But, this is a weird NASCAR track; come back later, and the shape is changed a bit, going from almost circular to more squashed and back to almost circular over , years. There actually is a ,year modulation, so almost circular-slightly squashed-almost circular-more squashed-almost circular really squashed-almost circular-some squashed For the tiny changes in the Earth orbit, this is only a tenth of a percent or so in total sun received.
You may remember from the description of obliquity that the North Pole is inclined a bit. In addition to this angle changing, the North Pole also wobbles. Imagine putting your feet against a metal stake in the ground, grabbing the stake with your hands, leaning out until your arms are straight, and then having a friend push you in circles around the stake.
Imagine a North Pole sticking up out of your head, extending your spine. If you bend your elbow and pull yourself toward the metal stake, your North Pole will point more nearly in the same direction as the metal stake, because you have changed your obliquity. Now, suppose you were doing this metal stake, friend and all on top of a NASCAR racer, with the sun in one of the towers in the infield.
Your friend would have to push you really slowly the drivers would make about 10, laps before you got halfway around the metal stake! But notice that you would slowly switch from being on the infield-side of the metal stake when the car was at the end of the track closest to the sun tower summertime for your North Pole, and wintertime for your South Pole , to being on the outside of the metal stake at that closest approach and on the near side of the metal stake at the farthest distance from the sun tower.
This is precession. Notice that if you are close in northern summer, you are far in northern winter, giving a big difference between seasons in the north, but that close in northern summer is close in southern winter, and far in northern winter is close in southern summer, so when the winter-summer difference is large in the north, the winter-summer difference is small in the south, and when the winter-summer difference is small in the north, it is large in the south.
Also, your friend is not pushing you with perfect consistency and, bizarrely enough, the whole track is actually turning slowly, so that the straightaways switch slowly from being mostly north- south to being mostly east-west and on around to north-south again , so that rather than making a full circle of your metal stake every 20, laps or so, you typically make a full circle after either 19, laps or 23, laps.
Thus, precession matters a lot when the orbit is very eccentric, and precession matters little when the orbit is nearly round. The most complete pre-computer treatment came from Milutin Milankovitch, so these are usually called Milankovitch cycles. Milankovitch predicted that, when ice-age cycles were understood, it would be found that the climate had varied with periods of 19, years, 23, years, 41, years and , years. Several decades later, when isotopic records of oceanic foraminifera were developed, these very periodicities were discovered — Milankovitch was right!
And, because the different cycles affect north and south, and poles and equator, differently, it is possible to tell where the main controls reside.
The leading interpretation now is that poles are more important than equator, and north more important than south. When Canada and Eurasia received little summer sun, ice grew and the world cooled globally; when the sun increased in the high latitudes of the north, the world warmed and the ice shrank.
Oddly enough, northern sun has been more important than southern sun, with cooling in the south during some times when sunshine was increasing there. Many people have tried to explain this odd behavior in many ways, but so far the only successful explanations involve carbon dioxide. The content is provided for information purposes only.
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In the process, they produce a valuable synthesis of patterns of change by latitude and environment. An excellent, provocative study on evolutionary change as it relates to extinction The dedicated will walk away with an infinitely deeper comprehension of how and which circumstances have affected the environment in major ways—leading the way toward a greater understanding of today's changing world. Sally Day, Fossil News This is a useful volume for graduate level students and scientific specialists I would certainly recommend it.
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