Similarly, according to Tena’s chronography, the summer solstice took place toward the end of the sixth month, called Etzalqualiztli. At that time, sunrise would have seemed to stand still at an azimuth of ca. 65°. Viewed from the top of the Templo Mayor, sunrise would have taken place behind Tepetlaoxtoc, in the western foothills of the Sierra de Patlachique, across the briny waters of Lake Texcoco where the Basin’s saltworks were . In coincidence, the first day of the seventh month, called Tecuilhuitontli, was devoted to a celebration in honor of Huixtocihuatl, the goddess of salt. Close to the summer solstice bearing, further east from the salty lakeshores, there were fertile agricultural terraces with cultivated milpas, or cornfields. Sahagún noted that in the eighth month, called Huey Tecuilhuitl, the goddess of fresh corn, Xilonen, or Chicomecoatl, was also celebrated. It does not seem coincidental that the name of Chiconcuac, a settlement found along this summer sunrise view, is derived from the name of this goddess. The winter solstice occurred close to the beginning of the 16th month, Atemoztli, a time in which sunrise seems to stand still at its southernmost azimuth of ca. 116°, on the northern slope of the Iztaccíhuatl volcano, the “sleeping woman” . According to Sahagún, the beginning of the following month, called Tititl, pipp mobile systems was devoted to celebrating Ilama Tecuhtli , also known as Tona .
The correlation between sunrise close to the woman-like volcano and the celebration of womanhood in general is striking. In summary, there seems to be a noteworthy association between some elements of the horizon calendar and the feasts and celebrations of each season: the arid spring equinox, when the sun rises behind Mount Tlaloc, was associated with Tlaloc, the god of water and rain. The summer solstice, when sunrise occurs behind the distant salty shores of Lake Texcoco, was associated with salt and summer corn. Finally, the winter equinox, when the sun rises at the side of Iztaccihuatl, the sleeping woman, was associated with womanhood and female gods.The previous analysis suggests a correlation between the Mexica calendar and the topographic elements of the Basin’s eastern horizon but leaves an important question unanswered, namely that of the calendric role of Mount Tlaloc. It seems very clear that the horizon calendar, as viewed from Tenochtitlan’s Templo Mayor, should have relied strongly on the date of the sun rising behind Mount Tlaloc, as this mountain could have provided, better than any other, the accuracy needed for the precise estimation of the length of the solar year and for leap year adjustments. However, none of the 16th century codices and manuscripts consulted for this study describe this phenomenon in a direct and clear manner, other than a general mention in Sahagún that at the beginning of the third month, close to the alignment date of sunrise with Mount Tlaloc, a feast was made to Tlaloc, the god of rains. If the alignment of sunrise with Mount Tlaloc was indeed an important calendric landmark when viewed from Templo Mayor, a clear mention could have been expected in the ancient codices, including the question of why did the Mexica not use the Mount Tlaloc alignment to mark the beginning of the new year.
The answer to this paradox may lie in the ruins of the ceremonial center found at Mount Tlaloc’s peak. The summit of Mount Tlaloc is crowned by a rectangular walled enclosure about 40 m east–west by 50 m north–south . This courtyard, or tetzacualo, consists of stone walls that have been estimated to have been 2 to 3 m high when originally built, with a ca. 94° east-west azimuth . The eastern side of the precinct opens to a 150 m-long, ca. 6 m-wide, walled straight causeway that has an azimuthal bearing of 101°55′, offset more than 8°southward from the roughly east–west bearing of the enclosure . Because the causeway runs down slope on the western side of the peak, some researchers have wondered whether the causeway was intentionally misaligned with the axis of the enclosure in order to accommodate a particular orientation to the setting sun . If viewed upslope, the azimuthal bearing of Mount Tlaloc’s causeway and the angular elevation of 4°02′ above the celestial horizon defines a point in the celestial sphere that aligns with the sun’s apparent position on February 23 to 24 each year. That is, an observer standing at the lower end of the causeway will see the rising sun appear in the center of the upper part of the stone ramp on February 23 or 24, after the last nemontemi day and in synchrony with the beginning of Basin’s new year as defined by Tena’s first chronology . The causeway seems to have been constructed as a calendric solar marker with a celestial bearing that allows for leap-year adjustments and indicates the end of the year and the beginning of a new solar year. The idea that the structure was used for precise astronomical observations is further reinforced by the fact that it seems to have had specific sight markers to avoid parallax error. Wicke and Horcasitas described that the causeway had a stone circle in its upper end where, presumably, a monolith could have stood. Correspondingly, it still has a stone square with an erect, 40-cm monolith in its lower end. Jointly, they could have been used as alignment markers to further improve alignment accuracy.
