Chapter 11, Image 21

Caption

Earth's axis rotates slowly around an imaginary center line, like a wobble in a spinning top. The main cycle of this process--known as precession--takes about 26,000 years, and it shifts the dates of perihelion and aphelion forward by about one day every 70 years. Thus, in about 13,000 years, Earth will be closest to the sun in July instead of January. This will intensify the seasonal changes in solar energy across the Northern Hemisphere and weaken them in the south. The stronger summer sun is likely to bolster the African and Asian monsoons, as was the case around 10,000 years ago (see p. 80).