Speaking of Nature: Why is winter coming?
I’m teaching AP Environmental Science once again and I’ve decided to cover the geologic history of the Earth early in the class because of the dramatic change of season that takes place at this time of year. As one of the regular assignments for the class, I have my students keep weekly nature journals. My hope is to get them unplugged, outside and away from their electronic devices long enough to breathe the fresh air and really pay attention to their surroundings. Some of their journal entries have given me great hope that it’s working.
This week, I gave them a theme. The assignment was to have them tie what they observe in their backyards to the Earth’s annual orbit around the sun. We have seasons here on Earth. Why? It’s a simple question, but quite a story can be told if you simply allow yourself to start thinking about it.
Meanwhile, here at home, I’ve been struggling to come up with a topic for this week’s column. Then it hit me. Why not try to fulfill the very assignment I have given to my students? Why not try to explain why we have seasons here on Earth? A quick survey of my archives suggests that I haven’t already covered this, but let me give it one more look. Nope, I don’t see anything. Game on! My regards go out to any professors of astronomy who may be in the audience. Please know that I am painfully aware of the fact that I am telling an extremely simplified version of an exceedingly complex tale.
Our story starts about 4.7 billion years ago. The Earth didn’t exist at that time, but the remains of a massive stellar explosion had drifted into our quiet little corner of the Milky Way galaxy and the cosmic flotsam and jetsam started to bunch together. A huge rotating cloud started to accrete and as it spun, it flattened out into a disc.
At the center of the disc, an immense cloud of hydrogen grew in size and density until the pressures and temperatures became so great that a fusion reaction started. Thus, the sun was born and around it the planets began to form.
Just as dust bunnies seem to magically appear under your bed, grains of dust in the rotating disc began to cling together. And as they got larger, the clumps drew more and more of the dust into themselves. Eventually, a few of these objects began to get bigger faster than the rest and they began to consume everything around them, clearing the paths that would eventually become their planetary orbits.
After about 30 million years, the young Earth was the dominant object in its orbit. The collisions became so violent and so frequent that the entire planet melted and, during a brief time called the “Iron Catastrophe,” all of the heavy metals sank to the planet’s core. Meanwhile, the lighter elements floated to Earth’s surface like lily pads on a pond. Then, just as things were starting to calm down, the Earth experienced the most titanic collision in its history.
It is hypothesized that another planet (somewhat smaller than the Earth) occupied the same basic orbit. This planet has been named Theia, which will make sense in just a second. Roughly the size of Mars, Theia hit the Earth slightly off center. This glancing blow was so energetic that it was able to blow the upper layer of the Earth into space. Theia’s core was absorbed by the Earth, which became larger as a result. And with its tremendous gravity, the Earth was able to keep a substantial amount of the ejected matter in orbit. This debris later coalesced into the moon. In Greek mythology, the mother of the moon was known as Theia.
This hypothesis was strongly supported when astronauts landed on the moon and gathered up rocks. When these rocks were chemically analyzed, it was discovered that they had the same basic composition as the rocks of Earth, with one key difference: the moon rocks had no iron. But this made a great deal of sense since the Earth’s iron was safely tucked away in its core at the time of the collision.
But this doesn’t explain the seasons, does it? Well, remember how Theia was off-center when it struck the Earth? This off-center collision was so stupendous that it actually tilted the axis of the Earth’s rotation. The best minds that have examined this tilt have determined that it measures about 23 degrees. This is the key to the Earth’s seasons, but it will take a thought experiment to understand why.
Imagine yourself floating at the center of circular swimming pool. You are the Sun. Now imagine a tennis ball that has been skewered with a chopstick is perched on the edge of the pool directly in front of you. One end of the chopstick is pointed toward you at a 23-degree tilt. The tennis ball is Earth and the end of the chopstick pointing at you represents the geographic North Pole. This particular orientation represents the Earth at the height of summer in the Northern Hemisphere (the summer solstice).
Now, imagine that the ball (the Earth) is rotating on its axis. Despite the spinning, the chopstick always remains pointed at you. This is why the sun never sets if you’re at the North Pole during the summertime. This also explains why the days are longer and warmer in the Northern Hemisphere during the summer months: much more of the hemisphere is exposed to solar radiation while the North Pole remains tilted toward the Sun.
Now, imagine that the tennis ball is moved around the edge of the pool to your immediate left. You’re still floating in the water, so you can easily turn to look at the ball, but upon doing so you notice that the Earth’s orientation has changed. In this position, which represents the Northern Hemisphere’s autumnal equinox, Earth’s rotational axis is perpendicular to the Sun, therefore its rotation produces days and nights of equal length.
Move the tennis ball along the edge of the pool so it once again lies to your immediate left. As you turn to gaze upon the tennis ball you notice that the Northern Hemisphere is pointing away from you and the other end of the chopstick (the South Pole) is now pointed at you with a 23-degree tilt. This orientation represents the winter solstice in the Northern Hemisphere; less of the hemisphere is exposed to solar radiation while the North Pole remains tilted away from the Sun.
This is, of course, the summer solstice in the Southern Hemisphere.
This is what is happening on Earth at this very moment. On Sept. 22, the first day of fall, the Earth was oriented with its axis perfectly perpendicular to the sun. We had 12 hours of day and 12 hours of night in a single 24-hour period. But to complete an orbit with a radius of 93 million miles in only 365 days, the Earth has to move … times 6.28 … divide by 365 … divide by 24 … about 67,000 mph!
As we hurtle through space, the North Pole is starting to turn away from the sun. The leaves are shutting down because the light is fading and the warmth of summer is failing. Autumn is a season of change and, as Ned Stark would say, winter is coming!
Bill Danielson, who has worked as a national park ranger, a wildlife biologist and a field researcher, is a high school chemistry and biology teacher. To contact Bill, visit www.speakingofnature.com.