The urban layout of Teotihuacan exhibits two slightly different orientations, which resulted from both astronomical and topographic criteria. The central part of the city, including the Avenue of the Dead, conforms to the orientation of the Sun Pyramid, while the southern part reproduces the orientation of the Ciudadela. The two constructions recorded sunrises and sunsets on particular dates, allowing the use of an observational calendar. The orientation of the Sun Pyramid was intended to record “the sunrises on February 11 and October 29 and sunsets on April 30 and August 13. The interval from February 11 and October 29, as well as from August 13 to April 30, is exactly 260 days”. The recorded intervals are multiples of 13 and 20 days, which were elementary periods of the Mesoamerican calendar. Furthermore, the Sun Pyramid is aligned to Cerro Gordo to the north, which means that it was purposefully built on a spot where a structure with a rectangular ground plan could satisfy both topographic and astronomical requirements. The artificial cave under the pyramid additionally attests to the importance of this spot.