(WHTM) Boo! It’s Halloween! The day associated things creepy and crawly, spooky and scary, witches, ghosts, goblins –

Et cetera.

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But what is it about this day that it’s celebrated for, well, anything? It’s actually all about astronomy, and the seasons, and the seasons within the seasons.

To begin at the very beginning, the Earth revolves around the Sun every 365 1/4 days, more or less. (That more or less keeps a lot of astronomers, mathematicians, calendar publishers, and clock makers very busy.)

The Earth also rotates every 24 hours, again more or less, so we have day and night. But the Earth is tilted by about 23 degrees in relation to the Sun. This means that instead of having the same amount of daylight and nighttime every day, the amounts change as the Earth revolves.

For one half of the year, then, the period of sunlight every day increases. During the other half of the year, it decreases.

There are two days of the year when we have exactly 12 hours of daylight, and 12 hours of night. These are called the equinoxes. Equinox comes from the Latin “aequus” (equal) and “nox” (night). For 2023, in the northern hemisphere where we are, the spring or vernal equinox is on March 20, and the autumn or autumnal equinox is on September 22.

We also have a day when we have the most daylight in a 24-hour period. This is the summer solstice. Then there’s the day when we have the fewest hours of daylight, which is the winter solstice. Once again we’re speaking Latin; “sol” (sun) and “sistere” (to pause or stand still). In 2023 the summer solstice is June 21, and the winter solstice is December 21. (Keep “more or less” in mind; the actual dates can vary from year to year.)

The two solstices and the two equinoxes are what are known as quarter days, since a quarter of a year passes from one to the next.

All well and good, you may say, but where does Halloween fit in? Well, that brings us to the other four days of note on the calendar, days that are equidistant (more or less) between a solstice and equinox, or an equinox and solstice. In 2023 these days are February 3, May 5, August 7, and November 7. Collectively these are called cross-quarter days. (Why they were dubbed cross-quarter days I don’t know, but I will note it rolls off the tongue a little more easily than “one-eighth day”.)

Over the centuries there have been many different ways of marking cross-quarter days, and some of these celebrations eventually got attached to specific dates, no matter when the quarter-day actually took place. For instance, February 2 in America is always Groundhog Day, regardless of when the February cross-quarter day takes place. May 5 is close to May Day (May 1) and August 7 – well, we actually don’t make a big deal about that cross-quarter day in America, but it’s known as Lammas Day elsewhere.

This leaves us with the fourth of the cross-quarter days, midway between the autumnal equinox and winter solstice. This year it’s on November 7, but the celebration of the day is locked in every year to October 31.

Happy Cross-Quarter-Day Halloween!