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Halloween is an astronomical holiday

Halloween: A giant full moon on the horizon, and a man walking in front of it, in silhouette, holding a jack'o'lantern.
View at EarthSky Community Photos. | Stojan Stojanovski in Debrca, Macedonia, caught this wonderful image on Halloween night 2020, when there was a full moon. Thank you, Stojan!

Halloween is a cross-quarter day

Halloween – short for All Hallows’ Eve – is an astronomical holiday. Sure, it’s the modern-day descendant of Samhain, a sacred festival of the ancient Celts and Druids in the British Isles. But it’s also a cross-quarter day. The cross-quarter days fall more or less midway between the equinoxes (when the sun sets due west) and solstices (when the sun sets at its most northern or southern point on the horizon). Halloween – October 31 – is approximately midway between our autumn (September) equinox and winter (December) solstice.

In other words, in traditional astronomy, there are eight major seasonal subdivisions of every year. They include the March and September equinoxes, the June and December solstices, and the intervening four cross-quarter days.

In modern times, the four cross-quarter days are often called Groundhog Day (February 2), May Day (May 1), Lammas (August 1) and – the most sinister cross-quarter day because it comes at a dark time of year – Halloween (October 31).

The 2022 lunar calendars are here! Order yours before they’re gone. Makes a great gift!

Glowing pumpkins with stars and a comet carved into them.
View at EarthSky Community Photos. | Victor C. Rogus in Sedona, Arizona, carved these cool astro-themed pumpkins for Halloween 2020. Notice the sun, the moon and 2020’s favorite comet, Comet NEOWISE. Thanks, Victor!

It falls at a dark time of year

For us in the Northern Hemisphere, Halloween is the darkest of the cross-quarter days, coming at a time of year when the days are growing shorter. Early people once said that the spirits of the dead wander from sunset until midnight around this cross-quarter day. After midnight – on November 1, now called All Saints’ Day – the ghosts are said to go back to rest.

The October 31 date for Halloween has been fixed by tradition. The true cross-quarter day falls on November 7, representing a discrepancy of about a week. According to the ancient Celts, a cross-quarter day marks the beginning – not the middle – of a season.

Diagram of Earth's orbit with equinoxes, solstices, and cross-quarter days marked.
Equinoxes, solstices and cross-quarter days are all hallmarks of Earth’s orbit around the sun. Halloween is the 4th cross-quarter day of the year. Illustration via NASA.

The Pleiades connection

It’s thought that the early forebear of Halloween – Samhain – happened on the night that the Pleiades star cluster culminated at midnight.

In other words, the Pleiades climbed to its highest point in the sky at midnight on or near the same date as this cross-quarter day. In our day, Halloween is fixed on October 31, though the midnight culmination of the Pleiades cluster now occurs on November 21.

Presuming the supposed connection between Samhain and the midnight culmination of the Pleiades, the two events took place on or near the same date in the 11th century (1001-1100) and 12th century (1101-1200). This was several centuries before the introduction of the Gregorian calendar.

At that time, when the Julian calendar was in use, the cross-quarter day and the midnight culmination of the Pleiades fell – amazingly enough – on or near October 31. But, then, the Julian calendar was about one week out of step with the seasons. Had the Gregorian calendar been in use back then, the date of the cross-quarter day celebration would have been November 7.

Calendar converter via Fourmilab

But Halloween is now fixed on October 31. Meanwhile, the true cross-quarter day now falls on or near November 7 and the midnight culmination of the Pleiades cluster on or near November 21.

Seven bright stars in glowing blue clouds against a starry background.
The Pleiades star cluster, also known as the Seven Sisters. This tiny, misty dipper is easy to pick out in the night sky. Photo via Dave Dehetre/ Flickr.

Bottom line: October 31, the present date for Halloween, marks the approximate midway point between the autumn equinox and the winter solstice. Halloween is one of the year’s four cross-quarter days.

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October 28, 2021
Astronomy Essentials

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