Almost a century ago, Rickards described the presence of a monolith with the figure of Tlaloc in the center of the tetzacualo and aligned with the causeway, as had been described earlier by Durán . Although the figure has been removed since , it could have functioned as yet another element for precise solar alignments . The importance of Mount Tlaloc as a solar observatory is enhanced by the fact that the two largest peaks of the Mexican Transversal Volcanic Axis east of the Basin of Mexico are visible from its peak and almost perfectly aligned. Viewed from the center of the stone courtyard, the nearest peak, Matlalcuéyetl or Malinche has an azimuth of 105°52.7′ while Citlaltépetl or Pico de Orizaba has an azimuth 105°26.5′. Because the azimuthal difference between the two peaks is less than the angular width of the sun’s disk, viewed at dawn they will seem like a single mountain with twoclose crests, where sunrise would be seen on February 10. In short, the causeway in Mount Tlaloc marks very precisely the beginning of the Mexica solar year, but the summit courtyard could have been used to identify a precise alignment 15 d before the beginning of the year, during Izcalli—the last month of the Mexica calendar. Ceramic fragments are common in and around the enclosure, and these fragments have been collected by archeologists and dated to the Mesoamerican Classical Period, early Toltec, and Mexica, suggesting that the site was used for ceremonies from the beginning of the Common Era to the collapse of the Mexica Empire in the 16th century . Although the constructions have not been dated with precision, early chroniclers reported that the sanctuary in Mount Tlaloc was used by the Toltecs before the 7th century CE and by the Chichimecs in the 12th century, before the arrival of the Aztecs to the Basin . It seems likely, then, industrial drying rack that the astronomical use and significance of the Mount Tlaloc causeway, and hence the beginning of the Mesoamerican calendar, preceded the founding of Tenochtitlan and the development of the Mexica civilization.Broda noted that the causeway of Mount Tlaloc points toward Mount Tepeyac, a hill that emerges from the Basin’s sediments south of the Sierra de Guadalupe, a range of basaltic mountains in the center of the Basin of Mexico. Indeed, when viewed from Tepeyac, Mount Tlaloc has an azimuth of 100°54′, very close to the bearing of the causeway on Tlaloc’s peak and an elevation of 2°38′ . Mount Tepeyac is the southernmost hill of the Sierra de Guadalupe, only 4 km northeast and 7 km east of the pre-Hispanic settlements of Tlatelolco and Azcapotzalco. According to Sahagún , the hill had been a place of worship and pilgrimage for the inhabitants of the Basin long before the Spanish Conquest. Broda’s observation suggests a visual alignment of calendric importance may have existed between the Tepeyac ranges and Mount Tlaloc. Indeed, sunrise alignment with Mount Tlaloc occurs on February 24 if viewed from Mount Tepeyac. Like the alignment in the summit’s causeway, the Mount Tepeyac solar alignment date corresponds with that of the causeway and also heralds the beginning of Tena’s new year . It can be hypothesized, then, that before the Mexica built the Templo Mayor, the inhabitants of the Basin of Mexico were using the alignment between Tepeyac and Mount Tlaloc as a fundamental landmark in their horizon calendar. They could have adjusted with precision their agricultural calendar to the solar year based on the sunrise alignment between Mount Tlaloc and Tepeyac.Agriculture was already well established in the Basin of Mexico by the first millennium BCE, largely around the Pre-classic Cuicuilco culture in the southwest of the Basin.
The Cuicuilco civilization collapsed in the 3rd century CE when the Xitle volcano became active and covered the whole south of the Basin under a mantle of lava . Broda has analyzed the horizon calendar as viewed from the main pyramid of Cuicuilco, built ca. 600 BCE, almost nine centuries before the apogee of the Mexica Empire. She concluded that the sunrise alignment with Mount Papayo on March 24, close to the equinox, “could have constituted a simple and effective mechanism to adjust for the true length of the solar year, which needed a correction of 1 d every 4 y.” Broda’s studies on Cuicuilco provide strong evidence suggesting that rigorous calendric calculations and leap-year adjustments wereat the heart of the development of Mesoamerican agricultural civilizations from very early times and were certainly very important in pre-Classical settlements. In addition to the equinoctial alignment of sunrise with Mount Papayo, the Cuicuilco observatory would have provided good calendric alignments with Mount Telapon and with the “head” of the “sleeping woman” profile of the Iztaccihuatl volcano . The latter date is very close to Tena’s estimate for the beginning of the Mexica calendric year and, because of Iztaccihuatl’s majestic proportions when viewed from the south of the Basin, could also have constituted an important landmark for calendric adjustments .Many early codices seem to validate the working hypothesis that Mount Tlaloc was instrumental in the establishment of the date of the Basin’s new year and in the adjustments necessary to keep the agricultural calendar in synchrony with the solar year. As discussed previously, Sahagún described how Atlcahualo, the first month of the year, was devoted to celebrate the Tlaloc gods of rain. Similarly, in Duran’s description of the nemontemi days, he reported that the year ended when a sign of the first day of the new year became visible above a mountain peak , suggesting the use of a landmark alignment to indicate the beginning of the new year. Similar associations between Mount Tlaloc and the first day of the new year are shown in other ancient codices, such as Codex Tovar, Codex Borbonicus, and the Wheel of Boban . The narrow historical relationship between the first month Atlcahualo and Mount Tlaloc has been recently described in detail by Broda . From an ecological perspective, it seems clear that the rugged eastern horizon of the Basin provided precise landmarks that would have allowed to adjust the xiuhpohualli, the count of the years, with the true solar calendar. Sahagún’s description of the feasts and ceremonies associated with some of the Mexica “months,” or 20-d periods, coincides well with themes from landmarks visible in the sunrise horizon from the Templo Mayor. Because of its position near the equinox, when viewed from the center of the Basin, Mount Tlaloc seems to have played a very important calendric role. The long causeway at the summit strongly suggests that the ceremonial structure was used as a solar landmark, aligning very precisely with the rising sun on February 23 to 24 and October 19 to 20. The same alignment is found if Mount Tlaloc is viewed from Mount Tepeyac, a holy site whose use as a sacred mount and solar observation post preceded the establishment of the Mexica civilization in the Basin.