Other Stars Than Ours

Natural History, April, 2001 by Anthony F. Aveni

Aztec astronomers had their own reasons for sky watching.

The Aztecs saw in the heavens the sustainers of life--the gods they sought to repay, with the blood of sacrifice, for bringing favorable rains, for keeping the earth from quaking, for spurring them on in battle. Among the gods was Black Tezcatlipoca, who ruled the night from his abode in the north, with its wheel (the Big Dipper). He presided over the cosmic ball court (Gemini) where the gods played a game to set the fate of humankind. He lit the fire sticks (Orion's belt) that brought warmth to the hearth. And at the end of every fifty-two-year calendrical cycle, Black Tezcatlipoca timed the rattlesnake's tail (the Pleiades) so that it passed overhead at midnight--a guarantee that the world would not come to an end but that humanity would be granted another epoch of life. The priests in Tenochtitlan, the Aztec capital, climbed to the top of their sky watchers' temple on the Hill of the Star to witness this auspicious sign. The Aztecs lived their sky, knowing that everything that happened on earth was the outcome of destiny. And they wrote it all down in their books.

Created in the late fifteenth century, the Codex Borgia, a seventy-six-page "screenfold" document of deerskin, is one of a handful of Aztec manuscripts that escaped the Spaniards' book burnings. "Paganism must be torn up by the roots from the hearts of these frail people!" wrote the Dominican friar Diego Duran, a sixteenth-century chronicler horrified by such documents, which often included graphic depictions of sacrifice and dismemberment. And in fact, some pages in the Codex Borgia do have illustrations of stabbing and blinding and of blood squirting from a torso with dangling intestines. But this random survivor from Mexico's past is also a mine of impressive information about Aztec astronomy.

Much of the first half of the codex prescribes the performance of "debt payment" rituals, as the sixteenth-century Franciscan priest Bernardino de Sahagun put it. These rituals were to be carried out with reference to a sacred calendar called the tonalpohualli, the "count of days." This round of time operated much like the Western calendar's cycling of seven named weekdays in tandem with each month's numbered days (Monday the 1st, Tuesday the 2nd, and so on), except that it cycled twenty signs (representing various animals, plants, and forces of nature) against thirteen numbers. This yielded a sequence of 260 uniquely named days (1 Crocodile, 2 Wind, 3 House, and so on), after which the sequence would begin again. The rest of the Codex Borgia is a pictorial narrative detailing the attributes of supernatural characters--probably patron gods of the ruling Aztec lineage for which the codex was prepared--and the world they inhabited prior to humanity's existence.

Recently I have been delving into certain pages in this little-understood codex--pages that resemble Mayan almanacs, which are well known for their precise prediction of astronomical events. My working hypothesis has been that the dates designated in the Codex Borgia for the performance of rituals were chosen by Aztec prognosticators, who calculated when the heavenly bodies would be in especially auspicious or dangerous positions. If we take just one page in the codex--page 28--as an example, we can begin to see how they put their astronomical expertise to use.

This page (reproduced at the opening of this article) depicts five similar figures, one for each of the cardinal directions (most likely starting with east at the lower right and running counterclockwise to north, west, and south), along with a central one representing the up-down axis, a standard dimension in Mesoamerican cosmology. The featured player is Tlaloc, the goggle-eyed, long-fanged god of rain and fertility, who appears at the top of each frame in a similar pose but with varied accoutrements. At lower right, for example, the rain he unleashes from each hand is studded with flint knives (hail); at upper right, it carries flowers; at lower left, it is accompanied by wind in the form of Quetzalcoatl, the Feathered Serpent. Absorbing the seasonal punishments and rewards doled out by the forces of nature is the maize crop. In each frame, the maize is personified as an aged female deity, who is shown in a field beneath Tlaloc, flanked by plants bearing ears of corn.

Tlaloc may be the star of the show in another way, because he has a link with Venus. A variety of recent studies of both Mayan and Aztec sculpture, carved .inscriptions, and architecture have led to the discovery of a cult known to scholars as the Tlaloc-Venus-War complex, so called because of the imagery worn by its practitioners and the timing of its rites to coincide with particular stations of the planet Venus.

One of the brightest objects in the night sky, Venus commanded the attention of ancient sky watchers, just as it does today. Part of its fascination is that it periodically shifts from appearing as an evening star that sets shortly after sunset to appearing as a morning star that rises just before dawn. Between these manifestations, it disappears from the night sky altogether. All this is a consequence of the fact that the planet's orbit is closer to the Sun than ours is, so that from our point of view Venus seems to move back and forth across the Sun's position. (The reason we sometimes can't see Venus is that it gets so "close" to the Sun that it is lost in the daylight.)