Above is a diagram of the setting of the Pleiades over the western horizon as viewed from the Upton chamber in Massachusetts, an astronomically-aligned stone site in New England, discussed in this previous post.
As shown in the map diagram that accompanies that previous post discussing the Upton site, the chamber itself is aligned to allow an observer in the chamber to look out towards the north-west, with a line of sight which proceeds out over the body of water today known as "The Mill Pond" (and to the south of the body of water known as "Pratt Pond") and up to the summit of Pratt Hill to the west, and a narrow band of sky above the crest of the ridgeline of Pratt Hill.
Also discussed in that previous post is the pioneering work done by authors and archaeoastronomers James W. Mavor, Jr. and Byron E. Dix, whose analysis of that site and many other important New England stone structures is contained in their 1989 text entitled Manitou: The Sacred Landscape of New England's Native Civilization.
They first surmised that Pratt Hill would likely contain a corresponding mound to enable siting from the chamber -- and upon exploring Pratt Hill in the predicted location they found not one but three mounds along the horizon as seen from the Upton chamber. These they designated A, B, and C (sometimes calling them Cairn A, Cairn B, and Cairn C).
They then examined setting positions of the summer solstice sun, as being the first thing to check based on the azimuth from the chamber to the mounds, and were rewarded with precise alignments over Cairn C, as described on pages 45-46, calculating that the rate of changes in the obliquity of earth's axis gave a probable construction date of AD 670 plus or minus 300 years. This in itself is strong evidence arguing that this site (and the many others in the New England region that are commonly called "root cellars" and attributed to early colonial settlers) was made prior to the arrival of Europeans after Columbus.
As Mr. Mavor and Mr. Dix explain, an even more exciting discovery awaited them as they turned their attention to the significance of mounds A and B. Here is their description:
Then came the astronomical breakthrough. With a tentative date in hand, we looked at possible events during the same time period that could have been marked by the long mounds A and B. Stars change their positions of rise and set due to precession of the equinoxes and proper motion by about one-half degree per century. This can provide a very precise dating technique. We discovered that in AD 710, Alcyone, the brightest of the Pleiades cluster of stars, grazed the top of mound A and set in the notch between mounds A and B. Also, the match between the combined widths of mounds A and B and the width on the horizon of the Pleiades visible to the unaided eye, the eight brightest stars, is precise. Alcyone, Electra, and Pleione set in the notch between mounds, whereas Merope set on the southern end of mound A, Atlas on the northern end of A, and Maia, Taygeta and Asterope on mound B. 46.
Above is a rough drawing of the more precise diagram found on page 47 of Manitou and showing the mounds visible on the horizon-line at the summit of Pratt Hill from the Upton chamber, with the setting of the Pleiades, which are perfectly delineated by mounds A and B.
Equally compelling, the authors note that they have "identified a total of seven large stone mounds near the summit of the hill and two about 300 yards to the north" (47). It is very interesting that the Pleiades are known as the "Seven Sisters" the world over, describing the seven stars plus the two "parents" Atlas and Pleione (who are following at a distance from the seven), just as shown in the diagram above and also as appears to be indicated by the mounds at the summit of Pratt Hill (only three of which are visible on the horizon-line from the Upton chamber, those being mounds A, B, and C). There are also stone rows in the vicinity of the cairns, remarkably similar to those at other important New England aligned sites (see one discussion of the possible significance of these in this previous post).
An astute observer looking at the diagram above will only count six "sisters" plus the two parents Atlas and Pleione, and this is because the seventh of the Seven Sisters is famously difficult to see with the naked eye, for reasons discussed by astronomer Steven Gibson here.
As you can see in this previous post discussing the Pleiades and containing a diagram that can help you locate them using Perseus and other landmarks in their vicinity, the diagram above and the description in Mavor and Dix does not include Celaeno, who would have already set when Electra is in the position shown above, and who is one of the faintest of the Seven Sisters (although Asterope or Sterope is composed of two close-together stars which individually are each fainter than faint Celaeno). This other previous post discussing the probable role of the Pleiades in the famous myth of Aphrodite and Ares being caught in a net prepared by Hephaestos also contains a diagram of the Pleiades showing Celaeno.
The reason that I am bringing up the Pleiades and the excellent work of James Mavor and Byron Dix on this particular day of the year is the fact that the Pleiades are also anciently associated with this particular night of the year, as those authors also discuss in their book (and as can be found in many other resources discussing the Pleiades in myth and ancient culture, such as the Seven Sisters of the Pleiades: Stories from Around the World, by Munya Andrews (who herself was taught about the Seven Sisters in the Dreamtime and the traditions of her people in Australia by her grandmother, as she relates in the book).
