In the heart of the Fall season, October brings beautiful blue skies, colorful leaves, crisp temperatures, and championship baseball. With the changing times of sunrise and sunset we also get more hours of darkness for astronomical observing. While these are fine reasons to make October a special month, for many it's the last day of October, Hallowe’en, that carries a special magic. But did you know that October 31st is also a magical astronomy day as well?
Hallowe’en is one of four special days of the calendar year that mark the midpoints of the seasons. You know three of them quite well: February 2 - Groundhog Day, May 1 - May Day, the least familiar one, August 1 - Lammas, and of course, Hallowe’en. They’re known as ‘cross-quarter’ days because they break each season in half, marking quarter segments of the year. In ancient Celtic agricultural societies, these midpoints were essential in detailed planning of the agricultural year. For the Celts, Hallowe’en was the last night - the death night - of the old year. On that last night, it was believed that spirits of the dead arose from graveyards and wandered the countryside from sunset to midnight, when they all returned to their eternal resting places. Some historians suggest that November 1, known as All Saints Day, was created by early British churches to replace the October 31 Celtic festival of the dead. All Souls Day, on November 2, is also said to have religious connections to ‘All Hallows’ Eve’ - Hallowe’en. All four cross-quarter days, along with the equinoxes and solstices, actually mark positions in the Earth’s orbit around the sun - so technically, they all connect back to astronomy!
This October, the magic of Hallowe’en will be heightened by the second full moon of the month. The first full moon occurred on October 1, so this second full moon goes by the folkloric title, the ‘Blue Moon.’ The term originated as a description of a very rare event, something that “happens only once in a blue moon.” It actually referred to the moon’s color being altered by tiny aerosol particulates blown into the atmosphere by volcanic eruptions which might cause the moon to appear blue. That is a rare occurrence. The ‘two full moons in a month’ reference originally came about as a result of a misinterpretation of the term ‘blue moon’ in an astronomy magazine decades ago. The magazine acknowledged that use of the term ‘blue moon’ for the instance of the second of two full moons occurring in a month was not correct and tried to correct the meaning, but by the time the correction came to print, the term had already spread far enough that it has become a definitive term used to describe the phenomenon. The ‘two full moons in a month’ phenomenon occurs every 2-3 years, whenever there are 13 full moons in twelve months. There's still another version: if a season has four full moons, then the third full moon is called the ‘blue moon’. The next time this will occur is next August 22.
So not only is October connected to astronomy through Hallowe’en, this year it has not one, but two full moons to add to the spook factor. Let us know if you see a figure on a broomstick fly in front of the full moon this year! Want to go deeper? Check out this article from Earth and Sky.