Why Halloween Is Really an Astronomy Holiday Tied to Earth's Orbit

Halloween has deep roots in folklore, but according to EarthSky, its heart is astronomical. The holiday on Oct. 31 is a cross-quarter day, a point on the calendar that falls roughly midway between an equinox and a solstice, and so it is tied to Earth's path around the sun.

EarthSky describes the date as the approximate midpoint between the September equinox and the December solstice. That places Halloween among four such markers spread through the year.

What a cross-quarter day is

Astronomers mark the year with equinoxes, when the sun sets due west, and solstices, when it sets at its most northern or southern point on the horizon. Cross-quarter days sit more or less halfway between each pair.

In traditional astronomy, that gives eight major seasonal subdivisions every year: the March and September equinoxes, the June and December solstices, and the four cross-quarter days in between.

EarthSky notes that these four days are now usually known by familiar names: Groundhog Day on Feb. 2, May Day on May 1, Lammas on Aug. 1 and Halloween on Oct. 31.

Why Halloween feels like the darkest of them

For people in the Northern Hemisphere, EarthSky says, Halloween is the darkest of the four cross-quarter days. It arrives while daylight is shrinking.

Old tradition held that the spirits of the dead roam from sunset until midnight around this time. After midnight, on Nov. 1, which is now called All Saints' Day, the ghosts were thought to return to rest.

The holiday itself descends from Samhain, a sacred festival of the ancient Celts and Druids in the British Isles, and its name is short for All Hallows' Eve, according to EarthSky.

A week off the true date

The Oct. 31 date is fixed by tradition and does not track the sky exactly. EarthSky reports that the true cross-quarter day falls on Nov. 7, a difference of about a week. The ancient Celts, it adds, treated a cross-quarter day as the start of a season rather than its middle.

The Pleiades connection

Some people believe an early forerunner of Halloween, Samhain, took place on the night the Pleiades star cluster reached its highest point in the sky at midnight. EarthSky presents this link as a belief, not an established fact.

Today that midnight peak of the Pleiades comes on Nov. 21, not Oct. 31. But if the connection is real, EarthSky says, the two events would have landed on or near the same date in the 11th century (1001-1100) and the 12th century (1101-1200), several centuries before the Gregorian calendar was introduced.

The explanation lies in the calendar of the time. The Julian calendar then in use was about one week out of step with the seasons. Under the Gregorian calendar, EarthSky says, the cross-quarter celebration would have fallen on Nov. 7.

What this means for stargazers and trick-or-treaters

For ordinary readers, the idea does not change anything about the evening. It offers a different way to see a familiar holiday: as a seasonal marker, one of four spaced through the year, that reflects how earlier generations watched the sky and kept track of the seasons.

EarthSky's bottom line is that Oct. 31 marks the approximate halfway point between the September equinox and the December solstice, which is why it calls Halloween an astronomy holiday and one of the year's four cross-quarter days.

Around Prague, based on reporting by EarthSky