We know little further of the astronomical attainments of the Aztecs. That they were acquainted with the cause of eclipses is evident from the representation, on their maps, of the disk of the moon projected on that of the sun.[199] Whether they had arranged a system of constellations is uncertain; though that they recognized some of the most obvious, as the Pleiades, for example, is evident from the fact that they regulated their festivals by them. We know of no astronomical instruments used by them, except the dial.[200] An immense circular block of carved stone, disinterred in 1790, in the great square of Mexico, has supplied an acute and learned scholar with the means of establishing some interesting facts in regard to Mexican science.[201] This colossal fragment, on which the calendar[202] is engraved, shows that they had the means of settling the hours of the day with precision, the periods of the solstices and of the equinoxes, and that of the transit of the sun across the zenith of Mexico.[203] We cannot contemplate the astronomical science of the Mexicans, so disproportioned to their progress in other walks of civilization, without astonishment. An acquaintance with some of the more obvious principles of astronomy is within the reach of the rudest people. With a little care, they may learn to connect the regular changes of the seasons with those of the place of the sun at his rising and setting. They may follow the march of the great luminary through the heavens, by watching the stars that first brighten on his evening track or fade in his morning beams. They may measure a revolution of the moon, by marking her phases, and may even form a general idea of the number of such revolutions in a solar year. But that they should be capable of accurately adjusting their festivals by the movements of the heavenly bodies, and should fix the true length of the tropical year, with a precision unknown to the great philosophers of antiquity, could be the result only of a long series of nice and patient observations, evincing no slight progress in civilization.[204] But whence could the rude inhabitants of these mountain-regions have derived this curious erudition? Not from the barbarous hordes who roamed over the higher latitudes of the North; nor from the more polished races on the Southern continent, with whom, it is apparent, they had no intercourse. If we are driven, in our embarrassment, like the greatest astronomer of our age, to seek the solution among the civilized communities of Asia, we shall still be perplexed by finding, amidst general resemblance of outline, sufficient discrepancy in the details to vindicate, in the judgments of many, the Aztec claim to originality.[205]