Ask a room full of people why summer is hot and winter is cold, and a lot of them will say Earth gets closer to the Sun in summer. It sounds logical. Closer to the fire, warmer you get. But there's a big problem with that idea.
Earth is actually closest to the Sun in January and farthest in July. The real cause of the seasons is Earth's tilt. Our planet's axis leans about 23.4 degrees, so as we travel around the Sun, the Northern Hemisphere spends part of the year tipped toward the Sun and part of the year tipped away. That lean controls how high the Sun climbs and how long the days last.
How a Tilt Makes Summer
When your half of the planet leans toward the Sun, the Sun rides high in the sky. Its rays hit the ground more directly, delivering more energy to each patch of land. The days are longer, too, so the ground has more hours to soak up heat.
In winter, the Sun stays low. Its rays come in at a slant, spreading the same light over a larger area, so each spot gets weaker, less direct sunlight. Combine that with short days, and things cool down. Together, these effects account for most of the seasonal change in both hemispheres.
This is also why the seasons are flipped south of the equator. When it's summer in Ohio, it's winter in Australia, because the Southern Hemisphere is leaning away from the Sun at the same moment.
What About Distance?
Earth's orbit isn't a perfect circle, so our distance from the Sun does change a bit. But the effect is small, about a 7 percent difference in sunlight over the year. And for the Northern Hemisphere, it actually works against the seasons, since we're nearest the Sun in the middle of our winter.
Near the equator, the tilt doesn't change the Sun's strength very much over the year. That's why tropical places don't really have a warm summer and a cold winter. Instead, their seasons are often wet and dry, as a belt of rainy weather shifts north and south.
Why the Hottest Days Come Late
If the Sun is strongest around the June solstice, why are July and August usually hotter? The answer is seasonal lag. The oceans and land take time to warm up and cool down, a bit like a pot of water on a stove. The peak of summer or winter can arrive up to about seven weeks after the solstice.
That's also why, in much of the Northern Hemisphere, February is usually colder than November, even though February has more daylight. And the first day of fall is typically much warmer than the first day of spring, even though both get similar sunlight.
A Tilt That Changes Slowly
Earth's tilt isn't fixed forever. Over long stretches of time, it wobbles between about 22.1 and 24.5 degrees, which changes how strong the seasons are. The Moon helps keep that tilt fairly steady.
The calendar shifts too. About ten thousand years from now, northern winter will come when Earth is farthest from the Sun, and northern summer when it's closest.
So the next time you shovel snow in January, remember that you're closer to the Sun than you'll be all year. Winter isn't about distance. It's simply a matter of which way our planet happens to be leaning.
