How Groundhog Day Ties to the Skies


This bizarre day is full of amazing sky-related stuff

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Groundhog Day. Just a bizarre tradition involving a marmot, right? Believe it or not, Groundhog Day is full of amazing sky-related stuff. Even its date relates to the heavens. Bob Berman explains the connection to astronomy and sky watching.

As the tradition goes, a Pennsylvania groundhog ignores crowds of humans closely gathered around him, and then, despite having a four ounce brain, gets involved with meteorology. If he sees his shadow, this produces so much anxiety that weather systems throughout North America somehow get altered, and we have six more weeks of winter. And it all revolves around the perceptions of a single woodchuck who serves as the country’s spokesrodent.

This business with groundhogs began with an old German custom—a reminder that Pennsylvania used to have lots of German immigrants. In the homeland, however, it was a badger who gave the signal, not a groundhog. 

Read more about the origins of Groundhog Day.

How Groundhog Day Connects to the Skies

Believe it or not, this does connect with the sky.

If the severity of the rest of winter boils down to whether or not it’s cloudy on February 2, we should explore the odds. Turns out, according to NOAA climatological data, Pennsylvania in early March gets lots of cloud cover—about 66%, statistically. This is actually one of the year’s cloudiest months.

Next comes the fact that scattered clouds fill in toward the horizon, so seeing the early morning Sun, and thus your own shadow, is even less likely then. Just going by the odds, the woodchuck is not likely to see a shadow, and we will not get six more weeks of winter.

 So, you want to predict the weather?

Of course, this is all sort of a hoax, since the start of spring at the equinox on March 20, happens almost exactly six weeks from now no matter what.  

The Midpoint of Winter

Anyway, February 2 is a surprisingly ancient custom, originally Candlemas, the 40th day of Christmas, one of the year’s four “cross quarter” days exactly centered in the seasons. That day falls midway between the winter solstice and the vernal equinox.

If you count the days between winter’s start on the December solstice and the beginning of spring on the March equinox, you’ll see that, yes, February 2 comes very close to being the midpoint. If it’s not exact, that’s because the precession of the equinoxes caused by Earth’s wobble slowly shifts these things, causing the dates to shift by a day or two every once in a while.

Marking the halfway point of those two yearly milestones, a cross quarter day provides the calendar with a badly needed “tick mark” the way “southeast” helps out a compass by filling in the place between east and south. 

Will the Groundhog See His Shadow?

Bottom-line, the odds are, Phil will NOT see his shadow, meaning winter’s continued duration will be unchanged (which, as we’ve seen, is still six weeks!).

Jokes aside, shadows are actually useful and amazing. If it’s sunny, then the ground is filled with innumerable round images of the Sun all overlapping each other. But you can block one of those sun images out, and see a black “negative” of the Sun. Here’s how.

  1. Dangle a pencil eraser or irregular piece of gum from a thread or bit of dental floss. Maybe attach the eraser to the floss with gum.
  2. Now hold it just above a light surface in sunlight. You see the irregular shadow cast by this irregular little object.
  3. Now slowly lift the string until the shadow turns perfectly round. You are now blocking out exactly one of the sun images that are all around you.
  4. You’re seeing a reversed “picture” of the Sun.

How cool is this? And the next clear evening around sunset, look east (opposite the setting sun) and look for a curved gray band hugging the horizon. That’s the shadow of our planet Earth, cast into space!

Shadows are amazing. Phil doesn’t know what he’s missing.

→ Read more about how Groundhog Day came to be.

About The Author

Bob Berman

Bob Berman, astronomer editor for The Old Farmer’s Almanac, covers everything under the Sun (and Moon)! Bob is the world’s most widely read astronomer and has written ten popular books. Read More from Bob Berman

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