Illustration of the Aztec Calendar. The most precious relic of Pre-Columbian culture in Mexico is the Aztec Calendar Stone ( measures thirteen feet in diameter and weighs twenty-four tons and carved into porphyritic basalt). A blend of real science and Aztec mythology. The face of the stone contains mythological and astrological figures and signs in geometrical order. The outer border contains two serpents which represent time and the chief Aztec gods. Within this border are the rays emanating from the central figure which represents Tonatiub, the Sun god. To the amazement of archaeologists, the stone, when deciphered, revealed a sophisticated knowledge of astronomy. Aztec astronomy, based chiefly on astrology, divided the solar year into 18 months of 20 days each.