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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.

This 23½-degree angle is why you have seasons. This is the whole reason why there is winter and spring and summer and fall. This tilt is why there is a time of year when plants are growing vigorously and another time when they are dormant. This slant of the Earth is the reason why January 3 and July 3 have a completely different feel. And this is why some times of year the Sun races across the sky and sets like a falling rock and at other times it just seems to hang up there all day long.

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.

FIGURE 3-7: Here is a close-up view of Earth’s 23.5-degree slant and how it affects the distribution of sunlight in the course of its yearlong revolution around the Sun.

These dates are often said to be the “first official days” of the various seasons, but take another look. Is December 21 really the first day of winter where you live? As my people say at the Go Figure Academy of Sciences, this is the kind of thing that can happen when you send an astronomer to do a meteorologist’s job! In most parts of the United States, if you haven’t been wearing your winter coat before December 21, you haven’t been keeping your promises to your mother. Likewise, by June 21, the day of the year when the Sun’s rays beat down directly over more of the Northern Hemisphere than any other, summertime has already become a pretty familiar feeling.

In the middle latitudes of the Northern Hemisphere, weather scientists generally think of the winter season as the months of December, January, and February, spring as March, April, and May, and so on. (From winter through autumn, Part 3 goes into the details of the weather effects of these seasonal changes.)