Seasons

It’s starting to get warmer here in Virginia! I love feeling the winter start to recede—winters here are mild compared to some places, but I’m not a fan of anything that makes my bones hurt, and that includes Virginia cold.

When you first start to think about why we have seasons here on Earth, you probably go to the same place my mind went as a young kid: distance.

“I can’t wait until the Earth gets close to the Sun again, so we can have another nice summer!”, you think. This seems very intuitive!

I think back to the first trip I took to Brazil, back in 2004. It was July, and Rio was about as cold as it ever gets (which is to say: still pretty warm). Here in the northern hemisphere, the December solstice (usually December 21st or 22nd) is the official start of winter. It’s the shortest day and longest night of the year, and it marks a turning point: days only get longer from here.

That’s a good technical name for why winter officially begins when it does, but why should it be that Brazil’s winter is in America’s summer, and vice versa?

Before we get too far into this, here is Seasons by Chris Cornell, a grunge classic you’ll probably either really enjoy or really hate:

Okay, so what’s happening here?

The main thing to know is that the Earth doesn’t spin straight up and down relative to the Sun. Instead, it’s off-kilter by about 23.5 degrees. This is called the axial tilt for short.

Of course, while the earth is spinning on its axis exactly once every day, it’s also going around the Sun exactly once every year. If you can imagine that constant tilt while the Earth is orbiting the Sun, you can get a good visualization of what’s going on.

Because of that tilt, different parts of the Earth get different amounts of direct sunlight at different points during the orbit. This 23.5 degree tilt doesn’t change as the Earth goes around the Sun.

When the northern hemisphere is facing the Sun, we have summer. Imagine that bright, warm light shining on the face of the planet, and you can understand why the days are longer and why it’s generally warmer.

Conversely, when the northern hemisphere is facing away from the sun, it’s winter.

Spring and fall seem like they’re just in-between seasons, but they have interesting astronomical raisons d'être.

For farmers, the spring equinox represented the moment when day and night were the same length. This meant that there would be more and more light from here on out, and for as long as anyone could remember, it signaled the beginning of the main growing season.

The risk of losing crops due to frost went down significantly as the soil began to warm for the year. Farmers knew which seeds to plant before the equinox, and which to plant after. Naturally, festivals around this day sprung up all over the ancient world.

You can read a little bit more about the spring equinox and how it factors into the ten days that disappeared:

10 Days That Disappeared

As I go for a walk on a nearby trail, I feel the sun on my skin, warming me up, and reminding me that the Earth has circled around the Sun far enough so that the northern hemisphere is tilted toward the Sun.

Summer is my favorite season. I can’t get enough of what others call oppressive heat, and it has pretty much always been that way for me. What about you? What’s your favorite season, and why?