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Thursday, December 2, 2021

Groundhog Day and Shadow Fun

🕐 2 min read


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

Midpoint of Winter

If you now count the days between winter’s start on the solstice of December 21, and the beginning of spring on March 19, 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.

But it’s close enough. That business with groundhogs began with an old German custom—remembering that Pennsylvania used to have lots of German immigrants.

Will the Groundhog See His Shadow?

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

If Punxsutawney Phil sees his shadow, he gets scared and jumps back in, and there’s 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.

Want to predict 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!).

Wow, talk about a futile exercise. But if you want to have some fun with it, listen to my five minute Astounding Universe groundhog podcast here or here: astoundinguniverse.com.

Shadow Fun

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 knows.

Enjoy more Groundhog day facts and folklore!

Source: Farmer's Almanac - Amazing Sky


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