Halloween: Celestial Origins
For some, Halloween is a day for scary stories, spooky costumes, and an overdose of sugar, but for others, the holiday is an important marker in astronomy. As Halloween arrives and trick-or-treaters pound excitedly on our doors. We see costumes of monsters, superheroes, politicians, and pop stars. But this year, keep track of how many astronomers show up at your house! Most likely, not one Galileo, Carl Sagan, or even a Neil deGrasse Tyson will come knocking. Believe it or not, though, Halloween has an astronomical origin. It all comes down to the seasons and our planet's annual orbit around the sun.
Halloween is a cross-quarter day, meaning it falls between a solstice and an equinox. In the United States, October 31 serves as the midpoint between the autumn equinox and winter solstice. However, the exact cross-quarter day would be November 7; October 31 has been adopted by tradition. We hardly ever think about these details; we just recognize these dates as the beginnings of new seasons, and that's that. But it wasn't always this way.
To ancient Germanic and Celtic societies, for example, 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, or the points midway between the equinoxes and solstices.
Using astronomy, the year is divided into eight categories: the equinoxes of spring (March 20), and fall (September 22), when the sun crosses the celestial equator on its way north and south; the solstices of winter (December 21), and summer (June 21), when the sun reaches its southern and northernmost point in the daytime sky; and four cross-quarter days, the midpoints between equinoxes and solstices. These cross-quarter days, which have become minor holidays, are February 2 (Groundhog Day), May 1 (May Day, also known as Beltane), August 1 (Lammas Day or Lughnasadh), and October 31 (Halloween).
This cross-quarter day was important to the ancient Celts. It was the day they celebrated their holiday Samhain, (prounounced Sowain) which would eventually become Halloween. This was summer's end. It marked the transition between summer and winter, light and dark, life and death. This was also the Celtic New Year's Eve, and people celebrated the occasion with a great fire festival to encourage the sun not to vanish. On this frightful 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. Celebrations also took place when there was a full Moon. The Moon’s cycle helped keep track of how much time had passed when there weren’t calendars. They also knew when to celebrate due to the appearance of the Pleiades, a star cluster.
In addition to the history of Halloween, the show will touch on astronomical objects invisible to the naked eye that can be associated with the holiday, such as a nebula shaped like a witch’s head, one like a black widow, and one like a ghost rising out of a cloud of dust. The show will also explore the night sky and show what planets, constellations, and stars will be out on your Halloween evening.