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Space Topics: Planetary AnalogsStars Above, Earth BelowAstronomy and Space Exploration in America's National Parks
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Moonrise over the Great Smoky Mountains
The first quarter Moon rises over the changing fall colors of the Great Smoky Mountains. Credit: Tyler Nordgren |
Smoke and Seasons
Gatlinburg, TN -- When I told people I was going to the Great Smoky Mountains National Park for my astronomy project, I got a lot of blank stares. It was as if people were trying to find a way of drawing my attention to the fact it was called the “Smokies” for a reason, as if I just hadn’t quite noticed it was there in the name. This really hit home when even the park ranger with whom I was supposed to be arranging my trip gave me this reaction. So at every step of my Smoky Mountain adventure I have had to justify why this park is included in my plans. After all, if the skies are cloudy, and the lush green woods within the park aren’t exactly a terrestrial analog of anything we’re aware of on any other terrestrial planets, then why on Earth am I even here?
I’m so glad you asked. I understand that as an astronomer I observe
the world in a slightly different way than most. Stand outside a meeting
of observational astronomers and you’ll notice that upon leaving
any building we all instinctively look up to check the sky (“Yup,
light cirrus is forming, it’s not going to be photometric tonight”).
I haven’t had an observing run in over a year and I still do this
every time. When I go to the Grand Canyon with my wife and everyone is
hiking down the Bright Angel Trail “oohing” and “ahhing” at
the spectacle of the big drop off one side of the path, my wife and I have
our backs turned and are busy looking at the rocks on the other side of
the trail. So let me take this opportunity to describe to you what I, as
an astronomer, see when I visit the Great Smoky Mountains.
I’m here in mid-October when the leaves are changing. The mountains are in full fall splendor radiating waves of yellows and oranges. This is one of the busiest seasons for the Smokies (and the throngs of people crowding the tacky tourist traps of the gateway community of Gatlinburg attest to that). Seasons are a purely astronomical phenomenon but one that most people misunderstand. The popular misconception is that summer and winter are due to the changing distance between the Earth and Sun. If it’s winter, we must be farther from the Sun than in summer. Plausible but wrong.
In reality, it’s the tilt of Earth on its axis that causes the seasons. We see this, particularly in southern states, when in summer months the noontime sun beats mercilessly down on us from high overhead; while in winter months it casts long shadows low in the sky through bare trees. As the Sun lowers, the sun’s energy is spread out over a wider area of Earth’s surface, leaving less energy for any one spot. The hemisphere cools. The changing altitude of the Sun at noon also translates into changing positions along the horizon for sunrise and sunset and changes in the amount of time the Sun is in the sky. With shorter days the hemisphere cools more and the leaves on the trees pick up on this. Many cultures, from all over the world, have built what we believe to be monumental observatories or calendars to measure the changing rise and set positions of the sun and so be able to forecast the arrival of the seasons. During my stay here I gaze out my window while eating my Grape Nuts at dawn to see if I can detect the slow southward progress of the Sun’s rising position against the distant hills. I can’t, but then again I didn’t bring my travel-size Stonehenge either.
Newfound Gap Road
The Newfound Gap Road runs through a blaze of oranges and yellows as the fall colors sweep up the mountainside. In the mountains trees at different elevations change colors at different times as the weather slowly turns colder due to the Earth's motion around the Sun. Credit: Tyler Nordgren |
Earth is not alone in having an axial tilt and so it is also not alone in having seasons. Mars is tilted by very nearly the same amount as the Earth. However, unlike Earth, Mars does have an appreciably elliptical orbit, and so it really does get closer and farther from the Sun. This has the effect of modifying the intensity of its seasons (which due to its longer year means each season lasts nearly twice as long as its Earthly counterpart). Southern hemisphere summer comes when Mars is close to the Sun, making for energetic winds which flare into global sandstorms as just weathered by the Spirit and Opportunity rovers. Uranus is tilted so much it lies on its side and each pole sees the Sun for only half of its year. At the equinoxes the planet points perpendicular to the Sun and one pole awakes from its long, cold, lightless winter, while the other pole closes up shop as the Uranian tourists set sail for sunnier climes not to be seen again for another 42 years. The next equinox comes next month, December 7, 2007.
What in the formation of the planets gave rise to these tilts? Whatever it is must account for why some planets have virtually no tilt, such as Mercury where craters at the poles have crater floors that exist in perpetual night, while a planet such as Uranus can roll along on its side like an enormous Big Wheel. The prevailing theory is that catastrophic impacts early in the formation of the solar system left many planets literally bowled-over in their aftermath. Poor Venus slowly rotates backwards from everyone else leading to the conjecture that it was beaten so badly it was flipped upside down and nearly brought to a stop.
In the last several decades planetary impacts have become the go-to theory for a number of exciting questions that all of us encounter daily, whether we know it or not. My first week of hiking along hilltops in the Smokies, the sky was a wonderful deep blue (so much for those horrible skies the name implied). Each afternoon I was treated to moonrise over a distant hill. The origin of the Moon is now thought to be due to a glancing blow by an ancient Mars-sized planet. In my rambles through the woods I came upon salamanders (one of the earliest types of creatures to crawl up out of the water and colonize the land), as well as frogs, lizards, squirrels, dear and bears. Overhead I heard the calls of innumerable birds, but the one thing I didn’t see were dinosaurs. The very lack of giant, ferocious man-eating dinosaurs (outside the wax museums of Gatlinburg and Dollywood) is again due to planetary impacts. My very presence in the mountains, and the animals I hope to see or avoid, is thanks to astronomical collisions.
And these mountains! The Appalachians are a beautiful examples of a dynamic planet. In those hills I see millions upon millions of years of planetary crust running and thrusting up against one another. Every time the European plate rammed up against the North American plate, mountains not unlike the Himalayas formed from the collision. Every time Pangaea parted and the plates went their separate ways, erosion tore them down again to leave but a distant reminder behind. Looking back to the Moon we can see evidence of the ancient supercontinent by looking at the rate at which the Moon is slowly spiraling way from the Earth. As tidal interactions between the Moon and Earth slow Earth’s rotation, the distance to the Moon increases. Running the movie backwards, the current rate of the Moon’s recession has it encountering the Earth far too recently. However, lump all the continents on one side of the Earth every so often and you change the rate at which the Moon slows the Earth down, and thus you slow the rate at which the Moon spirals off, and push back the date at which the Moon had to have formed. Even if we had no other evidence for Pangaea, the Moon I see over these mountains that are its remnant tells me it must have been so.
This what an astronomer sees. Oh, and the stars sure were pretty at night.
Night in the Smoky Mountains
Clouds, fog, moonlight and city lights all mix in the skies over the Great Smoky Mountains. Credit: Tyler Nordgren |
I am heading back to southern California this weekend. My house is underneath one of the smoke plumes currently blowing out there. I guess I’m going from the Great Smoky Mountains, to the Great Smoky Basin. I’m out there for a month before starting the winter leg of my project which will take me through Chaco Culture National Historic Park, Grand Canyon and Yellowstone Parks.
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