Groundhog Day and Shadow Fun

The Quirks of Groundhog Day

January 26, 2021
Groundhog Emerging

Groundhog Day. Just a bizarre tradition involving a marmot, right? Actually, Groundhog Day is based in astronomy and full of amazing sky-related stuff. Let’s have some shadow fun!

The Midpoint of Winter 

If you count the days between winter’s start on the December solstice (December 21, 2020), and the beginning of spring on the March equinox (March 20, 2021), 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.

But it’s close enough. That 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 how Groundhog Day came to be.

Will the Groundhog See His Shadow?

And the shadow thing? Here’s where things get strange.

If Punxsutawney Phil sees his shadow, his sciophobia kicks in, he dives back into his burrow, and there will be “six more weeks of winter.” 

Of course, there’s some humor here. As we’ve seen, there’s always six more weeks between February 2 and the vernal equinox in March!

It’s rigged in another way, too. Pennsylvania in early March gets lots of cloud cover—about 66%, statistically. So, you want to predict the weather? 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!).

Shadow Fun

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.

Enjoy more Groundhog day facts and folklore!

About This Blog

Welcome to “This Week’s Amazing Sky,” the Almanac’s hub for everything stargazing and astronomy. Bob Berman, longtime and famous astronomer for The Old Farmer’s Almanac, will help bring alive the wonders of our universe. From the beautiful stars and planets to magical auroras and eclipses, he covers everything under the Sun (and Moon)! Bob, the world’s mostly widely read astronomer, also has a new weekly podcast, Astounding Universe