The Reason for the Seasons


As a born Californian, I never saw seasons this dramatically until I went to college in Flagstaff, Arizona.

I remember, in my first year here, when I was taking a walk around campus with a few friends. We passed over a riverbed where water was gently trickling along. Green grass and brush lined the banks. The sight absolutely captivated me. I had never seen anything like it, even in the springtime.

Then winter hit in all its blizzarding glory. At night, the temperature dropped below freezing. Snow fell in flurries that contrasted beautifully with the night. By morning, snow banks over a foot high lined the footpaths. To say nothing of the state of my winter jacket!

Summer in Flagstaff is hot. And I mean hot. It’s sweltering. Everyone crowds under the nearest tree. I experienced two days of it during orientation, and I never want to be here in the summer again.

Flagstaff’s autumn isn’t quite like the red and golden season depicted above. It pours. The rain sweeps down from the skies in torrents, soaking you through to the bone within minutes of being outside. Don’t think you’re safe under an umbrella. Better get some waterproof slacks to cover up those jeans, or you’ll be freezing in your classes all day.

I remember learning that my good blogging friend, the Momma, experiences the opposite seasons. When it’s pouring over here, it’s all green and sunny in Australia. When it snows here, she’s getting summer—I only hope it’s not as sweltering as it is in Flagstaff!

But why? Why should Australia have seasons that are opposite those in America?

It all comes down to the Earth’s tilt.

Technically, this isn’t all about America and Australia. What’s happening is that the northern and southern hemispheres of the earth get opposite seasons, and America and Australia happen to be in opposite hemispheres.


Remember, everything north of the black line is the northern hemisphere and everything south of it is the southern hemisphere.

Seasons happen differently between the northern and southern hemispheres because the sun’s light reaches them differently. Why? Because the Earth is tilted just a little bit. As it orbits around the sun, it doesn’t stay perfectly upright. And over the course of any one year, it stays tilted in pretty much the same direction.


This is what the Earth’s tilt looks like from December through February. During these three months, the northern hemisphere experiences winter and the southern hemisphere experiences summer.

And a similar effect makes the seasons change throughout the year…


As you can see here, the Earth orbits eastward around the sun over the course of a year. The Earth stays tilted at about a 23.5 degree angle. In June, that means the northern hemisphere gets more sunlight, but the southern hemisphere is in wintertime. But in December, the seasons switch, and it’s summertime for the southern hemisphere.

You might also notice that the daylight/nighttime shading of the Earth is equal during March and September. During this time, the hemispheres get equal sunlight, and each experiences either spring or autumn.

Because the hemispheres get equal sunlight, the beginning days of autumn and spring are each called equinoxes. Notice they share the prefix equi-, which basically means “equal.”

And the beginning days of summer and winter are called solstices. The word “solstice” comes from the root word “sol,” meaning “sun.” They’re named that because starting on the day of each solstice, one of the hemispheres is getting more sun than the other.

There’s a common misconception that the seasons happen because the Earth swings in closer to the sun during the summer…


This isn’t true. It’s true that the Earth’s orbit isn’t exactly circular. It’s an ellipse, which is sort of a stretched out circle. I’ll be talking more about what elliptical orbits mean in later posts.

But the Earth’s orbit is actually so close to being circular that its eccentricity, or how elliptical it is, doesn’t actually make a difference to its seasons. If this was the reason for the seasons, then the northern and southern hemispheres would have the same seasons at the same times.

You might wonder, if Earth is all one planet, how does it matter whether one half gets more light than the other half? Well, to answer that question well, I’d have to give you an in-depth explanation of climate science. For now…just know that climate systems in the hemispheres are mostly separate, so they don’t share the sun’s heat and light.

To recap: the winter and summer solstices are the first days of winter and summer, and the vernal (spring) and autumnal equinoxes are the first days of spring and autumn.

You may have also noticed that over the course of a year, the sun rises and sets at slightly different places along the horizon…


This diagram shows an observer standing at about 32° north latitude—meaning, the northern hemisphere.

During the summer, the northern hemisphere is tilted toward the sun.

That makes the sun appear higher in the sky. It rises and sets farther north along the horizon.

At the summer solstice, it rises and sets as far north as it ever will be.

During the winter, the northern hemisphere is tilted away from the sun.

During this time, the sun appears lower in the sky…and rises and sets farther south on the horizon.

At the winter solstice, the sun is seen the farthest south that it’s going to get.

Interestingly enough, you can tell what season it is by how high the sun appears in the sky at noon. If it’s near the zenith—the point in the sky right above your head—then it’s probably the summertime. If it appears a bit lower, as shown in the image below, then it’s probably the wintertime.


This is a composite image of the sun’s arc across the sky from the moment it rises in the east—to the left—to the moment it sets in the west. Notice that the sun is nowhere near directly overhead. This means the season is probably winter.

Please be careful, though. Don’t try to look directly at the sun. I know it’s normal for your eyes to flinch away when you try, but just don’t try in the first place. And please, for the sake of your eyesight and your expensive camera, don’t try to take a picture like this unless you know what you’re doing. The sun is as dangerous as it is vital to life on Earth. It’s like water—you need it to live, but you could drown in too much of it.

That’s it for now on the seasons and the importance of the sun. Next time in astronomy, we’ll talk a bit more about climate and how theories work in science

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