It spends 9 hours above the horizon in the United States, which means fewer hours of daylight and more hours of night in northern lands (and a strong need for people to hold celebrations to cheer themselves up). On December 21, the Sun rises south of east and sets south of west. For observers in the Northern Hemisphere of Earth, the Sun spends about 15 hours above the horizon in the United States, meaning more hours of daylight. On June 21, the Sun rises north of east and sets north of west. The Sun’s Path in the Sky for Different Seasons. Like the straight-on light, the sunlight in June is more direct and intense in the Northern Hemisphere, and hence more effective at heating. But if you hold the flashlight at an angle (if the wall “leans out” of the beam), then the spot of light is more spread out. If you shine the flashlight straight on, you get an intense spot of light on the wall. You can get a similar effect by shining a flashlight onto a wall. When we lean into the Sun, sunlight hits us at a more direct angle and is more effective at heating Earth’s surface ( ). How does the Sun’s favoring one hemisphere translate into making it warmer for us down on the surface of Earth? There are two effects we need to consider. In spring and autumn, the two hemispheres receive more equal shares of sunlight. In December, during winter in the Northern Hemisphere, the Southern Hemisphere “leans into” the Sun and is illuminated more directly. In June, the Northern Hemisphere “leans into” the Sun, and those in the North experience summer and have longer days. We see Earth at different seasons as it circles the Sun. In September and March, Earth leans “sideways”-neither into the Sun nor away from it-so the two hemispheres are equally favored with sunshine. In December, the situation is reversed: the Southern Hemisphere leans into the Sun, and the Northern Hemisphere leans away. As Earth travels around the Sun, in June the Northern Hemisphere “leans into” the Sun and is more directly illuminated. Note that our axis continues to point the same direction in the sky throughout the year. shows Earth’s annual path around the Sun, with Earth’s axis tilted by 23.5°. And if distance were the governing factor, why would the two hemispheres have opposite seasons? As we shall show, the seasons are actually caused by the 23.5° tilt of Earth’s axis. To make matters worse for people in North America who hold this hypothesis, Earth is actually closest to the Sun in January, when the Northern Hemisphere is in the middle of winter. That’s not enough to cause significant variations in the Sun’s heating. Although Earth’s orbit around the Sun is an ellipse, its distance from the Sun varies by only about 3%. But the facts don’t bear out this hypothesis. This sounds reasonable at first: it should be colder when Earth is farther from the Sun. Many people have believed that the seasons were the result of the changing distance between Earth and the Sun. With these observed facts in mind, let us ask what causes the seasons. The difference between seasons gets more pronounced the farther north or south from the equator we travel, and the seasons in the Southern Hemisphere are the opposite of what we find on the northern half of Earth. We thus divide the year into seasons, each with its different amount of sunlight. One of the fundamental facts of life at Earth’s midlatitudes, where most of this book’s readers live, is that there are significant variations in the heat we receive from the Sun during the course of the year. Explain how seasonal differences on Earth vary with latitude.Describe how the tilt of Earth’s axis causes the seasons.By the end of this section, you will be able to:
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