Astronomical Halloween


Pumpkin bin supermarket sentinels, cob web fluff, fold out spiders, broom stick witches. Pumpkin patch, amusement park Fright Night, haunted house tours, behemoth inflatable yard ghosts, pop-up fireworks outlets – it must be Halloween.

This year, take a moment to ponder astronomical Halloween, one of four “cross- quarter days” in a year – a cross-quarter day falls midway between a equinox (sun sets due west) and the solstice (sun sets at most northern or southern point on the horizon). March and September equinoxes, June and December solstices plus one cross-quarter between each, makes eight astronomical sub-divisions in a year.

Astronomical Halloween, rooted in the ancient Celtic festival Samhain https://en.wikipedia.org/wiki/Samhain took cues from the Pleiades star cluster. Cosmically vigilant pagans celebrated Samhain on the night Pleiades reached its highest point in the sky, coincidentally falling at cross-quarter time. Trouble is – the 7th century Catholic church knew nothing of Pleiades or cross-quarter days. They declared November 1 All Saints Day (honoring any saints who didn’t have their own day), October 31 All Hallows Eve (mass for all who are hallowed) – set in stone dates based on a wonky Julian calendar.

Had the Gregorian calendar been applied, Halloween would fall on November 7.

A color-composite image of the Pleiades from the Digitized Sky Survey. Image credit: NASA/ESA/AURA/Caltech

A color-composite image of the Pleiades from the Digitized Sky Survey. Image via NASA/ESA/AURA/Caltech.