The Astronomical History Of Halloween

You might think of Halloween as being more supernatural than scientific, but it turns out that there is an astronomical explanation for why we celebrate Halloween when we do. The astronomy of Halloween, like that of so many other holidays, has to do with the changing seasons and, therefore, with Earth’s movement around the sun. So when you’re heading off to a party on Monday in full Harry Potter regalia (or, you know, whatever fun costume you’ve cooked up this year), take a moment to look to the stars and thank the galaxy for giving you an opportunity to finally wear your Quidditch uniform in public.

You may already know that our present-day culture uses four astronomical events to mark the beginning of each season: The summer and winter solstices, and the spring and autumnal equinoxes. The summer solstice is the longest day of the year, and winter solstice the shortest day of the year; on the equinoxes, day and night are approximately the same length. These variations in the length of days occur because the Earth is tilted in its orbit around the sun, giving one hemisphere more exposure to the sun than the other at any given time. We can thank this ever-changing orientation toward the sun’s light and warmth for the fact that we have seasons.

In addition to the solstices and equinoxes, there are four other events that make up the Earth’s calendar: Cross-quarter days, or the days that occur smack dab in the middle of the span between each solstice and equinox. And guess what? The cross-quarter day between the autumnal equinox and the winter solstice happens to occur right around October 31, Halloween.

Cross-quarter days were significant to a number of ancient cultures, including the Celts, who considered these days as the start of each season, rather than the midway point. The Celts, living in the U.K., Ireland, and northern France two thousand years ago, saw the cross-quarter day in October as the beginning of winter (and some scholars suggest that they may have regarded it as the beginning of the new year). The Celts celebrated this day with Samhain, a holiday that translates to “summer’s end.” The Old Farmer’s Almanac suggests that because Samhain represented the “death-night of the old year,” the Celts came to link it with graveyards and spirits of the dead. And those associations between death, ghosts, and the late-autumn cross-quarter day would trickle down into our modern holiday of Halloween.

Significantly, Halloween isn’t the only modern holiday that occurs on or near a cross-quarter day. Groundhog Day (February 2), May Day (May 1), and Lammas Day (August 1) also occur at the midway points between the solstices and the equinoxes.

EarthSky points out another astronomical aspect of Halloween: In the 11th and 12th centuries, Samhain (again, a distant precursor to Halloween) occurred in conjunction with the Pleiades star cluster (aka the Seven Sisters) reaching its zenith at midnight.

The orientation of the Earth to the sun and stars has shifted slowly over time, so the dates associated with these events have changed, too. Although we celebrate Halloween on October 31, the cross-quarter day now actually happens on or around November 7, and the Pleiades star cluster reaches its height at midnight near November 21.

What do those dates mean to you? I think they are the perfect excuse to buy huge quantities of on-sale candy after Halloween and hold another Halloween/Samhain/cross-quarter-day extravaganza on November 7. Might as well get some more use out of your costume, right?

Images: Jia Liu/Moment/Getty Images; Giphy (2)