Sky Watch: Halloween's celestial origins

"Trick or treat!"

It's a phrase I'm sure we'll all hear quite a bit this coming week, screamed by a variety of ghosts, goblins and ghouls pounding on our doors. You'll likely see costumes of monsters, superheroes, politicians and singers, but this year, keep track of how many astronomers show up at your house.

My guess is that you won't see any. Not one Galileo. Not one Sagan. Not even a Mammana, for heaven's sake! I'm always amazed by this fact, considering that Halloween -- believe it or not -- is a holiday with celestial origins.

It all comes down to the seasons and to our planet's annual orbit around the sun.

Today, we associate the beginning of each season with the equinoxes and solstices. We in the U.S. recently heard the TV weather reporters explain that autumn began on Sept. 23 -- the autumnal equinox -- when the sun crossed the celestial equator on its way south. And soon, we'll hear them say that winter will begin on Dec. 21 -- the winter solstice -- when the sun reaches its southernmost point in the daytime sky and once again begins its northward journey in the heavens.

We hardly ever think about these details; we just recognize these dates as the beginning of new seasons, and that's that. But it wasn't always this way.To ancient Germanic and Celtic societies, the equinoxes and solstices marked not the beginning of the seasons, but their midpoints. They knew the seasonal beginnings to occur on "cross-quarter" dates, the points midway between the equinoxes and the solstices.

Four cross-quarter dates exist throughout the year, and each has become a minor holiday: Feb. 2 (Groundhog Day), May 1 (May Day), Aug. 1 (Lammas Day) and Oct. 31 (Halloween).

To the Celts, winter began with Halloween or, as they called it, Samhain (summer's end), which marked the transition between summer and winter, light and dark, life and death. This was also Celtic New Year's Eve, when people celebrated with a great fire festival to encourage the sun not to vanish. On this evening, people danced around massive bonfires to repel demons, but left their doors open in hopes that kind spirits of loved ones might join them around their hearths. So where did all the costumed ghosts and goblins and ghouls of modern Halloween come in? Well, that originates far back in history as well. It was in later pagan and Christian traditions that, at this time of year, people went out in masks and robes to frighten away evil spirits; some even traveled from farm to farm carrying hollow turnips with candles inside and demanding food to honor an old god, Muck Olla.

It's quite easy to see how these ancient traditions have influenced our modern holiday customs around this time. So this Halloween, while you're dressing up in scary outfits or quietly pilfering chocolate bars from your kids' trick-or-treat bags, think about the origin of all these traditions. You might even consider attending your office Halloween party dressed as an astronomer . . . But if you should choose me as your subject, be forewarned that you'll likely send people shrieking out the door!