Home pageAstronomy LinksStars PageAstronomy Articles

November is the Month of the Pleiades

Pleiades Midnight Culmination ushers in November

Halloween falls after sunset on October 31, on the eve of November 1. This celebrated holiday, derived from the ancient Celtic festival known as Samhain, coincided with the midnight culmination of the Pleiades star cluster. Samhain also fell on the cross-quarter day that lies midway between the September equinox and the December solstice. In many cultures, this cross-quarter day is associated with the Day of the Dead.
The Pleiades star cluster - also known as the Seven Sisters - culminates at midnight on only one day of the year. At this time, the Pleiades is opposite the Sun. Therefore, this star cluster rises in the east around sunset and sets in the west around sunrise. When midway between rising and setting, the Pleiades culimates - reaches its highest point in the heavens - at midnight. One night before its midnight culmination, the Pleiades culminates 4 minutes after midnight; the night after the midnight culmination, the Pleiades culminates 4 minutes before midnight. Because the Pleiades cluster returns to the same place in the sky four minutes earlier with each passing night, the midnight culmination can only happen once a year.

Cross-quarter Day & Midnight Culmination Askew?

The age-old festival of Samhain apparently celebrates the midnight culmination of the Pleiades cluster AND the cross-quarter day as simultaneous, or near simultaneous events. Since the time period between the autumn equinox and the winter solstice represents approximately 90 days, that means about 45 days should lapse between the the September equinox and November 1, and about 45 days from November 1 to the December solstice.
For the fun of it, I went to this web site to find out the number of days between the September 23, 2007 equinox and November 1, 2007, and from November 1, 2007 to the December 22, 2007 solstice. My, was I ever shocked to find out that less than 40 days stand between the equinox and November 1, but more than 50 days separate November 1 from the December solstice. This is hardly a good ballpark reference for this 2007 cross-quarter date. November 7 coincides much more closely to the mid-season point.
But the midnight culmination seems even more askew, being 3 weeks out of kilter. In our time, the midnight culmination of the Pleiades occurs on or near November 21.

October 31 paying homage to the Julian calendar?

Let's presume that the ancient forebearer of Halloween, Samhain, celebrated the midnight culmination of the Pleiades and this cross-quarter day on the same night. That prompted me to look back in time, searching for a century when these two natural phenomena happened in close conjunction. It appears the two paired up on or near the same date around 1000 A.D. This is almost 6 centuries before the advent of the Gregorian calendar in 1582.
In the 11th century, the Julian calendar used back then was about one week out of step with the seasons. In the year 1000, the September solstice fell on September 17 and the December solstice fell on December 15. Amazingly, October 31, 1000, fell about midway between these equinox and solstice dates. Despite our use of the Gregorian calendar, we still celebrate Halloween on October 31. That may account for our present day one-week discrepancy between the date of Halloween/All Saints Day and the cross-quarter day.

Precession of Equinoxes Advances Culmination Date

As far as the Gregorian calendar is concerned, this mid-season cross-quarter day always falls on or near November 7. Some 1,000 years ago, the midnight culmination of the Pleiades and the cross-quarter day pretty much happened on the same date. At the present time, the midnight culmination of the Pleiades takes place on or near November 21, about two weeks after the cross-quarter day. This changing date for the midnight culmination is due to something called the precession of the equinoxes.
By the time the year 3000 comes rolling around, the midnight culmination of the Pleiades will happen in December, and December will take its turn as the month of the Pleiades.

copyright 2007 by Bruce McClure

October 2007 Feature * December 2007 Feature