The fact that the earth is tilted on its axis is what's responsible for giving us different seasons. And it's why the southern hemisphere is getting ready for winter at the same time the northern hemisphere is heading into summer. In this Science Update, you'll hear how that tilt got knocked into place.
What made Earth a Tilt-A-World? I'm Bob Hirshon and this is Science Update.
Every 24 hours, the earth spins around once on its axis—an axis that's tilted 23 degrees with respect to earth's orbit. Vijay Thurimella of Denver, Colorado, wants to know how earth got that way.
We consulted Clark Wilson, a Geophysicist at the University of Texas at Austin. He says earth's tilt came about early in the formation of the solar system, four and a half billion years ago. Back then, a lot of dust and rocks were floating around and crashing into each other. That debris eventually stuck together to form the planets.
That process is a little messy, and in the case of the earth, probably led to some big impacts that eventually tilted the axis to what it is, 23 degrees now.
One of those big impacts, for example, ejected a lot of debris that eventually coalesced to form the moon.
But Wilson says earth doesn't get knocked around much anymore.
Most of those objects are gone, and we're left with sort of little pieces, asteroids and so on, that would not significantly change the earth’s orbit or rotational axis direction now.
If a science question's got you off-axis, call us at 1-800-Why-Isit. Or e-mail us from our website, www.scienceupdate.com. For the American Association for the Advancement of Science, I'm Bob Hirshon.
Making Sense of the Research
First of all, let's be sure we're clear on what the earth's axis is. Basically it's an imaginary stick going through the center of the earth, if we define the center as "the point around which it rotates." In other words, picture the earth spinning like a top, straight up and down. Now picture a stick going right through the center of the earth. If the earth weren't tilted, it would rotate like that as it revolved around the sun, and we wouldn't have seasons—only areas that were colder (near the poles) and warmer (near the Equator).
But the earth is tilted, and that's why the seasons happen. When the Northern Hemisphere is pointed toward the sun, it gets more hours of sunlight. Temperatures rise, and you get summer in New York, while it's darker and cooler "down under" in Sydney. Six months later, the reverse is true, and it's the Southern Hemisphere that experiences summer. The 23-degree tilt also explains why changes in daylight during the seasons are very dramatic near the poles (which are flooded with sunlight all day long in summer and get virtually no light in mid-winter) but barely perceptible near the equator (where the sun shines more or less equally throughout the year).
Getting back to why the axis exists, it's mainly the result of the rough-and-tumble environment of the early solar system. Scientists believe that the sun and the eight planets formed by chunks of rock and debris that self-accumulated through gravity. In other words, objects collided and clumped together, which increased their gravitational pull, which in turn drew more objects in, which made the object even more gravitationally powerful, and so on until the solar system looks like a sun and eight fairly neat planets with not much stray junk flying around.
Of course, occasionally these forming objects happen to attract something that's big enough to knock it off-kilter. That's what probably happened to the earth, after it was already large enough to start rotating. Actually, Wilson says it probably took several substantial impacts to whack the earth into the position it's in today.
Incidentally, back in the "old days," the earth used to rotate a lot faster—once every 6 to 10 hours at the start of the solar system—and the moon's gravity has played a big role in slowing us down to 24. It's a good thing too, because a 6-hour day would certainly lead to an awfully hectic work week.
Now try and answer these questions:
- What does it mean to say the earth rotates on a 23 degree axis?
- How does this tilt create the seasons? Why are seasons more dramatic in some places than in others?
- What probably caused the axis to tilt the way it does?
- Leaving aside the question of the damage done by the impact, what would happen if today the earth's axis were knocked back to 0 degrees? What if it were tilted 45 degrees? 90 degrees? How would life on earth change? Would all parts of the earth be habitable? Cite about specific places in the world and how their climate and daylight patterns would be affected. (Use a globe for reference if you want.)
The Nine Planets, created by astronomer Bill Arnett, has this feature on the earth and its general characteristics.
Read the National Geographic article, Was Moon Born From Planet's Crash Into Earth? about the collision that may have created the moon.