a long time to learn, not only because of its great range, but because experience is necessary to form a correct judgment. Nevertheless, it is probable that by its flawless certainty he meant that unchanging laws govern the celestial phenomena, so that they may always be predicted without error when the laws are known. ------------------------------------------------------------------------ [161] See p. 156. ------------------------------------------------------------------------ His definition of astronomy was accurate, for it was not until the middle of the nineteenth century that physical astronomy began to take its place beside the old “astronomy of position,” and in Dante’s day practically the only subject open to research concerning the heavenly bodies was their movements. Of these, and especially the “prime motion” of the diurnal revolution, he was vividly conscious. His favourite name for the heavens is “wheels”—“le ruote magne,” “eterne ruote,” “stellate ruote”;[162] and he often refers to them as a standard of motion. Thus when with Beatrice he was rapt from the summit of Mount Purgatory, to express the swiftness of their flight he says it was almost as rapid as the movement of the sky—“veloci, quasi come il ciel vedete.”[163] Constantine moving the capital of the Roman Empire eastwards is described as turning the Roman Eagle contrary to the course of the sky—“contra il corso del ciel,”[164] and the sky itself is described as a sphere which is ceaselessly at play like a lively child:— ------------------------------------------------------------------------ [162] “The great wheels,” “eternal wheels,” “starry wheels.” [163] “Swift, almost as the heaven ye behold.” _Par._ ii. 21. [164] “Against the course of the sky.” _Par._ vi. 2. ... “la spera, Che sempre a guisa di fanciullo scherza.”[165] [165] ... “That sphere, Which aye in fashion of a child is playing.” _Purg._ xv. 2, 3. (_Longfellow_). ------------------------------------------------------------------------