The reason you get to experience winter, spring, summer, and fall is because the amount of sunlight hitting the earth’s surface varies throughout the year, and that’s a result of the planet’s tilt. Earth rotates around an imaginary axle—its axis—that runs from the North Pole to the South. If you picture Earth’s orbit as a flat plane around the Sun, that axis is tilted 23.5 degrees away from that plane, called the plane of the ecliptic. In other words, Earth is actually leaning to the side. This means that as Earth orbits the sun, sometimes the Northern Hemisphere is tilted toward the Sun and gets more sunlight, and sometimes the Southern Hemisphere is tilted toward the Sun and gets more sunlight.
If there were Martians, they’d experience even more seasonal variations than Earthlings. Mars’s orbit is more eccentric than any other planet (except Mercury), and it’s tilted on its axis at a slightly more acute angle than Earth. During a Martian winter, a complex carbon dioxide exchange with its polar ice caps causes the planet’s atmospheric pressure to drop about 25 percent lower than in summer.
Solar radiation has the most effect on the earth’s surface and atmosphere when it’s shining straight down. If you shine a flashlight on the middle of a beach ball, you’ll notice it makes a round circle, because all the light is focused on that one point. But shine it closer to the top and the beam spreads out into an oval such that there’s less light shining on any one spot. That’s what happens as Earth revolves around the Sun. From June to September, when the Northern Hemisphere is tilted toward the Sun, light shines more directly on it (which is why the Sun seems to be almost straight overhead) and heats up the surface, causing a run on air conditioners. In addition, these days are longer, so the Sun has more time to do its job.
From December to March, with the Northern Hemisphere tilted away from the Sun, there’s less light reaching any one place to warm it up, and there’s a run on space heaters. Days are shorter, so by the time the Sun has begun to warm up the atmosphere, it’s already descending toward the horizon. Also, with sunlight striking the planet at a lower angle, its rays have to penetrate through a lot more atmosphere to get to the surface, and there’s a greater chance that some of that light will get scattered by dust particles, reflected by clouds, or absorbed by gases. Another factor that keeps the surface of Earth from heating up in the winter is the presence of ice or snow, which is very reflective and can keep the Sun’s energy from ever reaching the ground.
If Earth didn’t tilt, the Sun would always shine directly down on the equator and the days and nights would always be the same length: twelve hours. There would be no seasons, and it would be very hot in the equatorial regions. North America would always receive about the same amount of light it gets between September and December, so most of the time it would probably feel like autumn.
Drawn on a piece of paper, the orbits of Earth and the other planets are made to look like perfect circles, but in truth, none of them are. Earth’s orbit is about 2 percent closer to the Sun in January than it is in July (other planets’ orbits are even more out-of-round), but as mentioned earlier, that small difference isn’t enough to affect the seasons very much. The atmosphere receives only about 7 percent more energy when closest to the Sun (perihelion) than it does when farthest away (aphelion). What does have an effect is the fact that the earth’s landmasses aren’t evenly distributed; the Northern Hemisphere has more land; the Southern Hemisphere has more water. Land heats more rapidly than water, so in July when the Sun is shining more directly on landmasses, the temperature of the entire globe is higher than it is in December, when the Sun shines down on the oceans to the south.
You can see that effect on a smaller scale when you visit the desert in the summertime. During the night, the temperature might get down to 60°F or so, but when the Sun comes up, it can rise to over 100°F. If you’re sailing on the ocean, however, the temperature might be only 75–80°F. Then, when the Sun goes down, the temperature will fall only a few degrees because water is great at absorbing and retaining heat.