Astronomy EssentialsHuman World

Halloween is an astronomy holiday. It’s a cross-quarter day

Halloween is a cross-quarter day

Sure, Halloween is the modern-day descendant of Samhain, a sacred festival of the ancient Celts and Druids in the British Isles. And, yes, Halloween is short for All Hallows’ Eve. But, at its heart, Halloween is an astronomy holiday. It’s a day rooted in Earth’s orbit around the sun. It’s a cross-quarter day and a testament to our ancestors’ deep understanding of the sky.

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 Northern Hemisphere autumn (September) equinox and winter (December) solstice In the Southern Hemisphere, the September equinox heralds spring and the December solstice, summer.

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 2024 lunar calendars are here! Best Christmas gifts in the universe! Check ’em out here.

Halloween falls at a dark time of year

For us in the Northern Hemisphere, Halloween is the darkest of the cross-quarter days. And it comes 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 supposedly go back to rest.

The October 31 date for Halloween is 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. Image via NASA.

The Pleiades connection

Some believe 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. Now, the midnight culmination of the Pleiades cluster now occurs on November 21, but Halloween is fixed on October 31.

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

Halloween: A giant full moon on horizon with small, silhouetted  man holding a jack-o-lantern.
View at EarthSky Community Photos. | Stojan Stojanovski in Debrca, Macedonia, caught this wonderful image on Halloween Night in 2020, when there was a full moon. Thank you, Stojan!

Pleiades is associated with Halloween

Large area of fuzzy blue cloudiness with bright dozens of bright white stars immersed within.
View at EarthSky Community Photos. | Irwin Seidman in Bruce Peninsula, Ontario, Canada, captured this telescopic view of the Pleiades star cluster on January 14, 2023. He wrote: “This 1 hour and 21 minute integrated exposure was captured at the Fox Observatory on the Bruce Peninsula (Ontario, Canada). Located about 444 light-years from Earth, Messier 45 (aka M45, The Pleiades, The Seven Sisters) is an asterism and open star cluster in the constellation Taurus. Reflection nebulae around hot blue luminous stars give the Pleiades its somewhat eerie and spectacular glow.” Thank you, Irwin!

Bottom line: October 31, the date for Halloween, marks the approximate midway point between the September equinox and the December solstice. So Halloween is an astronomy holiday, and one of the year’s four cross-quarter days.

Read about another cross-quarter day, Groundhog Day

May Day is a cross-quarter day

Enjoying EarthSky? Sign up for our free daily newsletter today!

October 31, 2023
Astronomy Essentials

Like what you read?
Subscribe and receive daily news delivered to your inbox.

Your email address will only be used for EarthSky content. Privacy Policy
Thank you! Your submission has been received!
Oops! Something went wrong while submitting the form.

More from 

Bruce McClure

View All