In astronomy, the Pleiades, or Seven Sisters (Messier object 45 or M45), is an open star cluster containing middle-aged hot B-type stars located in the constellation of Taurus. It is among the nearest star clusters to Earth and is the cluster most obvious to the naked eye in the night sky. Pleiades has several meanings in different cultures and traditions.
The cluster is dominated by hot blue and extremely luminous stars that have formed within the last 100 million years. Dust that forms a faint reflection nebulosity around the brightest stars was thought at first to be left over from the formation of the cluster (hence the alternate name Maia Nebula after the star Maia), but is now known to be an unrelated dust cloud in the interstellar medium that the stars are currently passing through. Astronomers estimate that the cluster will survive for about another 250 million years, after which it will disperse due to gravitational interactions with its galactic neighborhood.
With all that has so far been said of the precise mechanics of the Sheaf cycle, and of the fact that the Maya and Aztecs both held ceremonies to mark the completion of the Sheaf, one could be forgiven for thinking that 2.6 days prior to the end of every 52 calendar year period (18977.4 days), a most accurate conjunction would be in evidence between Venus and the Earth; for indeed, is not the purpose of the Mayan Calendar as with any calendar, to track and harmonise real astronomical cycles?
Well, apparently, according to historical records, the Aztecs and Maya, rather than set the end of each sheaf to coincide with an actual Earth-Venus conjunction, instead set it to coincide with an altogether completely different celestial event: a conjunction of the Earth and the Pleiades.
A closely packed group of seven stars, the Pleiades are sometimes referred to as the Seven Sisters (Image of The Pleiades ). But to such as the Aztecs, they were known as the Cabrillas. According to accounts provided by the early Spanish explorers of the 16th century, every 52 years the ceremony to mark the end of a sheaf period coincided with a precise observation of the Pleiades star group at midnight, when it appeared exactly upon the meridian. Now to be clear, in terms of a night-time observation, there is only one particular day of the year when such an event will happen; when one is able to observe the star cluster at the precise time of midnight, and at such a time when the stars themselves are located on the meridian line. At the precise moment though when this does happen, there is an active conjunction in evidence between the Pleiades, the Earth, and the sun. As indeed, at such a moment, exactly 180 degrees around the other side of the Earth, the Sun itself is at the ‘12 noon position’, and thus directly ‘behind’ the Earth and so lined up with the Pleiades.
In view of the above, it should be readily apparent then that there will be a notable discrepancy between the time interval of 32.5 Earth-Venus conjunctions (18977.4 days) and the time interval between two Earth-Pleiades conjunctions at 52 years apart. Indeed, the reason for this is that each of the two conjunction events involve, or rather rest upon, a different type of Earth year. In the case of the former, one is speaking of 52 calendar years, each of 365 days in length. In terms of the latter, 52 sidereal years, each of 365.256363 days. That this is so is due to the fact that the Pleiades, as a background star group, is susceptible to precession. One may note the difference between the two time intervals as follows:
Earth-Pleiades Cycle: 52 x 365.256363 = 18993.33088 days
Earth-Venus Cycle: (52 x 365) – 2.6 = 18977.4 days
18993.33088 – 18977.4 = 15.930876 days
In careful consideration of the above, there is nothing in and of itself that would suggest that a time interval of 52 sidereal years is of significance, purely from an examination of the basic time cycles that compose the Sheaf i.e. 365 and 260 days. Indeed, it is not clear at all how one could derive a time interval of 18993.33088 days from 18977.4 days, employing any sort of basic correction measure, with elegance or precision.
On the face of it therefore it would seem as if the two noted 52 year cycles are very much separate in terms of their actual physical duration, and that the Pleiades conjunction cycle is only loosely based upon the Sheaf. And yet, somehow over time, has taken precedence over it to become the celestial event to mark the end of such a period.
But why is this?
Just what made the 52 year Earth-Pleiades conjunction event so important to the Maya and the Aztecs?
A Day of Disaster
The key reason why an Earth-Pleiades conjunction was held to be of such importance to the Maya and the Aztecs, is that it was taken to be a signal that could potentially mark ‘the end of the world’. Indeed, the Central American Indians were of the firm belief, in accordance with their prophecies and oracles, from an era almost lost to history, that at some future date, the very movement of the heavens itself would cease, and that this would occur at the time of an Earth-Pleiades conjunction following the completion of a Sheaf cycle. Of course, exactly which future Sheaf would herald this day of disaster was not known, and this indeed had an important bearing upon the ceremonies of the Aztecs in particular. This is because at the very moment of the Pleiades-Earth conjunction at the end of a sheaf period, a human sacrifice was made by cutting out the heart of a captive warrior, with the belief that this very act would ‘keep the world turning’ and prevent the heavens from cessation of motion.
That the Aztecs and Maya conducted human sacrifice is rooted in an ancient myth concerning the very creation of the world, and of its setting into motion. An account of the myth essentially held that in a far off age when the world was first formed, a council of gods was assembled about a sacred fire. And that these gods sacrificed themselves by throwing themselves into the flames, which led to the sun making its first appearance on the horizon and being set into motion. Over the course of time following the successive transmission of this ancient myth, the Aztecs and the Maya began to believe at some point that such sacrifices were periodically necessary in order to actually maintain celestial motion. And to this end, they sought to recreate the original sacrifice of the gods with a ritual ceremony of their own.
Lasting for a total of five days during the end of each sheaf period, the whole of the populace in the surrounding villages of the Mayan-Aztec Empire would partake in a ritual extinguishing of all of the fires throughout the community. They would then take to the hills at the culmination of the ceremony at midnight, when the critical observation of the Earth conjuncting the Pleiades was to occur. At the moment of the alignment the Indian priests would stab the warrior to death and cut out his still beating heart. They would then start a new fire within his chest cavity and bathe his heart within the flames. If at the precise moment of the sacrifice the Pleiades failed to cross over the meridian at midnight then the Indians knew that the world would end as the heavens had ceased to move, and their sacrifice had not been sufficient. If however the Pleiades continued on its course then this was taken to be a sign that the sacrifice had been sufficient and that the world would last at the very least for another 52 years, at which time yet another sacrifice would of course have to be made.