As Mavor and Dix explain:
The Pleiades have been admired and critically observed in all ages of world history, second among the heavenly bodies only to the sun and moon. According to the historical astronomical literature, their heliacal rise and set and their midnight culmination have marked festivals, seasons, and calendars throughout the world. The Greeks saw this group of tightly interconnected quivering stars as a flight of doves, carriers of ambrosia to the infant Zeus, and used it for orienting their temples. The date of the midnight culmination was observed in the ancient Druids' rites of November first, and it became the traditional date of the Witches Sabbath or Black Sabbath in medieval Europe. This tradition has come down to the modern world as Halloween, though the midnight culmination has now slipped to November 21. 52.
By "midnight culmination," Mr. Mavor and Mr. Dix refer to the transit of the stars, or the time when they reach their highest point in their arcing journey across the sky (every object in our sky appears to trace out an arc, caused of course by the turning of the earth, and the highest point in that arc is the transit point or the culmination point -- for the sun, we call this point "high noon"). In other words, the Pleiades reached their highest point at midnight at this important day at one time long in the past, but the delaying action of precession now causes that midnight culmination to take place later in the month of November.
Even more interesting is the discussion of the quartz and water elements in the Upton site and their significance to other Pleiades-aligned sites in other parts of the world, which Mr. Mavor and Mr. Dix explain on the page following the passage just quoted:
The role of the Pleiades in the life of the Inca is known through historical records and surviving structures. The Inca saw this cluster of stars as a universal mother who gave birth to the other stars, as well as to the new sun of the June solstice and the new year. The earth mother, the sun and the Pleiades were also related in Inca myth by association with crystal, or quartz: a crystal fell into the water before the sun god emerged, and the rising of the sun from a spring was considered to be a birth not only from the water but also from the Pleiades. Further, these stars were related to water in the forms of springs and rain. All of these elements were brought together at the Coriancha Sun Temple, the most magnificent structure in Cuzco, originally sheathed in gold [note: this temple is usually called the Coricancha and spelled thus -- this is perhaps a typo by the publisher in the Mavor and Dix text]. There the rise of the Pleiades over a basin of water, also used for libations to the sun god, and the June solstice sunrise are marked by structural alignments as seen from a single observation point at the great gate. We have already pointed out that all of these elements are likewise present at Upton, where there is a single observation point for the summer solstice sunset and the Pleiades set, and the Pleiades set sightline passes over a lake. And quartz from the nearby quarry is frequently used in the stone row. Thus hydrography and topography at the Upton site indicate a cosmology in which the Pleiades and water are parts, with the Pleiades representing an earth mother, observed from a stone chamber buried in the earth. 53.
As stated previously, the Pleiades are not culminating at midnight right now (although thousands of years ago they were), but rather are reaching that highest point just before three in the morning and doing so about four minutes earlier each night, on their way to a midnight culmination. The reason for the four minutes earlier each night are discussed in this previous post on Orion and precession.
At this particular moment as this post is published, the nearly full moon is almost right on top of the Pleiades, effectively drowning them out from view. Tomorrow night (Halloween) the moon will be lower, and as it continues to wane the Pleiades will become more and more visible and easy to locate in the eastern sky after sunset (this diagram from the always-excellent night-sky diagrams at Sky & Telescope gives some visual detail). Look above the "V"-shaped Hyades that make up the Bull of Taurus and which are diagrammed in this previous post (bright Jupiter is still in Taurus -- the opening of the top of the "V" directs you to Jupiter, making the Hyades and hence the Pleiades easy to locate once the moon moves out of the way), as well as with some step-by-step instructions guiding you to find the Hyades and the Pleiades in this previous post from this very same time of the year, one year ago.
While many argue that the midnight culmination is the ancient origin that led to Halloween, I believe it is also important to point out that this festival marks a solar "cross-quarter day" in the annual yearly cycle, and that this significant transition towards winter is just as likely to be the impetus for Halloween. This previous post on Beltane (a cross-quarter day in May) discusses Samhain, the cross-quarter day most closely associated with modern Halloween, and cites the arguments that some have cogently made that if summer solstice is called "Midsummer" and winter solstice is called "Midwinter," then the cross-quarter days on either side of them were probably thought of as the start and end of summer or winter (you sometimes hear people declare that summer begins on summer solstice or that winter begins on winter solstice, but the tradition of calling the solstices mid summer or winter belies that interpretation).
In any event, now is an excellent time of year to begin enjoying the beautiful spectacle of the Pleiades, and to consider their tremendous importance around the world. It is also a good time of year to think with appreciation upon the work of James Mavor and Byron Dix in opening up new perspectives on the Pleiades with their analysis of the Upton site.