The Aztecs calculated their civil year by the solar; they divided it into eighteen months of twenty days each, and added five complimentary days, as in Egypt, to make up the complete number of three hundred and sixty-five. After the last of these months the five nemontemi or "useless days" were intercalated, and, belonging to no particular month, were regarded as unlucky, by the superstitious natives. Their week consisted of five days, the last of which was the market day; and a month was composed of four of these weeks. As the tropical year is composed of about six hours more than three hundred and sixty-five days, they lost a day every fourth year, which they supplied, not at the termination of that period, but at the expiration of their cycle of fifty-two years, when they intercalated the twelve days and a half that were lost. Thus it was found, at the period of the Spanish conquest, that their computation of time corresponded with the European, as calculated by the most accurate astronomers.
At the end of the Aztec or Toltec cycle of fifty-two years,—for it is not accurately ascertained to which of the tribes the astronomical science of Tenochtitlan is to be attributed,—these primitive children of the New