According to the National Retail Foundation, the most popular Halloween costume of 2005 was a princess. Next were witches, Spiderman, assorted monsters and Darth Vader: complete list.

Notice something missing? Astronomers! Hubble didn't even crack the top 20. And that's funny, because Halloween is an astronomical holiday.

It has to do with seasons: Halloween is a "cross-quarter date," approximately midway between an equinox and a solstice. There are four cross-quarter dates throughout the year, and each is a minor holiday: Groundhog Day (Feb. 2nd), May Day (May 1st), Lammas Day (Aug. 1st), and Halloween (Oct. 31st).

Right: Red crosses mark the year's cross-quarter dates. [More]

Long ago, "the Celts of the British Isles used cross-quarter days to mark the beginnings of seasons," says John Mosley of the Griffith Observatory in Los Angeles. "Winter began with Halloween, [or as they called it, 'Samhain']. Halloween marked the transition between summer and winter, light and dark -- and life and death."

"On that one night, according to folklore, those who had died during the previous year returned for a final visit to their former homes. People set out food and lit fires to aid them on their journey -- but remained on guard for mischief the spirits might do."

And, so, something astronomical became something spooky. It's not the first time. Have you heard that comets are bad omens? Or that a full moon brings out werewolves? Astronomy and superstition are old friends.

Web Links:

The History of Halloween -- (The History Channel)

The Wheel of the Year -- "Weather Doctor" Keith C. Heidorn explains the ancient origins of Halloween and cross-quarter dates.

Cross-quarter Halloween -- (Clark Planetarium) more about Halloween and cross-quarter dates.