The Latin works, _De Monarchia_, _De Vulgari Eloquentia_, the _Eclogues_, and the _Letters_, do not give much scope for astronomical references; yet there are many similes drawn especially from the sun and moon, and the thought of the spheres as instruments of God’s Will in bringing about events on earth is constantly recurring. In the curious work _Quæstio de Aqua et Terra_, Dante argues learnedly about the spheres, the orbit of the moon, and her effect on the tides, and the influences of the stars. The book purports to have been written at Verona in 1320 as the outcome of a discussion in which Dante had taken part as to the respective heights of land and ocean, a problem which had been dealt with by Ristoro d’ Arezzo some forty years earlier. Its authenticity has been questioned, partly under the mistaken idea that facts such as gravity, the spherical form of Earth, and the connection of tides with the moon were not known until much later; but as a matter of fact the discussion is quite in the manner of Dante’s day, and no facts or theories are put forward which he could not have learned from books with which we believe he was acquainted.[157] The internal evidence of his authorship is strong, and belief in it seems to be gaining ground among experts.[158] ------------------------------------------------------------------------ [157] The spherical form of Earth, and the action of gravity at the earth’s surface, were commonplaces with the Greeks, as we have seen in Part I. of this book. Posidonius, Strabo, and other classical writers speak of the tides as following the revolution of the heavens, and having periods similar to those of the moon; Albertus Magnus and Aquinas ascribe them to the influence of the moon, and so does Dante himself in _Par._ xvi. 83. [158] See Moore, _Studies in Dante_, II. “The Genuineness of the _Quæstio de Aqua et Terra_,” for a complete discussion of the question. ------------------------------------------------------------------------ Finally, the _Divine Comedy_, the work of Dante’s maturity, which took him so many years to write, and for which he had studied ever since he closed the _Vita Nuova_ with the resolve to write more worthily one day of Beatrice,[159] focuses in one unique and finished work the thoughts and ideals, the knowledge and fancies, of the poet and his age. The subject of the poem, taken in its literal sense, is cosmical, for it describes a journey in which the author penetrates to the centre of the universe, and passes from planet to planet until he reaches the outermost sphere. Moreover, the journey is assumed to take a definite space of time, and the passing of the hours by day and by night is indicated by the successive positions of sun or stars, and the phases and movements of the moon. The _Divine Comedy_ contains more than twice as many allusions to the heavenly bodies as all the other works of Dante put together.