sun also passed the zenith. Here was a visible, convenient mechanism to signal the start of the year. Tying the sun to the stars was different from commencing the calendar at one of the solstices, which mark the sun's most northerly or southerly passage. The Pleiades, being both prominent and in the right place at the right time, would have been the ideal asterism for the daykeepers who advised the rulers of Teotihuacan, and this determined the orientation of the whole city. But the orientation of Teotihuacan may have been a natural choice because of another curious coincidence. The intervals of time marked out by its axis were ideally tuned to the base-20 body-count (fingers-and-toes) which was in widespread use throughout Mesoamerica. By accident of its location, forty days, or twice the body count, separate sunrise along Teotichuacan's axis and sunrise on the day of solar zenith passage. Who put this heavenly ordained plan into action, when, and why? What sequence of events led to the arrangement of an entire city according to heavenly dictates? Great significance would no doubt have been attached to nature's signals at the moment of initiation of the earth's cycle of fertility. In the vicinity of the Valley of Mexico the first thunder is heard in the time leading up to the first overhead passage of the sun. Even today, around  May 1st dark clouds gather in the east over Mt Tlaloc, home of the Aztec rain-god. They built a shrine at its summit where