You already know, that the sun is pretty important. It is the center of the solar system after all. You also know that our planet earth revolves around the sun, making its orbit every 365 days. And remember: earth isn't taking that lap while it's standing straight up and down. Instead, it's tilted on its axis. The invisible line around which our planet spins. Put together the earth's tilt on its axis and the orbit it makes around the sun and you get the yearly pattern we call seasons. Let's see how.

Since the earth is tilted, for part of the year, one of the hemispheres, which is half of the earth, is leaning toward the sun and the other part of the year, it's leaning away. Let's follow the northern hemisphere once around the sun to see how this works. 

In June, the northern hemisphere is tilted towards the sun. This means that it's getting a lot of direct sunlight, light that's hitting it straight on.
If you ever sat directly underneath a bulb, you know that things can get pretty hot and that's exactly what's happening to the northern hemisphere. It's summertime and the living is easy. Temperatures are warm and days are long.

In December, though, the northern hemisphere is tilted away from the sun. It's getting indirect sunlight, meaning light is hitting it at an angle. Indirect sunlight means cooler temperatures, shorter days and, for lots of folks, hot cocoa and bundling up since it's winter.

But how can the angle of the sun's light make a difference between hot and cold? Well, try this little trick with a flashlight: get a flashlight and dim the lights in your room a little bit. If you turn the flashlight on and point it straight-down onto your desk, you'll see a small bright concentrated circle of light. That's kind of how sunlight hits the norther hemisphere during the summer: bright and intense. Now move the flashlight down at an angle and point it at the top of your desk. See how the light isn't as bright and as less intense where it falls? That's like the sunlight we get in winter. 

But what about spring and autumn? During these two seasons the earth's orbit causes the northern hemisphere to be neither tilted toward the sun nor away from it. So temperatures during the spring and fall are more moderate, not too hot and not too cold, since the entire globe is getting about the same amount of direct sunlight at once.

Now let's take a look at how the amount of sunlight affects temperatures in the northern hemisphere over the course of a whole year. An easy way to show this yearly pattern is by using a graph. This graph shows the average high temperature in each month for one year in Toronto, Canada, where I live. Looking at the graph, we see that during December, January and February, when the northern hemisphere is getting very little direct sunlight, temperatures are low - and in the month of June, July and August, when the tilt of the earth on its axis is causing Toronto to get direct sunlight, the temperature are much higher. Proof positive that something is going on here and that something is this:

The season that you're experiencing right this very minute is caused in part by the amount of direct sunlight you're getting. So, seasons are caused by the earth's tilt on its axis as it cruises around the sun in its orbit. When one hemisphere gets more direct sunlight, it's summer there. Temperatures are warmer, days are longer and nights are shorter. And when it gets more indirect sunlight, it's winter. Temperatures are cooler, days are shorter and nights are longer. 
And now you know what causes summer, spring, autumn and winter.

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