In the light of the recent excavations of the Templo Mayor in downtown Mexico City, we explore the problem of the role of astronomy, calendar, and the landscape in the design and orientation of the building and of the city in general. We employ ethnohistoric data relating to the foundation myth of Tenochtitlan as a means of generating hypotheses concerning astronomical orientation that can be tested by reference to the archaeological record. We find that eastward-looking observations (implied in dismantling and reconstructing the myth) that took place around the time of the equinox may have been related to an attempt to transform a true east orientation from the natural environment into the architecture via a line that passed through the center of the Temple of Huitzilopochtli (the more southerly temple of the pair constituting the top of the Templo Mayor). It also is possible that the notch between the twin temples served a calendrical/orientational function. Evidence is presented to support the view that the mountain cult of Tlaloc, represented in the environment on the periphery of the Valley of Mexico by Mount Tlaloc, also may have directly influenced the orientation of the building and that it was part of a scheme for marking out days of the calendar by reference to the position of the rising sun at intervals of 20 days from the spring equinox. In this regard, we discuss the connection between the Templo Mayor and an enclosure containing offertory chambers atop Mount Tlaloc, which is located on a line extended to the visible horizon 44 km east of the ceremonial center. The ethnohistoric record implies that this place had been used for sacrifices to the rain god after whom the other of the twin temples of the Templo Mayor was named.
