A furry little rodent crawls out of its den, is scared by its own shadow, and that means we'll have six more weeks of winter? Sounds like silly superstition, doesn't it? But several parts of the Groundhog Day legend do in fact have a basis in science and careful observation of nature:
Astronomy: Before calendars came into widespread use, Europeans divided the year into four parts, using the Winter Solstice (the shortest day of the year, around December 21), the Summer Solstice (the longest day, roughly June 21), and the Spring and Fall Equinoxes (days when night and day are exactly equal, March 21 and September 21) as signposts. The days halfway in-between these four milestones are called cross-quarter days.
February 2 lies midway between the Winter Solstice and the Spring Equinox. Some ancient Europeans considered it the first day of spring (a bit early, if you ask me!) Romans held a purification festival called Februus at this time, to prepare their fields for planting. (The festival's torchlight parade later evolved into the Christian feast of Candlemas.) The Celts of Ireland celebrated Imbolc on this day, a fertility ritual associated with ewes preparing to give birth.
Animal behavior: Ancient peoples observed animal behavior for clues to changes in the weather. The reappearance of hibernating or inactive animals like badgers, hedgehogs and even bears was a sign of winter's end. When German settlers came to Pennsylvania in the 1700s, they chose the groundhog as the local harbinger of spring.
Even today, we watch for animals to indicate the change of seasons. Geese flying south is a sure sign of fall. In the Midwest and Northeast, we wait for the first robin of spring. Californians wait for the swallows to return to Capistrano. And Ohio residents know it's spring when the buzzards come back to Hinkley.
Meteorology: Ever notice how bright, clear winter days are often very cold? That's because they are caused by high pressure systems. Areas of high pressure pull cold air down from the north. They also sweep away any clouds that might have provided insulation.
Farmers in ancient Europe noticed this relationship, and developed various legends and practices around it. Some of the most noteworthy involved February 2, the old cross-quarter day. An old Scottish poem says:
If Candlemas Day is bright and clear,
There'll be two winters in the year.
Other cultures have similar sayings. Of course, the weather on one particular day won't determine the weather for the next six weeks. But, in general, if winter has had a lot of cold, sunny days, one can expect the pattern—and the chilly weather—to continue.
So, Groundhog Day merges three elements of pre-scientific knowledge. It takes a sign of spring — animals emerging from hibernation — and a sign of winter — clear, cold days — and combines them with a date that has astronomical significance.
Unfortunately, each of these elements works better as a general rule of thumb than as a concrete prediction. Put them together in a specific way, and you end up with a superstition: there's no evidence of winters being any harsher or milder after the groundhog sees his shadow. However, each of these elements individually is a testament to ancient people's ability to carefully observe their environment and recognize patterns — which is pretty much what science is all about.
(Researching this entry, I came across a lot of fun sites related to Groundhog Day. The Punxsutawny Groundhog Club has a page on the history of the holiday. Stormfax Weather Information has some interesting information. And The Holiday Spot has lots of links you can follow. Different sites sometimes give different information -- but with a holiday as old and widespread and changeable as this one, there are some things we may never pin down for sure.)