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It’s not today! You can still enjoy one last weekend of summer! But the first day of fall is coming up Monday, and for why? Why does this happen? Why do the seasons change? Why do the leaves turn color? (At least, they do in New England, where I’m writing this; ymmv.

Well, this is a hint: why is it that in the summer, a sunny day makes you hot and gives you a sunburn if you stay outside in it, but in the winter, it can be sunny and still be super cold?

Here’s the short answer: the seasons change because the axis of the Earth is tilted, and so in the summer, the sun’s rays enter the atmosphere at a different angle from the angle they enter at in the winter. When the sunlight has to go through more atmosphere, less of it reaches you in the end.

More about why the seasons change 
What is sunlight made of?
More about the Sun
What is the atmosphere?
Why is the sky blue?

And some easy projects:

Why is the sky blue?
Why do my ears pop?
 Sunny but cold

Just remember: it’s NOT because the sunlight has to go farther to reach the Earth. People often say that, but the added distance is not big enough to matter. It’s all about how far the sunlight has to go through the Earth’s atmosphere.