Astronomy is the oldest of the sciences, so it’s not surprising it’s found its way into the social side of many cultures. The winter solstice and Christmas are deeply connected as is the date of Easter and the spring equinox. Did you know that Halloween also has astronomical roots? A contraction of “All Hallows Eve,” Halloween is a cross-quarter day, so called because it falls midway between the fall equinox or first day of fall and the winter solstice, which marks the start of winter.
Cross-quarter days come from ancient Celtic tradition and mark important festivals and observances. The other cross-quarter days — with their more familiar, mostly Christian equivalents — are Imbolc (Candlemas) on Feb. 2nd; Beltane (May Day) on May 1 and Lughnasadh (Lammas) on Aug. 1.
Halloween is directly related to the Celtic Samhain (pronounced SOW-in) that marked the end of the harvest season and the beginning of winter with its long nights and associations with darkness and decay. People believed that during Samhain the border between the living world and that of dead spirits was easier to cross. Back in medieval times, places were set at the dinner table for deceased family and friends and their presence invoked in rituals.
I’ve also read that the Pleiades star cluster was an important celestial presence on Halloween night because it supposedly culminated (reached its highest point in the sky) at midnight on that date. But if you check a star map, you’ll see that doesn’t happen these days until 2 a.m. Daylight Saving Time or 1 a.m. standard time. Instead, midnight culmination occurs on Nov. 21st. Why the difference of an hour?
The old Julian calendar in use since Roman times was slightly inaccurate because it didn’t take into account the true amount of time it took Earth to go once around the sun. They used 365¼ days when it’s actually 365.256 days. Close but no cigar.
As a result, the dates of Christmas and other important events drifted over the centuries from the times of the seasons. By the 1500s, the celebration dates and true seasonal dates were out of sync by 10 days, a bit intolerable. But if allowed to continue for additional centuries, Christmas would still happen on Dec. 25th, but eventually take place in the middle of summer!
In October 1582, Pope Gregory XIII instituted what’s now known as the Gregorian Calendar. To rectify the situation and line up seasons and celebrations properly, he deleted 10 days from the month. The Julian calendar day October 4, 1582 was followed by the first day of the Gregorian calendar on October 15, 1582.
Many countries were slow to adopt the new system. England and the American colonies held off until 1752. That year, September 2, 1752 was immediately followed by September 14, 1752. It wasn’t very popular back then especially with Protestants, who saw the move as a way for the Catholic Church to reassert their authority. Eventually, the sense of it became obvious, and now its use is universal.
But let’s get back to the Pleiades. In Roman times, long before the Gregorian calendar took effect, the Pleiades really did culminate around midnight on Halloween, but by shifting the calendar date forward in 1582, it delayed the Pleiades’ culmination by just about an hour. And that’s how Halloween and the midnight Pleiades got out of sync.
While the candy is dropping into your child’s pumpkin basket, may the Pleiades light the way.