The Shrinking Tropics

More and more folks are finding the tropics a great place to visit. It offers a refreshing break from our winters, and it offers the flavor of native cultures and the romance of foreign sun and beaches. The amusing irony to this is that while more and more folks flock to the tropics, there are less and less tropics to flock to! The fact is that each year the tropics of Capricorn and Cancer shrink about 14.8 meters towards the equator.

We all know the equator is the imaginary line running around the middle of the Earth, 0 degrees latitude. The tropic of Cancer is an imaginary line which runs parallel to the equator, 23.5 degrees north latitude. It is the line at which the Sun shines directly overhead on the summer solstice. 23.5 degrees north is the farthest north the sun can shine directly overhead. The tropic of Capricorn is an imaginary line at 23.5 degrees south latitude and is the southern counterpart of the tropic of Cancer. On the winter solstice the sun is directly overhead 22.5 degrees south of the equator.

The 23.5 degrees comes about from the fact that the Earth is ?tilted? 23.5 degrees from the plane of the Earth?s orbit. If you were to measure the angle between the Earth?s equator and the plane of the solar system, you would measure a 23.5 degree angle. This tilt is called obliquity. The reason that the topics are shrinking is because the obliquity of the Earth is getting smaller.

Obliquity is not the same as the Earth's axial tilt. We have talked previously about prcession, the 26,000 year cycle of the Earth?s axis (its just like the wobble of a spinning top.) What causes the wobble is the gravitational tugs from the Sun and Moon on the Earth. While this is happening, the orbital plane itself is precessing, from the gravitational pull of all the other planets in the system. The period of this action is 71,000 years.

Together, the precession of the axis and the orbital plane cause the obliquity of the Earth to oscillate with a period of about 41,000 years. These oscillations cause the topics to shrink and expand. While the wobble of the Earth's axis traces out a rather large circle in the sky, the change in obliquity is much more subtle, with only a plus or minus 1.3 degree variation. But the effect of that change on the topics is that movement of 14.8 meters per year (during some parts of the cycle, that rate of change is faster than others, we are currently at the fastest pace.)

So what does all this wobbling and oscillating mean? The answer is yes, the climate on our planet is affected by these periodic changes. Studies have shown a direct relationship between the obliquity changes and the smaller ice ages the Earth periodically goes through. When the obliquity is low, the polar regions receive less sunlight, and more ice tends to build up. When the obliquity is high, the polar regions get more sunlight and the ice tends to melt.

As it turns out there are many cycles relating to the Earth's rotation and its orbit. It is believed that all of them have some effect on climate. As we learn more, we will no doubt find missing pieces to the puzzle of Earth's past. For now we'll just continue to wobble and oscillate and rotate and revolve and ... does anyone have anything for motion sickness?