Why Groundhog Day has its roots in astronomy

Groundhog Day is what's known in astronomy as a "cross-quarter day." Here's what that means

By Nicole Karlis

Senior Writer

Published February 2, 2023 5:30AM (EST)

Groundhog in Space (Photo illustration by Salon/Getty Images)
Groundhog in Space (Photo illustration by Salon/Getty Images)

As one of the few — okay, probably the only — Western tradition that revolves around a marmot's morning ritual, Groundhog Day is a particularly bizarre North American superstition. Each year, on the morning of Feb. 2, it is said that if a groundhog comes out of its burrow and observes its shadow because the skies are clear, it will retreat out of fear, at which point winter continues for six weeks longer. If it does not observe its shadow due to overcast skies, spring will come early. The superstition comes from Pennsylvania Dutch culture, which makes sense given that the groundhog's range is largely confined to eastern U.S. states like Pennsylvania as well as Canada. (Nowadays, much of the press coverage of Groundhog Day revolves not around any old groundhog, but a specific groundhog in Punxsutawney, Pennsylvania, nicknamed Punxsutawney Phil, and for whom the town holds a large festival annually.)

While clear skies usually mean winter is still near, and clouds in the sky mean the opposite, there isn't anything scientific about this tradition. In fact, The National Oceanic and Atmospheric Administration (NOAA) has reported that using a groundhog as a weather forecaster doesn't really work most of the time. In the last 10 years, Punxsutawney Phil been correct about 40 percent with his predictions. Phil — or rather, the succession of western Pennyslvanian groundhogs who have borne the honorific of Punxsutawney Phil over more than a century — doesn't have a good track record prior to that either, and has been consistently inconsistent since he got the gig in the late 19th century. 

"In 1887, when he made his debut as the official groundhog forecaster for the entire country, Phil saw his shadow," NOAA explained. "His first prediction of six more weeks of winter was accurate for a few regions, but it came up short for several others."

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NOAA says predicting the arrival of springtime for an entire country is nearly impossible because of such varied regional climates, and Phil's track record is a testament to that. And while Groundhog Day is ostensibly a meteorological event, the tradition has roots — surprisingly — in astronomy.

So what could a tradition about a burrowing land mammal possibly have to do with the motions of the stars and planets?

Groundhog Day is what astronomers call a "cross-quarter day," and these days were used by the ancient Celts to signify markers of a new season.

The origins of the tradition, as explained by History.com, stem from an ancient Christian celebration called Candlemas where clergy would distribute candles for winter. These candles were meant to represent how long and cold the winter would be that year. Germans took this concept and replaced candles with an animal: the hedgehog. However, hedgehogs are native only to Europe, Asia, and Africa; they do not naturally occur in the Americas. Once Germans settled in Pennsylvania, the tradition morphed to using a groundhog as the weatherman instead.

Despite the evolution of Groundhog Day, the date — February 2 — has stayed the same. And it's not a random date that the Christians chose; it was significant because it fell in the middle of the December solstice and the March equinox — the first of a few astronomical origins to Groundhog Day.

As Berkeley astronomer Bryan Méndez writes, it is likely not a coincidence that Groundhog Day coincides with the ancient Celtic festival called Imbolc.

"Imbolc is observed on February 1 to celebrate the start of spring at the cross-quarter day, which now occurs on February 3 in the Gregorian calendar," Méndez says. "Some traditions of Imbolc held it as a day to divine when the coming of spring would be, considering a sunny Imbolc to indicate a late start to the spring season; this was very likely an influence on the Candlemas/Badger Day traditions that came later."

Groundhog Day is what astronomers call a "cross-quarter day," and these days were used by the ancient Celts to signify markers of a new season. As Ohio State University's Astronomer Richard Pogge notes, The Celtic Solar Calendar and Japanese Luni-Solar Calendars used the cross-quarter days to mark the beginning of the various seasons. 

"The Spring and Fall Equinoxes are the times of equal-length day and night and the Summer and Winter Solstices are the longest and shortest day, respectively; in current usage these each define the official beginning of a season," Harvard astronomer Avi Loeb explained to Salon. As an example, he noted that summer begins around June 21st. "However, a less-used parallel system holds that June 21st is actually Midsummer's Day, which then requires the start of summer to be in early May." 

In other words, a cross-quarter day — the midpoint between two seasons.

"Cross-Quarter Days mark the middle of each season under our current system, or seasonal boundaries under the alternative system," Loeb elaborated. "Due to the insertion of a Leap Day on February 29th every four years, the exact dates of these eight astronomical events shift back and forth, with a total range of about 54 hours."

This is all to say that Groundhog Day is yet another example of how ancient humans' understanding of astronomy influences our lives today — much more than, say, the random convulsions of a ground-burrowing mammal in Pennsylvania.

By Nicole Karlis

Nicole Karlis is a senior writer at Salon, specializing in health and science. Tweet her @nicolekarlis.

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