Sunday, February 01, 2009

Happy Groundhog Day!

Every year, I write an article about how Groundhog Day is actually an astronomical holiday. Why do I do it? Am I stuck in some time warp where I am forced to write the same entry over and over until I get it right? Is it because I am lazy and don't want to write new material? Or is it because I have a point that I think is somewhat important that I want to get across?

The answers are: Not that I'm aware of, maybe, and yes, respectively. (And also that I think it is amusing that an arcane holiday involving men in silk hats dragging a mean-spirited rodent out of a hole in a log is actually related to astronomy. Thankfully these men don't harm the critter, though the critter routinely harms them.)

For those who don't want to read my previous posts on this austere holiday, let me summarize. Groundhog Day is (roughly) on a "cross-quarter" day, or the day exactly halfway between a solstice and an equinox. In this case, we are halfway between the winter solstice and the spring equinox, which means that winter is half over. Since there are 13 weeks in each season, there are six-and-a-half weeks left in winter (whether or not the varmint sees its shadow).

My purpose for re-telling this story every year is that, in this day and age, many of us are quite insulated from the motions of the Earth, whereas just a couple of centuries ago, these motions were crucial to life. At the winter solstice in late December, Earth's North Pole was pointed as far away from the sun as it gets. For those of us in the Northern Hemisphere, this means that the sun appeared low in the sky. Because the sun's rays were coming in at an angle, the energy and warmth from those rays were spread out more. Those spread-out rays were thus not able to warm the Earth as efficiently, and the hemisphere dipped into winter.

Since the winter solstice, the sun has been starting to climb higher in the sky, bit by bit. If we spent more time outdoors, this would be painfully obvious to us, and would have been a reason to celebrate: a higher sun means more direct energy which means warmer weather is coming! But since most of us live and work indoors, we don't notice this sign of the coming spring.

For those of us in the Southern Hemisphere, the sun has been almost imperceptibly sinking in the sky for the past several weeks, but now that sinking will start to become more noticeable, and autumn is only six weeks away.

Confused? You're not alone. It's hard to picture how this all works. It's not impossible, but it's hard. But you can easily observe the changing of the sun's position in the sky without needing sensitive measuring equipment. If you go out for some fresh air at roughly the same time every day, say over a lunch break, mentally measure the length of your shadow. (If you have a brick or block walkway where you can measure how many bricks "tall" your shadow is, this is all the easier!) Over the next few months, once a week or so re-measure your shadow at the same time of day. You'll notice a surprising difference!

Or, if you routinely are able to see the sunrise or sunset (maybe during a commute, or as you are waiting for the bus, or while you are eating dinner), make a mental note of where along the horizon the sun sets. Use a distant house, tree, hill, or other landmark as a reference. I've been able to see noticeable differences in just one or two days.

The Earth's motion around the sun is the root cause of these changes. We can't feel the orbit, but we can see the consequences. Our length of day. weather, and seasons all change because of this orbital motion. Next to the Earth's daily rotation, our orbit around the sun is the most fundamental impact of space on us and all life on Earth. I'd urge you to take a little time to try one of these experiments I mentioned. And, when you successfully detect the change in the sun's position, you'll be experiencing some of the most basic but most important astronomy anyone can study.

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