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.
Spreading the beam
The closer together the rays of sunshine, the more intense the energy. This idea helps explain why the energy from the Sun is weaker when it shines on the polar regions of Earth than at the Equator. These regions remain cold even though the Arctic and Antarctica, at the North and South Poles, get many hours of daily sunshine during the summers in the Northern and Southern Hemispheres. It has to do with the angle at which the sunlight strikes.
Everybody notices this effect of the sunshine between the different seasons, of course. Unless they live in the Tropics, the region of the world along the Equator, where the Sun is more or less directly overhead all year long. For most of the world, the winter Sun that comes glancing in at a low angle is a pretty weak sister to the summer Sun that spends a lot of time directly overhead. Figure 3-7 illustrates the point. The next section of this chapter explains the cause of this seasonal angle.
A MATTER OF SOME GRAVITY
Hmmm … let me see now… .
The time from the Summer solstice to the Winter solstice would be — yup, that checks out, 182 and 183 days between them, close enough.
And from the spring equinox to the autumnal equinox — oops, what’s going on here? Between March 20 and September 22 are 186 days, and between September 22 and March 20 are 179 days.
Sure enough, it has to do with the elliptical shape of Earth’s orbit around the Sun (refer to Figure 3-6). This is gravity at work — the pull of the mass of the Sun on the Earth. When the Earth is closer to the Sun, the pull of gravity is stronger. Because it is farther from the Sun from March 20 to September 22, Earth travels more slowly during that loop of its orbit.
This means that summers are seven days longer in the Northern Hemisphere than in the Southern Hemisphere. Do they know this in Australia? Is this legal?
The sunlight’s angle affects its intensity in another way. The more directly the Sun is over your head, the less of Earth’s atmosphere it has to penetrate. Various things in the atmosphere filter out or scatter some of the incoming rays, so the more atmosphere it has to travel through, the more filtering and scattering takes place. (Chapter 15 says a lot about these optical effects.)
Tilting at the seasons
Earth is out of kilter. You might expect a well-behaved planet to stand up straight, and after 5 billion years or so, to act its age. But nope, not Earth. What can you do? Always it’s got this slant to it, like a slouchy teenager, as if it’s leaning against something. The angle of this tilt — the difference between where its poles are and where they would be if it were upright in relation to the Sun — is 23.5 degrees. When you come to think of it, this is quite a slant.
If Earth were upright in relation to the Sun, still there would be weather, because still there would be cold air near the poles and warm air near the Equator for the atmosphere to contend with. And still there would be the cool and warm variations of night and day. But without the tilt, there would be no seasons.
My people at the Go Figure Academy of Sciences tell me that in a truly upright world, life as you know it would be very different. For one thing, everywhere on Earth all year long would get the same amount of daylight and darkness — exactly 12 hours. For another, there would be no tourist seasons.
As Figure 3-7 shows, this arrangement that gives the Earth the same slant in relation to the Sun throughout the year produces some interesting dates.
On about March 20, the vernal (or spring) equinox, and again on or about September 22, the autumnal (or fall) equinox, it happens that daylight and darkness is distributed evenly around the world — each lasting 12 hours.
Direct sunlight reaches its most northerly point on or about June 21, the summer solstice, over the Tropic of Cancer, an imaginary latitude line 23.5 degrees north of the Equator. In the Northern Hemisphere, this is sometimes called “the longest day” because it is the day of most daylight.
Likewise, on or about December 21, the beam of direct sunlight has reached its most southerly point, over the Tropic of Capricorn, 23.5 degrees south of the Equator. In the Northern Hemisphere, this is “the shortest day,” the day of least daylight. By the same token, at the other end of Earth’s tilt, in the Southern Hemisphere, these “longest” and “shortest” days are reversed.