The Insignificance of Groundhog Day

There should be a latitudinal cutoff point for the significance of groundhog day.  For those of you who have been living in a cave for the past 30 years, let me reiterate the rules of this absurd event.  Basically every year on February 2nd, Groundhog Day, a groundhog (in each region) is supposed to come out of its hole, look around and take note of whether it sees its shadow or not.  If it sees its shadow then there will be 6 more weeks of winter.  If he doesn’t, an early spring will result.

First of all, I don’t quite understand the significance of the whole shadow thing.  And how can one particular day being sunny or not affect the weather six weeks into the future?  And why a groundhog?  Why can’t it be something cool like an owl or a porcupine?