October is the first full month of fall and includes the traditional Halloween holiday. As with numerous other holidays, there are known astronomical connections with it going back beyond the contemporary costumes, jack-o-lanterns and trick-or-treaters.
Halloween is a seasonal holiday, but how did it come to be?
Our four seasons are traditionally marked by the two equinoxes and two solstices. In our modern world, they mark the starting date of our seasons. Equinoxes mark spring and fall and the solstices mark summer and winter. Thus last month, the equinox on Sept. 22 marked the beginning of autumn in the northern hemisphere, and spring in the southern hemisphere.
Lesser known are four other dates midway between the seasonal equinoxes and solstices. These are referred to as "cross-quarter days." According to Bruce McClure (EarthSky.org), These dates have been handed down to our modern era where they currently are celebrated as minor holidays on Groundhog Day (February 2), May Day (May 1), Lammas (August 1) and Halloween (October 31).
According to LiveScience.com, Ohio State astronomer Richard Poggee traces the origins of Halloween back to civilizations living in the British islands in the pre-Christian era. For the Celts and Druids — not to mention Shinto Japanese as well — these cross-quarter days were not the midpoints of each season. No, quite the opposite was the case. In these ancient cultures, the seasons began on the cross-quarter days and the Equinoxes and Solstices marked their midpoints, in contrast to our currently observed custom.
The Celts celebrated a sacred fall festival known as Samhain, meaning summer's end. For the Celts, Samhain was like New Year's Eve. Occurring in late October it was likely also meant to celebrate the harvest.
The Celtic observation is described by Luke Gilkerson (ExperienceAstronomy.com) as occurring like this in the green hills of Ireland: They believed that during the season a veil between this world and the next was so thin that spirits could move freely back and forth. The ritual feasts were held to which the souls of departed loved ones were invited to attend.
Various rituals, offerings and bonfires were used for keeping the evil souls away.
Another astronomical tie-in for this occasion was the naked eye star cluster known as the Pleiades (a.k.a. Messier 45) in the constellation Taurus the Bull. The veil between worlds became thinner at midnight and was thinnest on the night when the Pleiades culminated — or reached their highest point above the horizon — precisely at midnight.
However, due to precession of the Earth's axis, changes in the calendar system and the fact that the date of celebration has become fixed on the calendar, the Pleiades do not culminate at midnight on the night of the dead like it used to.
When the Celts eventually conquered Rome they kept the holiday alive. When Rome later converted to Christianity, "All Saints Day" was moved from May 13 to November 1, replacing the night of the dead, thus making October 31 "All Hallow's Eve" which, due to eventual rebranding, became known later as Halloween.
One other astronomical tie-in is this. If you have a clear horizon to the west on Halloween night, then around 7 p.m. when the sky is dark you will see the bright star Arcturus, orange as a pumpkin, low in the western sky.
Astronomers tend to give objects names reminiscent of what they look like. They don't always resemble their namesake, but often do.
Perhaps the most recognizable object for Halloween is a faint nebula known as the "Witch Head" located in the constellation Eridanus the River, and only about three degrees west-northwest of Orion's knee star Rigel. The crooked nose, pointed chin, shrieking gaping mouth, ear and eye socket are obvious features that our brains recognize.