The history and science of Groundhog’s Day

by Katie Bowell, Curator of Cultural Interpretation

Groundhog’s Day is upon us and it seems the rodent community has been pulling the wool…er…fur over our eyes. Here’s why: Tradition holds that, on February 2nd, if a groundhog emerges from its burrow and sees its shadow, we’ll have six more weeks of winter. If there’s no shadow, spring is on its way. Now, groundhogs are good at many things – climbing trees, whistling, and inspiring tongue twisters (how much wood could a woodchuck chuck?), but they probably need to take weather prediction off their resumes.

Groundhogs

The tradition of the groundhog and its shadow began as a Pennsylvania Dutch custom, and has similarities to the medieval Catholic holiday Candelmas and the Pagan festival of Imbolc. Other animals have also been used to predict spring, including the badger, bear, and hedgehog. It’s easy to understand why those hibernating animals would be hallmarks of spring, but why do we check to see what a groundhog is doing on February 2nd?

First, we need to know a little background on the solstices and equinoxes. On June 21st of this year, the North Pole will be as far tilted towards the sun as it gets, and here in the Northern Hemisphere we’ll have the longest amount of daylight/day for the year. That’s our summer solstice. Six months later, on December 21st, the South Pole will be tilted as close to the sun as it gets and the Northern Hemisphere will have the shortest amount of daylight/day for the year – our winter solstice. On March 20th and September 22, halfway between each of those dates, are the spring and fall equinoxes. On those dates, everyone in the world has the same amount of daylight and night. Since none of those dates are near February 2nd, where am I going with this? Well, while we pay attention to the solstices and equinoxes to mark the beginnings of our seasons, early Northern Europeans also paid attention to the dates between those events – called cross-quarter days. Their seasons began on those days, and the beginning of February is pretty close to being between the winter solstice and the spring equinox, and would have marked the beginning of the next season for pagan Europeans.

So what does this have to do with shadows? February also marks the beginning of being able to see the sun taking a higher path across the sky. This has been happening since the winter solstice, but it’s difficult to see when it’s just beginning. Changes in the sun’s path across the sky also mean changes in the shadows we see: the higher the sun, the shorter the shadow. And, finally, winter days in which you can see your shadow tend to be colder because there are no clouds present to insulate the earth.

Put all the pieces together and you can see how the tradition of checking on a hibernating animal on the cross-quarter day between the winter solstice and spring equinox and its ability to produce a shadow could have developed. However, whether or not anyone sees their shadow on February 2nd, spring still won’t start until March 20th. And while it’s a fun tradition, if the groundhogs are anything like me I bet they’d like the chance to sleep in a bit longer…

Update: Per reader Robin’s request, I did a little digging into the accuracy of groundhogs on Groundhog’s Day. According to StormFax Weather Almanac, Punxsutawney Phil has been accurate 39% of the time since 1887.

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1 Response to “The history and science of Groundhog’s Day”


  1. 1 Robyn February 2, 2010 at 12:18 pm

    Interesting history behind it… but now we need to know what kind of historical accuracy these rodents have had! 😉


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