What is the real deal with Groundhog Day and Candlemas?

Groundhog Day is a popular tradition celebrated on February 2 in the United States and Canada.  According to legend, it is the day the groundhog first comes out of his burrow after the long winter.  

If it is cloudy when the critter exits his hole, he remains outside, effectively signifying the end of winter weather.

However, if the sun is shining, and the groundhog sees his own shadow, he will head back into his home, and winter weather will continue for six more weeks.

The tradition stems from similar beliefs associated with the early days of the Christian church in Europe.  Candlemas Day is celebrated forty days after the birth of Christ.  An old English poem makes it sound pretty clear:

If Candlemas be fair and bright,
Come, Winter, have another flight;
If Candlemas brings clouds and rain,
Go Winter, and come not again.

According to an old Scotch couplet:

If Candlemas Day is bright and clear,
There'll be twa (two) winters in the year.

Another variation of the Scottish rhyme:

If Candlemas day be dry and fair,
The half o' winter to come and mair,
If Candlemas day be wet and foul,
The half of winter's gone at Yule.

Germans, who settled in Pennsylvania, brought with them the custom of letting a groundhog determine if the weather was fair or foul.