Other Stars Than Ours

Natural History, April, 2001 by Anthony F. Aveni

Venus completes its entire appearing-disappearing-reappearing-disappearing act every 583.92 days. Coincidentally, Venus runs through five such cycles in the course of almost exactly eight solar years (a solar year is 365.2422 days). As a consequence, whatever we see Venus doing in the sky on a given date in our calendar, we can expect to see the planet do again, eight years later, on exactly or almost exactly the same day of the same month. For example, Venus begins to appear as a morning star around April 1, 2001, and around April 1, 2009, it will put on a repeat performance. The Aztecs, who had a 365-day calendar by which they measured the solar year, were well aware of this mathematical meshing, which added to Venus's attraction as an astronomical phenomenon.

At the turn of the twentieth century, the visionary German scholar Eduard Seler, who drew on his knowledge of Babylonian astronomical texts, suggested that page 28 in the Codex Borgia may have referred to events in the Venus cycle. He declared himself unable, however, "to discover a law for the days." He was referring to the dates, some of them effaced, that were written along the base of each of the five framed Tlalocs. Fortunately, not only has decipherment of these symbols improved since Seler's time, but high-speed computers now enable us to easily calculate the astronomical events to which ancient dates may correspond.

Within each frame, two tonalpohualli (ritual calendar) days are specified, as well as a third day that identifies the particular 365-day year in which the two days fall. Inconveniently for us, however, central Mexican calendars provided no long-term running count for the years, as their Classic Mayan counterparts did. Instead they tell us only the sign and number for the 360th day of the year they are referring to. (It's as if we identified 2001 only as "the year Tuesday-the-25th" because in 2001, Christmas falls on a Tuesday.) But this shorthand presented no problem to the Aztecs, since the same tonalpohualli day did not coincide again with the 360th day of the year for a long while.

Indeed, exactly how long did it take for a year name to be repeated? One might guess 260 years, since there are that many distinct days in the tonalpohualli cycle, with its twenty day signs and thirteen numbers. But when the twenty day signs are rotated in order against 365-day solar years, only four signs happen to fall on the 360th day--Reed, Flint, House, and Rabbit, after which we come back to Reed. These four, cycled against the thirteen numbers, provide fifty-two unique names for years. Thus the year name in which a person was born would not recur before he or she reached the age of fifty-two.

In each frame on page 28 of the codex, one of the day names serves to refer to the year. We can distinguish it because the day sign is superimposed over a sort of trapezoidal symbol, which essentially translates as "this is the year." Deciphering the year names on this page, we get the following: upper right, 2 Flint; upper left, 3 House; center, 5 Reed. Although the year names for the bottom two Tlaloc figures are partly effaced, it's a good guess that we're dealing with a sequence of five consecutive years, beginning at the lower right panel, continuing counterclockwise, and ending at the center: 1 Reed, 2 Flint, 3 House, 4 Rabbit, 5 Reed.