Happy Groundhog's Day

groundhog sees shadow

Every year on February 2nd people gather to see if Punxsutawney Phil, the famous groundhog from Punxsutawney, Pennsylvania, will see his shadow, signaling that winter will last for another 6 weeks.  If the groundhog does not see his shadow, then summer is right around the corner.  While people wait with bated breath physicists see another application.  What is a shadow and where in this folklore is the science?

A shadow defined by Webster’s Dictionary is “partial darkness or obscurity within a part of space from which rays from a source of light are cut off by an interposed opaque body.”  Basically, it’s an area where the rays of light are blocked by an object.  Light travels in a straight line and if something is in the way that light can’t travel through, the light rays stop and a shadow is cast behind the object that stopped the light.  The edge of the shadow is where the light rays were able to continue to travel because they didn’t run into the object that stopped the light. Imagine rolling a ball into a wall.  Once the ball hits the wall, the ball stops and cannot pass behind the wall.  In this analogy the ball is like a light ray and the wall is like the object.  When the ball can’t pass through the wall it is the same as a light ray being unable to pass through an object.  When this happens, there wouldn’t be light behind the object.

On an overcast day the groundhog wouldn’t see his shadow, but it would see the shadow of all of the clouds that are in the sky.  On a sunny day the groundhog would see his shadow because the clouds wouldn’t block the rays of light and the groundhog would block the ones that hit him.  On Groundhog’s Day many people who haven’t seen a sunny day in many months would love to see the sun come out, but not at the risk of Punxsutawney Phil seeing his shadow and signaling six more weeks of winter. Instead, fingers are crossed for overcast skies and the shadow of clouds engulfing the home of Punxsutawney Phil.