Читать книгу Weather For Dummies - John D. Cox - Страница 61

If you told me that it takes a year for the Earth to travel completely around the Sun, and that a year is 365 days, you would be accurate enough for most purposes. But I might not want to set my clock by yours. Did you remember Leap Year — the fact that you add a 29th day to February every four years? This makes up for the fact that the complete revolution of Earth’s orbit around the Sun actually takes 365¼ days.

There’s something else about Earth’s orbit of the Sun that is a little, well, irregular. If you look at it closely, you will see that it is not really a circle — that is, the Sun is not in the center of Earth’s orbital path. Instead, it is off to one side. The shape of the orbit is elliptical, which means that at some times during the year the Earth is actually closer to the Sun than at other times.

This state of affairs might lead you to think — as some people do — that Earth’s elliptical orbit is responsible for the fact that some times of year are warmer than others — that summer might be caused by the fact that the Earth and Sun are closest together at that time of year. This is a completely mistaken idea, and you should wash it out of your mind immediately. In fact, I’m sorry I brought it up!

If you have any doubts, consider this fact: The Earth is closest to the Sun every year on January 3. I don’t know about you, but I live in California, and while we have had some pretty nice January days, January 3 has never felt much of anything like summer. If I lived in the Southern Hemisphere, south of the Equator, where January 3 is in the summertime, I might think differently about this, of course. But still I would be wrong. Earth being closest to the Sun at that time of year is not responsible for the fact that it is summertime there either.

Here’s what it means: On or about January 3, the Earth and the Sun are a mere 91 million miles apart. Six months later, on July 3, at the opposite side of the elliptical orbit, when they are farthest apart, the distance has stretched to 94 million miles. This is about a 3 percent difference in the Earth-Sun distance from one time of year to another.

It has an impact on the intensity of sunshine reaching Earth, no doubt about it. Scientists have figured out that Earth gets 7 percent more heat energy from the Sun on January 3 than it does on July 3. This is because on July 3, even though it is mid-summer in the Northern Hemisphere, the sun’s rays are traveling a little farther and so are slightly more spread out than they are on January 3. But this small difference does not account for the seasons. The angle that they strike a particular place on Earth makes a lot more difference to the intensity of the Sun's rays. As Figure 3-6 illustrates, the angle is what the seasons are all about.

FIGURE 3-6: In the Northern Hemisphere’s summer, the Sun’s rays are more intense as they strike the atmosphere more directly overhead, while in winter they strike at a greater angle and travel through more atmosphere.