Once a year, a burrowing rodent is lifted in front of a crowd and asked to comment on the coming weather. It is the only annual holiday built around a member of the squirrel family, and the woodchuck did not volunteer.
The tradition in one paragraph
Every 2 February, a groundhog is brought out before an assembled crowd. If it is said to see its shadow, tradition holds that six more weeks of winter follow. If not, spring is declared to be on the way. The most famous participant is Punxsutawney Phil in Pennsylvania, where the ceremony has been running since the late 1880s.
And yes - the groundhog on the stage is a woodchuck. Same animal, different name.
Where it came from
The custom descends from older European traditions marking the midpoint between the winter solstice and the spring equinox, in which the behaviour of animals emerging from winter dormancy was read as a seasonal indicator. German-speaking settlers carried the practice to Pennsylvania, where the local burrowing mammal was substituted for the European one.
The woodchuck got the job largely because it was the most conspicuous hibernator available.
Does it work?
No. Reviews of the record consistently find that the shadow ceremony performs no better than chance at predicting the length of winter, which is roughly what you would expect from a method that depends on whether a particular morning happens to be overcast.
That is not really a criticism. Nobody attends for the meteorology.
The kernel of sense underneath
There is a real signal buried in the folklore, even if the ceremony does not capture it. When a hibernating animal genuinely emerges depends on soil temperature at burrow depth, which does track the progress of the season.
A woodchuck that surfaces early has, in a limited sense, detected something about the year. It simply has nothing to do with shadows, and everything to do with how warm the ground is a metre down.
Frequently asked
Is Punxsutawney Phil a woodchuck?
Yes. Groundhog and woodchuck are two names for the same species, Marmota monax, so Groundhog Day's most famous participant is a woodchuck by any naming convention.
Does Groundhog Day actually predict the weather?
No. Analyses of the record find the shadow ceremony performs no better than chance at forecasting the length of winter. It is folklore rather than meteorology.
Why is a groundhog used to predict spring?
The custom descends from older European traditions that read the behaviour of animals emerging from winter dormancy as a seasonal sign. German-speaking settlers brought the practice to Pennsylvania, where the groundhog was the most conspicuous local hibernator.