Science · 6 pages · 12 min
The axis leans by about twenty-three degrees, and that lean, not the distance, makes summer.
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Seasons happen because Earth's axis leans by about twenty-three degrees relative to the plane of its orbit, not because we move much closer to the Sun. When a hemisphere leans toward the Sun, its days run longer and the Sun climbs higher, so the ground receives more concentrated light for more hours.
The northern summer is not the time when Earth is nearest the Sun. The closest approach falls in early January, during the northern winter, which is the wrong way around for a distance theory of the northern seasons.
The mechanism is more particular than the slogan. A tilt toward the Sun means the midday Sun stands higher, so the same beam covers less ground and each patch gets more energy. The tilt also keeps the Sun above the horizon longer, and those two changes, height and day length, reinforce each other through the warm season.
Follow it one step further. Half a year later the same hemisphere leans away. The midday Sun stays low, the beam arrives at a slant and spreads out, the days are shorter, and the ground runs a deficit even though the planet as a whole is not much farther from the Sun.
The case worth keeping in mind is specific. Southern seasons run the opposite way on the same tilt, so January is summer in Australia while it is winter in Europe. One distance from the Sun cannot make both of those true, and the lean can.
That case has a boundary. Distance does change a little through the year, and the sunlight is slightly stronger at the closest approach than at the farthest. That variation is real and small beside the effect of the tilt, which is why the hemisphere leaning toward the Sun has summer even when the planet is on the farther side of the orbit.
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